diff --git a/docs/subsystems/events-system.md b/docs/subsystems/events-system.md
index 76207f0164..ef3a2571fe 100644
--- a/docs/subsystems/events-system.md
+++ b/docs/subsystems/events-system.md
@@ -381,9 +381,9 @@ node test fixtures and our OpenAPI documentation.
 #### Node testing
 
 Once you've completed backend testing, be sure to add an example event
-in `web/tests/lib/events.js`, a test of the
+in `web/tests/lib/events.cjs`, a test of the
 `server_events_dispatch.js` code for that event in
-`web/tests/dispatch.test.js`, and verify your example
+`web/tests/dispatch.test.cjs`, and verify your example
 against the two versions of the schema that you declared above using
 `tools/check-schemas`.
 
diff --git a/docs/subsystems/markdown.md b/docs/subsystems/markdown.md
index f532e010cc..2589a20986 100644
--- a/docs/subsystems/markdown.md
+++ b/docs/subsystems/markdown.md
@@ -41,7 +41,7 @@ message is sent). As a result, we try to make sure that
 The Python-Markdown implementation is tested by
 `zerver/tests/test_markdown.py`, and the marked.js implementation and
 `markdown.contains_backend_only_syntax` are tested by
-`web/tests/markdown.test.js`.
+`web/tests/markdown.test.cjs`.
 
 A shared set of fixed test data ("test fixtures") is present in
 `zerver/tests/fixtures/markdown_test_cases.json`, and is automatically used
diff --git a/docs/testing/testing-with-node.md b/docs/testing/testing-with-node.md
index f755777298..cd5ce1ff8c 100644
--- a/docs/testing/testing-with-node.md
+++ b/docs/testing/testing-with-node.md
@@ -18,7 +18,7 @@ name when debugging something.
 
 The JS unit tests are written to work with node. You can find them
 in `web/tests`. Here is an example test from
-`web/tests/stream_data.test.js`:
+`web/tests/stream_data.test.cjs`:
 
 ```js
 (function test_get_by_id() {
@@ -45,7 +45,7 @@ there are, you should strive to follow the patterns of the existing tests
 and add your own tests.
 
 A good first test to read is
-[example1.js](https://github.com/zulip/zulip/blob/main/web/tests/example1.test.js).
+[example1.test.cjs](https://github.com/zulip/zulip/blob/main/web/tests/example1.test.cjs).
 (And then there are several other example files.)
 
 ## How the node tests work
@@ -60,10 +60,10 @@ those slow down the tests a lot, and often don't add much value.
 Instead, the preferred model for our unit tests is to mock DOM
 manipulations (which in Zulip are almost exclusively done via
 `jQuery`) using a custom library
-[zjquery](https://github.com/zulip/zulip/blob/main/web/tests/lib/zjquery.js).
+[zjquery](https://github.com/zulip/zulip/blob/main/web/tests/lib/zjquery.cjs).
 
 The
-[unit test file](https://github.com/zulip/zulip/blob/main/web/tests/zjquery.test.js)
+[unit test file](https://github.com/zulip/zulip/blob/main/web/tests/zjquery.test.cjs)
 for `zjquery` is designed to be also serve as nice documentation for
 how to use `zjquery`, and is **highly recommended reading** for anyone
 working on or debugging the Zulip node tests.
@@ -149,7 +149,7 @@ narrow_state.stream = function () {
 
 ## Creating new test modules
 
-The test runner (`index.js`) automatically runs all .js files in the
+The test runner (`index.cjs`) automatically runs all .test.cjs files in the
 `web/tests` directory, so you can simply start editing a file
 in that directory to create a new test.
 
@@ -278,7 +278,7 @@ These instructions assume you're using the Vagrant development environment.
       1. **Set the `Node.js interpreter path` to `/usr/local/bin/node`**
       1. Hit `OK` 2 times to get back to the `Run/Debug Configurations` window.
    1. Under `Working Directory` select the root `zulip` directory.
-   1. Under `JavaScript file`, enter `web/tests/lib/index.js`
+   1. Under `JavaScript file`, enter `web/tests/lib/index.cjs`
       -- this is the root script for Zulip's node unit tests.
 
 Congratulations! You've now set up the integration.
@@ -288,7 +288,7 @@ Congratulations! You've now set up the integration.
 To use Webstorm to debug a given node test file, do the following:
 
 1. Under `Application parameters` choose the node test file that you
-   are trying to test (e.g., `web/tests/message_store.test.js`).
+   are trying to test (e.g., `web/tests/message_store.test.cjs`).
 1. Under `Path Mappings`, set `Project Root` to `/srv/zulip`
    (i.e. where the `zulip` Git repository is mounted in the Vagrant guest).
 1. Use the WebStorm debugger; see [this overview][webstorm-debugging]
diff --git a/docs/tutorials/new-feature-tutorial.md b/docs/tutorials/new-feature-tutorial.md
index 54a2035fc1..75ce9d3826 100644
--- a/docs/tutorials/new-feature-tutorial.md
+++ b/docs/tutorials/new-feature-tutorial.md
@@ -62,7 +62,7 @@ organization in Zulip). The following files are involved in the process:
 
 - `web/e2e-tests/admin.test.ts`: end-to-end tests for the organization
   admin settings pages.
-- `web/tests/dispatch.test.js`
+- `web/tests/dispatch.test.cjs`
 
 **Documentation**
 
@@ -668,11 +668,11 @@ frontend tests: [node-based unit tests](../testing/testing-with-node.md) and
 
 At the minimum, if you created a new function to update UI in
 `settings_org.ts`, you will need to mock that function in
-`web/tests/dispatch.test.js`. Add the name of the UI
+`web/tests/dispatch.test.cjs`. Add the name of the UI
 function you created to the following object with `noop` as the value:
 
 ```js
-// web/tests/dispatch.test.js
+// web/tests/dispatch.test.cjs
 
 set_global('settings_org', {
     update_email_change_display: noop,
diff --git a/tools/check-schemas b/tools/check-schemas
index 0b890008cd..444ff90bf4 100755
--- a/tools/check-schemas
+++ b/tools/check-schemas
@@ -23,7 +23,7 @@ TOOLS_DIR = os.path.dirname(os.path.abspath(__file__))
 sys.path.insert(0, os.path.dirname(TOOLS_DIR))
 ROOT_DIR = os.path.dirname(TOOLS_DIR)
 
-EVENTS_JS = "web/tests/lib/events.js"
+EVENTS_JS = "web/tests/lib/events.cjs"
 
 # check for the venv
 from tools.lib import sanity_check
diff --git a/tools/linter_lib/custom_check.py b/tools/linter_lib/custom_check.py
index f3efcfa046..b8ba0a91dc 100644
--- a/tools/linter_lib/custom_check.py
+++ b/tools/linter_lib/custom_check.py
@@ -207,7 +207,7 @@ js_rules = RuleList(
             "exclude_pattern": r"(const |\S)style ?=",
             "description": "Avoid using the `style=` attribute; we prefer styling in CSS files",
             "exclude": {
-                "web/tests/copy_and_paste.test.js",
+                "web/tests/copy_and_paste.test.cjs",
             },
             "good_lines": ["#my-style {color: blue;}", "const style =", 'some_style = "test"'],
             "bad_lines": ['<p style="color: blue;">Foo</p>', 'style = "color: blue;"'],
diff --git a/tools/node_lib/dump_fixtures.js b/tools/node_lib/dump_fixtures.js
index f27b3368a5..8df0004121 100644
--- a/tools/node_lib/dump_fixtures.js
+++ b/tools/node_lib/dump_fixtures.js
@@ -1,5 +1,5 @@
 "use strict";
 
-const events = require("../../web/tests/lib/events.js");
+const events = require("../../web/tests/lib/events.cjs");
 
 console.info(JSON.stringify(events.fixtures, null, 4));
diff --git a/tools/test-js-with-node b/tools/test-js-with-node
index bd98f777f2..830a9bb65d 100755
--- a/tools/test-js-with-node
+++ b/tools/test-js-with-node
@@ -20,7 +20,7 @@ sanity_check.check_venv(__file__)
 import orjson
 from zulint.printer import BOLDRED, CYAN, ENDC, GREEN
 
-INDEX_JS = os.path.join(ROOT_DIR, "web/tests/lib/index.js")
+INDEX_JS = os.path.join(ROOT_DIR, "web/tests/lib/index.cjs")
 NODE_COVERAGE_PATH = os.path.join(ROOT_DIR, "var/node-coverage/coverage-final.json")
 
 # Ideally, we wouldn't need this line, but it seems to be required to
@@ -28,9 +28,9 @@ NODE_COVERAGE_PATH = os.path.join(ROOT_DIR, "var/node-coverage/coverage-final.js
 os.chdir(ROOT_DIR)
 
 USAGE = """
-    tools/test-js-with-node                                - to run all tests
-    tools/test-js-with-node util.test.js activity.test.js  - to run just a couple tests
-    tools/test-js-with-node --coverage                     - to generate coverage report
+    tools/test-js-with-node                                  - to run all tests
+    tools/test-js-with-node util.test.cjs activity.test.cjs  - to run just a couple tests
+    tools/test-js-with-node --coverage                       - to generate coverage report
     """
 
 
@@ -291,14 +291,14 @@ EXEMPT_FILES = make_set(
         "web/src/zcommand.ts",
         "web/src/zform.js",
         "web/src/zulip_test.ts",
-        "web/tests/lib/example_user.js",
-        "web/tests/lib/mdiff.js",
-        "web/tests/lib/real_jquery.js",
-        "web/tests/lib/zjquery_element.js",
-        "web/tests/lib/zpage_billing_params.js",
+        "web/tests/lib/example_user.cjs",
+        "web/tests/lib/mdiff.cjs",
+        "web/tests/lib/real_jquery.cjs",
+        "web/tests/lib/zjquery_element.cjs",
+        "web/tests/lib/zpage_billing_params.cjs",
         # There are some important functions which are not called right now but will
         # be reused when we add tests for dropdown widget so it doesn't make sense to remove them.
-        "web/tests/recent_view.test.js",
+        "web/tests/recent_view.test.cjs",
     ]
 )
 
@@ -362,8 +362,8 @@ def print_error(msg: str) -> None:
 
 def clean_file(orig_fn: str) -> str:
     fn = orig_fn
-    if not fn.endswith(".test.js"):
-        fn += ".test.js"
+    if not fn.endswith(".test.cjs"):
+        fn += ".test.cjs"
     if "web/tests/" not in fn:
         fn = os.path.join(ROOT_DIR, "web", "tests", fn)
     fn = os.path.abspath(fn)
@@ -381,7 +381,7 @@ def clean_files(fns: list[str]) -> list[str]:
 def run_tests_via_node_js() -> int:
     os.environ["TZ"] = "UTC"
 
-    # The index.js test runner is the real "driver" here, and we launch
+    # The index.cjs test runner is the real "driver" here, and we launch
     # with either nyc or node, depending on whether we want coverage
     # reports.  Running under nyc is slower and creates funny
     # tracebacks, so you generally want to get coverage reports only
@@ -393,7 +393,7 @@ def run_tests_via_node_js() -> int:
         parallel = 1
         files = individual_files
     else:
-        files = sorted(glob.glob(os.path.join(ROOT_DIR, "web/tests/*.test.js")))
+        files = sorted(glob.glob(os.path.join(ROOT_DIR, "web/tests/*.test.cjs")))
 
     test_files = clean_files(files)
 
@@ -417,14 +417,15 @@ def run_tests_via_node_js() -> int:
         coverage_dir = os.path.join(ROOT_DIR, "var/node-coverage")
 
         nyc = os.path.join(ROOT_DIR, "node_modules/.bin/nyc")
-        command = [nyc, "--extension", ".hbs", "--extension", ".ts"]
+        command = [nyc]
+        command += [f"--extension={ext}" for ext in [".cjs", ".cts", ".hbs", ".mjs", ".mts", ".ts"]]
         command += ["--report-dir", coverage_dir]
         command += ["--temp-directory", coverage_dir]
         command += ["-r=lcov", "-r=json", "-r=text-summary"]
         command += node_tests_cmd
     else:
         # Normal testing, no coverage analysis.
-        # Run the index.js test runner, which runs all the other tests.
+        # Run the index.cjs test runner, which runs all the other tests.
         command = node_tests_cmd
 
     try:
@@ -473,8 +474,8 @@ def enforce_proper_coverage(coverage_json: Any) -> bool:
         *glob.glob("web/src/*.js"),
         *glob.glob("web/src/*.ts"),
         *glob.glob("web/src/billing/*.js"),
-        *glob.glob("web/tests/*.js"),
-        *glob.glob("web/tests/lib/*.js"),
+        *glob.glob("web/tests/*.cjs"),
+        *glob.glob("web/tests/lib/*.cjs"),
     }
     missing_files = sorted(EXEMPT_FILES - all_js_files)
     assert not missing_files, f"Missing files should be removed from EXEMPT_FILES: {missing_files}"
diff --git a/web/tests/activity.test.js b/web/tests/activity.test.cjs
similarity index 99%
rename from web/tests/activity.test.js
rename to web/tests/activity.test.cjs
index 4b660070ff..7bd51aedf0 100644
--- a/web/tests/activity.test.js
+++ b/web/tests/activity.test.cjs
@@ -8,12 +8,12 @@ const {
     buddy_list_add_user_matching_view,
     buddy_list_add_other_user,
     stub_buddy_list_elements,
-} = require("./lib/buddy_list.js");
-const {mock_esm, set_global, with_overrides, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const $ = require("./lib/zjquery.js");
-const {page_params} = require("./lib/zpage_params.js");
+} = require("./lib/buddy_list.cjs");
+const {mock_esm, set_global, with_overrides, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const $ = require("./lib/zjquery.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 const $window_stub = $.create("window-stub");
 set_global("to_$", () => $window_stub);
diff --git a/web/tests/alert_words.test.js b/web/tests/alert_words.test.cjs
similarity index 98%
rename from web/tests/alert_words.test.js
rename to web/tests/alert_words.test.cjs
index 1c003a317c..0d788ec8ec 100644
--- a/web/tests/alert_words.test.js
+++ b/web/tests/alert_words.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {set_global, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 set_global("page_params", {
     is_spectator: false,
diff --git a/web/tests/alert_words_ui.test.js b/web/tests/alert_words_ui.test.cjs
similarity index 95%
rename from web/tests/alert_words_ui.test.js
rename to web/tests/alert_words_ui.test.cjs
index 6a8b027fc9..595a473155 100644
--- a/web/tests/alert_words_ui.test.js
+++ b/web/tests/alert_words_ui.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {$t} = require("./lib/i18n.js");
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {$t} = require("./lib/i18n.cjs");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const channel = mock_esm("../src/channel");
 
diff --git a/web/tests/blueslip_stacktrace.test.js b/web/tests/blueslip_stacktrace.test.cjs
similarity index 94%
rename from web/tests/blueslip_stacktrace.test.js
rename to web/tests/blueslip_stacktrace.test.cjs
index 726b611335..1e2f800edb 100644
--- a/web/tests/blueslip_stacktrace.test.js
+++ b/web/tests/blueslip_stacktrace.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const blueslip_stacktrace = zrequire("blueslip_stacktrace");
 
diff --git a/web/tests/bot_data.test.js b/web/tests/bot_data.test.cjs
similarity index 98%
rename from web/tests/bot_data.test.js
rename to web/tests/bot_data.test.cjs
index bfd55f7724..bc4840fc88 100644
--- a/web/tests/bot_data.test.js
+++ b/web/tests/bot_data.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const bot_data = zrequire("bot_data");
 
diff --git a/web/tests/browser_history.test.js b/web/tests/browser_history.test.cjs
similarity index 92%
rename from web/tests/browser_history.test.js
rename to web/tests/browser_history.test.cjs
index b99b7647a6..58bc36f0d0 100644
--- a/web/tests/browser_history.test.js
+++ b/web/tests/browser_history.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {make_stub} = require("./lib/stub.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {make_stub} = require("./lib/stub.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 window.location.hash = "#bogus";
 
diff --git a/web/tests/buddy_data.test.js b/web/tests/buddy_data.test.cjs
similarity index 98%
rename from web/tests/buddy_data.test.js
rename to web/tests/buddy_data.test.cjs
index 4dda0e31d9..515edc46f7 100644
--- a/web/tests/buddy_data.test.js
+++ b/web/tests/buddy_data.test.cjs
@@ -4,9 +4,9 @@ const assert = require("node:assert/strict");
 
 const _ = require("lodash");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 mock_esm("../src/settings_data", {
     user_can_access_all_other_users: () => true,
@@ -35,7 +35,7 @@ const user_settings = {};
 initialize_user_settings({user_settings});
 
 // The buddy_data module is mostly tested indirectly through
-// activity.test.js, but we should feel free to add direct tests
+// activity.test.cjs, but we should feel free to add direct tests
 // here.
 
 const selma = {
diff --git a/web/tests/buddy_list.test.js b/web/tests/buddy_list.test.cjs
similarity index 97%
rename from web/tests/buddy_list.test.js
rename to web/tests/buddy_list.test.cjs
index 99cf3c22aa..14bce71b30 100644
--- a/web/tests/buddy_list.test.js
+++ b/web/tests/buddy_list.test.cjs
@@ -10,11 +10,11 @@ const {
     buddy_list_add_user_matching_view,
     buddy_list_add_other_user,
     stub_buddy_list_elements,
-} = require("./lib/buddy_list.js");
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const $ = require("./lib/zjquery.js");
+} = require("./lib/buddy_list.cjs");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const padded_widget = mock_esm("../src/padded_widget");
 const message_viewport = mock_esm("../src/message_viewport");
diff --git a/web/tests/channel.test.js b/web/tests/channel.test.cjs
similarity index 98%
rename from web/tests/channel.test.js
rename to web/tests/channel.test.cjs
index f3466def0f..2d5fe1b341 100644
--- a/web/tests/channel.test.js
+++ b/web/tests/channel.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_jquery, mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {mock_jquery, mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 const xhr_401 = {
     status: 401,
diff --git a/web/tests/color_data.test.js b/web/tests/color_data.test.cjs
similarity index 89%
rename from web/tests/color_data.test.js
rename to web/tests/color_data.test.cjs
index a658e74256..034eeed686 100644
--- a/web/tests/color_data.test.js
+++ b/web/tests/color_data.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const color_data = zrequire("color_data");
 
diff --git a/web/tests/common.test.js b/web/tests/common.test.cjs
similarity index 98%
rename from web/tests/common.test.js
rename to web/tests/common.test.cjs
index c6715fd35e..f08ace2ec0 100644
--- a/web/tests/common.test.js
+++ b/web/tests/common.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 mock_esm("tippy.js", {
     default(arg) {
diff --git a/web/tests/components.test.js b/web/tests/components.test.cjs
similarity index 97%
rename from web/tests/components.test.js
rename to web/tests/components.test.cjs
index d355750c96..3307c6a548 100644
--- a/web/tests/components.test.js
+++ b/web/tests/components.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {$t} = require("./lib/i18n.js");
-const {mock_jquery, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const {$t} = require("./lib/i18n.cjs");
+const {mock_jquery, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 let env;
 
diff --git a/web/tests/compose.test.js b/web/tests/compose.test.cjs
similarity index 99%
rename from web/tests/compose.test.js
rename to web/tests/compose.test.cjs
index bd9798a321..530d615de1 100644
--- a/web/tests/compose.test.js
+++ b/web/tests/compose.test.cjs
@@ -4,11 +4,11 @@ const assert = require("node:assert/strict");
 
 const MockDate = require("mockdate");
 
-const {mock_banners} = require("./lib/compose_banner.js");
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {mock_banners} = require("./lib/compose_banner.cjs");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 const user_groups = zrequire("user_groups");
 
diff --git a/web/tests/compose_actions.test.js b/web/tests/compose_actions.test.cjs
similarity index 98%
rename from web/tests/compose_actions.test.js
rename to web/tests/compose_actions.test.cjs
index 1fb2aaf221..242a128299 100644
--- a/web/tests/compose_actions.test.js
+++ b/web/tests/compose_actions.test.cjs
@@ -2,11 +2,11 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_banners} = require("./lib/compose_banner.js");
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const $ = require("./lib/zjquery.js");
+const {mock_banners} = require("./lib/compose_banner.cjs");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const user_groups = zrequire("user_groups");
 
diff --git a/web/tests/compose_closed_ui.test.js b/web/tests/compose_closed_ui.test.cjs
similarity index 97%
rename from web/tests/compose_closed_ui.test.js
rename to web/tests/compose_closed_ui.test.cjs
index 43b1030a02..b761ad54f5 100644
--- a/web/tests/compose_closed_ui.test.js
+++ b/web/tests/compose_closed_ui.test.cjs
@@ -3,9 +3,9 @@
 // Setup
 const assert = require("node:assert/strict");
 
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 // Mocking and stubbing things
 set_global("document", "document-stub");
diff --git a/web/tests/compose_fade.test.js b/web/tests/compose_fade.test.cjs
similarity index 96%
rename from web/tests/compose_fade.test.js
rename to web/tests/compose_fade.test.cjs
index 621a39682b..4b3334bcc9 100644
--- a/web/tests/compose_fade.test.js
+++ b/web/tests/compose_fade.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_jquery, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {mock_jquery, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 mock_jquery((selector) => {
     switch (selector) {
diff --git a/web/tests/compose_pm_pill.test.js b/web/tests/compose_pm_pill.test.cjs
similarity index 97%
rename from web/tests/compose_pm_pill.test.js
rename to web/tests/compose_pm_pill.test.cjs
index 91f1545485..80661a53e7 100644
--- a/web/tests/compose_pm_pill.test.js
+++ b/web/tests/compose_pm_pill.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const input_pill = mock_esm("../src/input_pill");
 const people = zrequire("people");
diff --git a/web/tests/compose_state.test.js b/web/tests/compose_state.test.cjs
similarity index 91%
rename from web/tests/compose_state.test.js
rename to web/tests/compose_state.test.cjs
index 5aa4c4f7ce..0b2b935cb6 100644
--- a/web/tests/compose_state.test.js
+++ b/web/tests/compose_state.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const compose_pm_pill = mock_esm("../src/compose_pm_pill");
 
diff --git a/web/tests/compose_ui.test.js b/web/tests/compose_ui.test.cjs
similarity index 99%
rename from web/tests/compose_ui.test.js
rename to web/tests/compose_ui.test.cjs
index e8014e940f..b60f7c5d57 100644
--- a/web/tests/compose_ui.test.js
+++ b/web/tests/compose_ui.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {$t} = require("./lib/i18n.js");
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {$t} = require("./lib/i18n.cjs");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 set_global("navigator", {});
 
diff --git a/web/tests/compose_validate.test.js b/web/tests/compose_validate.test.cjs
similarity index 99%
rename from web/tests/compose_validate.test.js
rename to web/tests/compose_validate.test.cjs
index 2cde480ab9..daa86e480f 100644
--- a/web/tests/compose_validate.test.js
+++ b/web/tests/compose_validate.test.cjs
@@ -2,12 +2,12 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_banners} = require("./lib/compose_banner.js");
-const {$t} = require("./lib/i18n.js");
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const $ = require("./lib/zjquery.js");
+const {mock_banners} = require("./lib/compose_banner.cjs");
+const {$t} = require("./lib/i18n.cjs");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const compose_banner = zrequire("compose_banner");
 const compose_pm_pill = zrequire("compose_pm_pill");
diff --git a/web/tests/compose_video.test.js b/web/tests/compose_video.test.cjs
similarity index 97%
rename from web/tests/compose_video.test.js
rename to web/tests/compose_video.test.cjs
index 3c9f8adee7..bb951ce4ee 100644
--- a/web/tests/compose_video.test.js
+++ b/web/tests/compose_video.test.cjs
@@ -2,11 +2,11 @@
 
 const assert = require("node:assert/strict");
 
-const events = require("./lib/events.js");
-const {mock_esm, set_global, with_overrides, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
-const {page_params} = require("./lib/zpage_params.js");
+const events = require("./lib/events.cjs");
+const {mock_esm, set_global, with_overrides, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 const channel = mock_esm("../src/channel");
 const compose_closed_ui = mock_esm("../src/compose_closed_ui");
diff --git a/web/tests/composebox_typeahead.test.js b/web/tests/composebox_typeahead.test.cjs
similarity index 99%
rename from web/tests/composebox_typeahead.test.js
rename to web/tests/composebox_typeahead.test.cjs
index 0a91faa61f..9ebded8755 100644
--- a/web/tests/composebox_typeahead.test.js
+++ b/web/tests/composebox_typeahead.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_banners} = require("./lib/compose_banner.js");
-const {mock_esm, set_global, with_overrides, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_banners} = require("./lib/compose_banner.cjs");
+const {mock_esm, set_global, with_overrides, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 let autosize_called;
 
diff --git a/web/tests/copy_and_paste.test.js b/web/tests/copy_and_paste.test.cjs
similarity index 99%
rename from web/tests/copy_and_paste.test.js
rename to web/tests/copy_and_paste.test.cjs
index ad15371bf6..1feb8a674e 100644
--- a/web/tests/copy_and_paste.test.js
+++ b/web/tests/copy_and_paste.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const copy_and_paste = zrequire("copy_and_paste");
 const stream_data = zrequire("stream_data");
diff --git a/web/tests/deprecated_feature_notice.test.js b/web/tests/deprecated_feature_notice.test.cjs
similarity index 82%
rename from web/tests/deprecated_feature_notice.test.js
rename to web/tests/deprecated_feature_notice.test.cjs
index feafb42d32..ae5f03a755 100644
--- a/web/tests/deprecated_feature_notice.test.js
+++ b/web/tests/deprecated_feature_notice.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {set_global, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 set_global("navigator", {
     userAgent: "",
diff --git a/web/tests/dispatch.test.js b/web/tests/dispatch.test.cjs
similarity index 99%
rename from web/tests/dispatch.test.js
rename to web/tests/dispatch.test.cjs
index 7732a4cfab..0202960b7e 100644
--- a/web/tests/dispatch.test.js
+++ b/web/tests/dispatch.test.cjs
@@ -2,13 +2,13 @@
 
 const assert = require("node:assert/strict");
 
-const events = require("./lib/events.js");
-const {mock_esm, set_global, with_overrides, zrequire} = require("./lib/namespace.js");
-const {make_stub} = require("./lib/stub.js");
-const {run_test, noop} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const $ = require("./lib/zjquery.js");
-const {page_params} = require("./lib/zpage_params.js");
+const events = require("./lib/events.cjs");
+const {mock_esm, set_global, with_overrides, zrequire} = require("./lib/namespace.cjs");
+const {make_stub} = require("./lib/stub.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const $ = require("./lib/zjquery.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 const event_fixtures = events.fixtures;
 const test_message = events.test_message;
@@ -865,7 +865,7 @@ run_test("submessage", ({override}) => {
     });
 });
 
-// For subscriptions, see dispatch_subs.test.js
+// For subscriptions, see dispatch_subs.test.cjs
 
 run_test("typing", ({override}) => {
     // Simulate that we are not typing.
diff --git a/web/tests/dispatch_subs.test.js b/web/tests/dispatch_subs.test.cjs
similarity index 97%
rename from web/tests/dispatch_subs.test.js
rename to web/tests/dispatch_subs.test.cjs
index 8c59f90c16..2e4812e002 100644
--- a/web/tests/dispatch_subs.test.js
+++ b/web/tests/dispatch_subs.test.cjs
@@ -2,11 +2,11 @@
 
 const assert = require("node:assert/strict");
 
-const events = require("./lib/events.js");
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {make_stub} = require("./lib/stub.js");
-const {run_test, noop} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const events = require("./lib/events.cjs");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {make_stub} = require("./lib/stub.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 const event_fixtures = events.fixtures;
 const test_user = events.test_user;
diff --git a/web/tests/drafts.test.js b/web/tests/drafts.test.cjs
similarity index 99%
rename from web/tests/drafts.test.js
rename to web/tests/drafts.test.cjs
index 0214b5c2a3..b33f82dc64 100644
--- a/web/tests/drafts.test.js
+++ b/web/tests/drafts.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_banners} = require("./lib/compose_banner.js");
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_banners} = require("./lib/compose_banner.cjs");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const user_pill = mock_esm("../src/user_pill");
 const messages_overlay_ui = mock_esm("../src/messages_overlay_ui");
diff --git a/web/tests/echo.test.js b/web/tests/echo.test.cjs
similarity index 98%
rename from web/tests/echo.test.js
rename to web/tests/echo.test.cjs
index 4829ac40cf..95d4b055f8 100644
--- a/web/tests/echo.test.js
+++ b/web/tests/echo.test.cjs
@@ -4,9 +4,9 @@ const assert = require("node:assert/strict");
 
 const MockDate = require("mockdate");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {make_stub} = require("./lib/stub.js");
-const {run_test, noop} = require("./lib/test.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {make_stub} = require("./lib/stub.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
 
 const compose_notifications = mock_esm("../src/compose_notifications");
 const markdown = mock_esm("../src/markdown");
diff --git a/web/tests/emoji.test.js b/web/tests/emoji.test.cjs
similarity index 95%
rename from web/tests/emoji.test.js
rename to web/tests/emoji.test.cjs
index 7703dd4186..6f1281724c 100644
--- a/web/tests/emoji.test.js
+++ b/web/tests/emoji.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const events = require("./lib/events.js");
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const events = require("./lib/events.cjs");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 const emoji_codes = zrequire("../../static/generated/emoji/emoji_codes.json");
 
diff --git a/web/tests/emoji_picker.test.js b/web/tests/emoji_picker.test.cjs
similarity index 96%
rename from web/tests/emoji_picker.test.js
rename to web/tests/emoji_picker.test.cjs
index 10b1d997e7..f8f873344b 100644
--- a/web/tests/emoji_picker.test.js
+++ b/web/tests/emoji_picker.test.cjs
@@ -4,8 +4,8 @@ const assert = require("node:assert/strict");
 
 const _ = require("lodash");
 
-const {zrequire, set_global} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire, set_global} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const emoji = zrequire("emoji");
 const emoji_picker = zrequire("emoji_picker");
diff --git a/web/tests/example1.test.js b/web/tests/example1.test.cjs
similarity index 88%
rename from web/tests/example1.test.js
rename to web/tests/example1.test.cjs
index 7dcf052a53..5886932dd8 100644
--- a/web/tests/example1.test.js
+++ b/web/tests/example1.test.cjs
@@ -9,10 +9,10 @@
 // become clear as you keep reading.
 const assert = require("node:assert/strict");
 
-const {make_stream} = require("./lib/example_stream.js");
-const {make_user} = require("./lib/example_user.js");
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {make_stream} = require("./lib/example_stream.cjs");
+const {make_user} = require("./lib/example_user.cjs");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 // We will use our special zrequire helper to import the
 // Zulip code. We use zrequire instead of require,
@@ -43,7 +43,7 @@ const isaac = make_user({
 
 // The `people` object is a very fundamental object in the
 // Zulip app.  You can learn a lot more about it by reading
-// the tests in people.test.js in the same directory as this file.
+// the tests in people.test.cjs in the same directory as this file.
 
 // Let's exercise the code and use assert to verify it works!
 assert.ok(!people.is_known_user_id(isaac.user_id));
@@ -74,4 +74,4 @@ run_test("verify stream_data persists stream color", () => {
     const sub = stream_data.get_sub_by_name("Denmark");
     assert.equal(sub.color, "a1a1a1");
 });
-// See example2.test.js in this directory.
+// See example2.test.cjs in this directory.
diff --git a/web/tests/example2.test.js b/web/tests/example2.test.cjs
similarity index 93%
rename from web/tests/example2.test.js
rename to web/tests/example2.test.cjs
index 36be9a3ff3..aae5b4365e 100644
--- a/web/tests/example2.test.js
+++ b/web/tests/example2.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {make_stream} = require("./lib/example_stream.js");
-const {make_user} = require("./lib/example_user.js");
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {make_stream} = require("./lib/example_stream.cjs");
+const {make_user} = require("./lib/example_user.cjs");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 // Hopefully the basic patterns for testing data-oriented modules
 // are starting to become apparent.  To reinforce that, we will present
diff --git a/web/tests/example3.test.js b/web/tests/example3.test.cjs
similarity index 94%
rename from web/tests/example3.test.js
rename to web/tests/example3.test.cjs
index 22594da629..a1026d67d0 100644
--- a/web/tests/example3.test.js
+++ b/web/tests/example3.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {make_stream} = require("./lib/example_stream.js");
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {make_stream} = require("./lib/example_stream.cjs");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 // In the Zulip app you can narrow your message stream by topic, by
 // sender, by direct message recipient, by search keywords, etc.
diff --git a/web/tests/example4.test.js b/web/tests/example4.test.cjs
similarity index 95%
rename from web/tests/example4.test.js
rename to web/tests/example4.test.cjs
index 74f0848051..9513bfb776 100644
--- a/web/tests/example4.test.js
+++ b/web/tests/example4.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {make_bot} = require("./lib/example_user.js");
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
+const {make_bot} = require("./lib/example_user.cjs");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
 
 /*
 
@@ -24,7 +24,7 @@ const {run_test, noop} = require("./lib/test.js");
     you are interested, all of these objects have test
     suites that have 100% line coverage on the modules
     that implement those objects.  For example, you can look
-    at people.test.js in this directory for more tests on the
+    at people.test.cjs in this directory for more tests on the
     people object.
 
     We can quickly review some testing concepts:
diff --git a/web/tests/example5.test.js b/web/tests/example5.test.cjs
similarity index 96%
rename from web/tests/example5.test.js
rename to web/tests/example5.test.cjs
index f1858fe93d..b3bd1df8f1 100644
--- a/web/tests/example5.test.js
+++ b/web/tests/example5.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {make_user} = require("./lib/example_user.js");
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
+const {make_user} = require("./lib/example_user.cjs");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
 
 /*
    Our test from an earlier example verifies that the update events
diff --git a/web/tests/example6.test.js b/web/tests/example6.test.cjs
similarity index 95%
rename from web/tests/example6.test.js
rename to web/tests/example6.test.cjs
index bff63293be..556aa31ef3 100644
--- a/web/tests/example6.test.js
+++ b/web/tests/example6.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {make_stub} = require("./lib/stub.js");
-const {run_test, noop} = require("./lib/test.js");
+const {make_stub} = require("./lib/stub.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
 
 /*
     The previous example was a bit extreme.  Generally we just
diff --git a/web/tests/example7.test.js b/web/tests/example7.test.cjs
similarity index 97%
rename from web/tests/example7.test.js
rename to web/tests/example7.test.cjs
index 698878cc13..b095f95a01 100644
--- a/web/tests/example7.test.js
+++ b/web/tests/example7.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {make_stream} = require("./lib/example_stream.js");
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {make_stream} = require("./lib/example_stream.cjs");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 /*
 
diff --git a/web/tests/example8.test.js b/web/tests/example8.test.cjs
similarity index 96%
rename from web/tests/example8.test.js
rename to web/tests/example8.test.cjs
index 5d804cd8bc..f72bc85731 100644
--- a/web/tests/example8.test.js
+++ b/web/tests/example8.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {make_user} = require("./lib/example_user.js");
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {make_user} = require("./lib/example_user.cjs");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 mock_esm("../src/settings_data", {
     user_can_access_all_other_users: () => true,
diff --git a/web/tests/fenced_code.test.js b/web/tests/fenced_code.test.cjs
similarity index 94%
rename from web/tests/fenced_code.test.js
rename to web/tests/fenced_code.test.cjs
index 986107c92f..77f894cb21 100644
--- a/web/tests/fenced_code.test.js
+++ b/web/tests/fenced_code.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const fenced_code = zrequire("../shared/src/fenced_code");
 
diff --git a/web/tests/fetch_status.test.js b/web/tests/fetch_status.test.cjs
similarity index 97%
rename from web/tests/fetch_status.test.js
rename to web/tests/fetch_status.test.cjs
index 52d0ecaaa2..6438f2cf1c 100644
--- a/web/tests/fetch_status.test.js
+++ b/web/tests/fetch_status.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 mock_esm("../src/message_feed_loading", {
     hide_loading_older() {},
diff --git a/web/tests/filter.test.js b/web/tests/filter.test.cjs
similarity index 99%
rename from web/tests/filter.test.js
rename to web/tests/filter.test.cjs
index 30b0e182ee..5dbc0cfe04 100644
--- a/web/tests/filter.test.js
+++ b/web/tests/filter.test.cjs
@@ -4,11 +4,11 @@ const assert = require("node:assert/strict");
 
 const {parseOneAddress} = require("email-addresses");
 
-const {mock_esm, with_overrides, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const $ = require("./lib/zjquery.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {mock_esm, with_overrides, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const $ = require("./lib/zjquery.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 const message_store = mock_esm("../src/message_store");
 const user_topics = mock_esm("../src/user_topics");
diff --git a/web/tests/fold_dict.test.js b/web/tests/fold_dict.test.cjs
similarity index 95%
rename from web/tests/fold_dict.test.js
rename to web/tests/fold_dict.test.cjs
index 75a3b995f1..81e2bc5e4e 100644
--- a/web/tests/fold_dict.test.js
+++ b/web/tests/fold_dict.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const {FoldDict} = zrequire("fold_dict");
 
diff --git a/web/tests/gear_menu_util.test.js b/web/tests/gear_menu_util.test.cjs
similarity index 97%
rename from web/tests/gear_menu_util.test.js
rename to web/tests/gear_menu_util.test.cjs
index c7119c59ec..d6fc8a8556 100644
--- a/web/tests/gear_menu_util.test.js
+++ b/web/tests/gear_menu_util.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const gear_menu_util = zrequire("gear_menu_util");
 const {set_realm} = zrequire("state_data");
diff --git a/web/tests/hash_util.test.js b/web/tests/hash_util.test.cjs
similarity index 98%
rename from web/tests/hash_util.test.js
rename to web/tests/hash_util.test.cjs
index ba20605cde..852b5f0e2f 100644
--- a/web/tests/hash_util.test.js
+++ b/web/tests/hash_util.test.cjs
@@ -4,8 +4,8 @@ const assert = require("node:assert/strict");
 
 const message_link_test_cases = require("../../zerver/tests/fixtures/message_link_test_cases.json");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const hash_parser = zrequire("hash_parser");
 const hash_util = zrequire("hash_util");
diff --git a/web/tests/hashchange.test.js b/web/tests/hashchange.test.cjs
similarity index 99%
rename from web/tests/hashchange.test.js
rename to web/tests/hashchange.test.cjs
index c9da4b2012..0c32f666fb 100644
--- a/web/tests/hashchange.test.js
+++ b/web/tests/hashchange.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 let $window_stub;
 set_global("to_$", () => $window_stub);
diff --git a/web/tests/hotkey.test.js b/web/tests/hotkey.test.cjs
similarity index 99%
rename from web/tests/hotkey.test.js
rename to web/tests/hotkey.test.cjs
index d702b9db52..523faadcbf 100644
--- a/web/tests/hotkey.test.js
+++ b/web/tests/hotkey.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, set_global, with_overrides, zrequire} = require("./lib/namespace.js");
-const {make_stub} = require("./lib/stub.js");
-const {run_test} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, set_global, with_overrides, zrequire} = require("./lib/namespace.cjs");
+const {make_stub} = require("./lib/stub.cjs");
+const {run_test} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 // Important note on these tests:
 
diff --git a/web/tests/i18n.test.js b/web/tests/i18n.test.cjs
similarity index 97%
rename from web/tests/i18n.test.js
rename to web/tests/i18n.test.cjs
index 212009892a..c06fc1c8ad 100644
--- a/web/tests/i18n.test.js
+++ b/web/tests/i18n.test.cjs
@@ -4,9 +4,9 @@ const assert = require("node:assert/strict");
 
 const _ = require("lodash");
 
-const {unmock_module, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {unmock_module, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 // We download our translations in `page_params` (which
 // are for the user's chosen language), so we simulate
diff --git a/web/tests/input_pill.test.js b/web/tests/input_pill.test.cjs
similarity index 99%
rename from web/tests/input_pill.test.js
rename to web/tests/input_pill.test.cjs
index 1c897daa3c..6cd2cf2f64 100644
--- a/web/tests/input_pill.test.js
+++ b/web/tests/input_pill.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 set_global("document", {});
 class ClipboardEvent {}
diff --git a/web/tests/internal_url.test.js b/web/tests/internal_url.test.cjs
similarity index 95%
rename from web/tests/internal_url.test.js
rename to web/tests/internal_url.test.cjs
index f5a0098324..7eb31fcbf3 100644
--- a/web/tests/internal_url.test.js
+++ b/web/tests/internal_url.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const internal_url = zrequire("../shared/src/internal_url");
 
diff --git a/web/tests/keydown_util.test.js b/web/tests/keydown_util.test.cjs
similarity index 92%
rename from web/tests/keydown_util.test.js
rename to web/tests/keydown_util.test.cjs
index cf62358387..d8dc715f93 100644
--- a/web/tests/keydown_util.test.js
+++ b/web/tests/keydown_util.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const keydown_util = zrequire("keydown_util");
 
diff --git a/web/tests/lazy_set.test.js b/web/tests/lazy_set.test.cjs
similarity index 83%
rename from web/tests/lazy_set.test.js
rename to web/tests/lazy_set.test.cjs
index 066e18eb5f..e01f82b20a 100644
--- a/web/tests/lazy_set.test.js
+++ b/web/tests/lazy_set.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 const {LazySet} = zrequire("lazy_set");
 
diff --git a/web/tests/left_sidebar_navigation_area.test.js b/web/tests/left_sidebar_navigation_area.test.cjs
similarity index 98%
rename from web/tests/left_sidebar_navigation_area.test.js
rename to web/tests/left_sidebar_navigation_area.test.cjs
index 20089431fa..80c38877fc 100644
--- a/web/tests/left_sidebar_navigation_area.test.js
+++ b/web/tests/left_sidebar_navigation_area.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 mock_esm("../src/resize", {
     resize_stream_filters_container() {},
diff --git a/web/tests/lib/buddy_list.js b/web/tests/lib/buddy_list.cjs
similarity index 95%
rename from web/tests/lib/buddy_list.js
rename to web/tests/lib/buddy_list.cjs
index 6c6a3809a8..7b427854d2 100644
--- a/web/tests/lib/buddy_list.js
+++ b/web/tests/lib/buddy_list.cjs
@@ -1,7 +1,7 @@
 "use strict";
 
-const {noop} = require("./test.js");
-const $ = require("./zjquery.js");
+const {noop} = require("./test.cjs");
+const $ = require("./zjquery.cjs");
 
 let users_matching_view = [];
 exports.buddy_list_add_user_matching_view = (user_id, $stub) => {
diff --git a/web/tests/lib/compose_banner.js b/web/tests/lib/compose_banner.cjs
similarity index 95%
rename from web/tests/lib/compose_banner.js
rename to web/tests/lib/compose_banner.cjs
index d7abe56b9b..c02c8a377b 100644
--- a/web/tests/lib/compose_banner.js
+++ b/web/tests/lib/compose_banner.cjs
@@ -2,8 +2,8 @@
 
 const compose_banner = require("../../src/compose_banner.ts");
 
-const {noop} = require("./test.js");
-const $ = require("./zjquery.js");
+const {noop} = require("./test.cjs");
+const $ = require("./zjquery.cjs");
 
 exports.mock_banners = () => {
     // zjquery doesn't support `remove`, which is used when clearing the compose box.
diff --git a/web/tests/lib/events.js b/web/tests/lib/events.cjs
similarity index 100%
rename from web/tests/lib/events.js
rename to web/tests/lib/events.cjs
diff --git a/web/tests/lib/example_stream.js b/web/tests/lib/example_stream.cjs
similarity index 100%
rename from web/tests/lib/example_stream.js
rename to web/tests/lib/example_stream.cjs
diff --git a/web/tests/lib/example_user.js b/web/tests/lib/example_user.cjs
similarity index 100%
rename from web/tests/lib/example_user.js
rename to web/tests/lib/example_user.cjs
diff --git a/web/tests/lib/handlebars.js b/web/tests/lib/handlebars.cjs
similarity index 100%
rename from web/tests/lib/handlebars.js
rename to web/tests/lib/handlebars.cjs
diff --git a/web/tests/lib/i18n.js b/web/tests/lib/i18n.cjs
similarity index 100%
rename from web/tests/lib/i18n.js
rename to web/tests/lib/i18n.cjs
diff --git a/web/tests/lib/index.js b/web/tests/lib/index.cjs
similarity index 89%
rename from web/tests/lib/index.js
rename to web/tests/lib/index.cjs
index 3f0ecd5a45..21728b569e 100644
--- a/web/tests/lib/index.js
+++ b/web/tests/lib/index.cjs
@@ -8,14 +8,14 @@ require("handlebars/runtime.js");
 const {JSDOM} = require("jsdom");
 const _ = require("lodash");
 
-const handlebars = require("./handlebars.js");
-const stub_i18n = require("./i18n.js");
-const namespace = require("./namespace.js");
-const test = require("./test.js");
-const blueslip = require("./zblueslip.js");
-const zjquery = require("./zjquery.js");
-const zpage_billing_params = require("./zpage_billing_params.js");
-const zpage_params = require("./zpage_params.js");
+const handlebars = require("./handlebars.cjs");
+const stub_i18n = require("./i18n.cjs");
+const namespace = require("./namespace.cjs");
+const test = require("./test.cjs");
+const blueslip = require("./zblueslip.cjs");
+const zjquery = require("./zjquery.cjs");
+const zpage_billing_params = require("./zpage_billing_params.cjs");
+const zpage_params = require("./zpage_params.cjs");
 
 process.env.NODE_ENV = "test";
 
@@ -88,7 +88,7 @@ require("../../src/templates.js"); // register Zulip extensions
 async function run_one_module(file) {
     zjquery.clear_initialize_function();
     zjquery.clear_all_elements();
-    console.info("running test " + path.basename(file, ".test.js"));
+    console.info("running test " + path.basename(file, ".test.cjs"));
     test.set_current_file_name(file);
     test.suite.length = 0;
     require(file);
diff --git a/web/tests/lib/markdown_assert.js b/web/tests/lib/markdown_assert.cjs
similarity index 99%
rename from web/tests/lib/markdown_assert.js
rename to web/tests/lib/markdown_assert.cjs
index e13c4d0d8e..37337ca946 100644
--- a/web/tests/lib/markdown_assert.js
+++ b/web/tests/lib/markdown_assert.cjs
@@ -27,7 +27,7 @@ const assert = require("node:assert/strict");
 const {JSDOM} = require("jsdom");
 const _ = require("lodash");
 
-const mdiff = require("./mdiff.js");
+const mdiff = require("./mdiff.cjs");
 
 // Module-level global instance of MarkdownComparer, initialized when needed
 let _markdownComparerInstance = null;
diff --git a/web/tests/lib/mdiff.js b/web/tests/lib/mdiff.cjs
similarity index 100%
rename from web/tests/lib/mdiff.js
rename to web/tests/lib/mdiff.cjs
diff --git a/web/tests/lib/namespace.js b/web/tests/lib/namespace.cjs
similarity index 99%
rename from web/tests/lib/namespace.js
rename to web/tests/lib/namespace.cjs
index eecf58ff4d..b87f21c787 100644
--- a/web/tests/lib/namespace.js
+++ b/web/tests/lib/namespace.cjs
@@ -6,7 +6,7 @@ const path = require("node:path");
 
 const callsites = require("callsites");
 
-const $ = require("./zjquery.js");
+const $ = require("./zjquery.cjs");
 
 const new_globals = new Set();
 let old_globals = {};
@@ -18,7 +18,7 @@ const used_module_mocks = new Set();
 const used_templates = new Set();
 
 const jquery_path = require.resolve("jquery");
-const real_jquery_path = require.resolve("./real_jquery.js");
+const real_jquery_path = require.resolve("./real_jquery.cjs");
 
 let in_mid_render = false;
 let jquery_function;
diff --git a/web/tests/lib/real_jquery.js b/web/tests/lib/real_jquery.cjs
similarity index 100%
rename from web/tests/lib/real_jquery.js
rename to web/tests/lib/real_jquery.cjs
diff --git a/web/tests/lib/stub.js b/web/tests/lib/stub.cjs
similarity index 100%
rename from web/tests/lib/stub.js
rename to web/tests/lib/stub.cjs
diff --git a/web/tests/lib/test.js b/web/tests/lib/test.cjs
similarity index 82%
rename from web/tests/lib/test.js
rename to web/tests/lib/test.cjs
index 1ae5f2fdec..d29b1a2ee0 100644
--- a/web/tests/lib/test.js
+++ b/web/tests/lib/test.cjs
@@ -1,10 +1,10 @@
 "use strict";
 
-const namespace = require("./namespace.js");
-const zblueslip = require("./zblueslip.js");
-const $ = require("./zjquery.js");
-const zpage_billing_params = require("./zpage_billing_params.js");
-const zpage_params = require("./zpage_params.js");
+const namespace = require("./namespace.cjs");
+const zblueslip = require("./zblueslip.cjs");
+const $ = require("./zjquery.cjs");
+const zpage_billing_params = require("./zpage_billing_params.cjs");
+const zpage_params = require("./zpage_params.cjs");
 
 let current_file_name;
 let verbose = false;
diff --git a/web/tests/lib/zblueslip.js b/web/tests/lib/zblueslip.cjs
similarity index 100%
rename from web/tests/lib/zblueslip.js
rename to web/tests/lib/zblueslip.cjs
diff --git a/web/tests/lib/zjquery.js b/web/tests/lib/zjquery.cjs
similarity index 98%
rename from web/tests/lib/zjquery.js
rename to web/tests/lib/zjquery.cjs
index 69cf5d76cd..3a2c7e2c47 100644
--- a/web/tests/lib/zjquery.js
+++ b/web/tests/lib/zjquery.cjs
@@ -8,8 +8,8 @@ const assert = require("node:assert/strict");
     and then subsequent calls to $("#foo") get the
     same instance.
 */
-const FakeElement = require("./zjquery_element.js");
-const FakeEvent = require("./zjquery_event.js");
+const FakeElement = require("./zjquery_element.cjs");
+const FakeEvent = require("./zjquery_event.cjs");
 
 function verify_selector_for_zulip(selector) {
     const is_valid =
diff --git a/web/tests/lib/zjquery_element.js b/web/tests/lib/zjquery_element.cjs
similarity index 99%
rename from web/tests/lib/zjquery_element.js
rename to web/tests/lib/zjquery_element.cjs
index 300e68ce8f..6b32d86e0a 100644
--- a/web/tests/lib/zjquery_element.js
+++ b/web/tests/lib/zjquery_element.cjs
@@ -2,7 +2,7 @@
 
 const assert = require("node:assert/strict");
 
-const FakeEvent = require("./zjquery_event.js");
+const FakeEvent = require("./zjquery_event.cjs");
 
 const noop = function () {};
 
diff --git a/web/tests/lib/zjquery_event.js b/web/tests/lib/zjquery_event.cjs
similarity index 100%
rename from web/tests/lib/zjquery_event.js
rename to web/tests/lib/zjquery_event.cjs
diff --git a/web/tests/lib/zpage_billing_params.js b/web/tests/lib/zpage_billing_params.cjs
similarity index 100%
rename from web/tests/lib/zpage_billing_params.js
rename to web/tests/lib/zpage_billing_params.cjs
diff --git a/web/tests/lib/zpage_params.js b/web/tests/lib/zpage_params.cjs
similarity index 100%
rename from web/tests/lib/zpage_params.js
rename to web/tests/lib/zpage_params.cjs
diff --git a/web/tests/linkifiers.test.js b/web/tests/linkifiers.test.cjs
similarity index 92%
rename from web/tests/linkifiers.test.js
rename to web/tests/linkifiers.test.cjs
index 15a885a53d..e953a6c910 100644
--- a/web/tests/linkifiers.test.js
+++ b/web/tests/linkifiers.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 const linkifiers = zrequire("linkifiers");
 
diff --git a/web/tests/list_cursor.test.js b/web/tests/list_cursor.test.cjs
similarity index 94%
rename from web/tests/list_cursor.test.js
rename to web/tests/list_cursor.test.cjs
index d1f68e8efb..7abb20dd3e 100644
--- a/web/tests/list_cursor.test.js
+++ b/web/tests/list_cursor.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const $ = require("./lib/zjquery.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const {ListCursor} = zrequire("list_cursor");
 
diff --git a/web/tests/list_widget.test.js b/web/tests/list_widget.test.cjs
similarity index 99%
rename from web/tests/list_widget.test.js
rename to web/tests/list_widget.test.cjs
index 4df7feb616..6cac822976 100644
--- a/web/tests/list_widget.test.js
+++ b/web/tests/list_widget.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, mock_jquery, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, mock_jquery, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 // We need these stubs to get by instanceof checks.
 // The ListWidget library allows you to insert objects
diff --git a/web/tests/markdown.test.js b/web/tests/markdown.test.cjs
similarity index 99%
rename from web/tests/markdown.test.js
rename to web/tests/markdown.test.cjs
index 5f8f522ebd..d8d675e0f6 100644
--- a/web/tests/markdown.test.js
+++ b/web/tests/markdown.test.cjs
@@ -6,10 +6,10 @@ const katex = require("katex");
 
 const markdown_test_cases = require("../../zerver/tests/fixtures/markdown_test_cases.json");
 
-const markdown_assert = require("./lib/markdown_assert.js");
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const {page_params} = require("./lib/zpage_params.js");
+const markdown_assert = require("./lib/markdown_assert.cjs");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 const example_realm_linkifiers = [
     {
diff --git a/web/tests/markdown_parse.test.js b/web/tests/markdown_parse.test.cjs
similarity index 99%
rename from web/tests/markdown_parse.test.js
rename to web/tests/markdown_parse.test.cjs
index 56962f30f9..73a3570ee4 100644
--- a/web/tests/markdown_parse.test.js
+++ b/web/tests/markdown_parse.test.cjs
@@ -4,8 +4,8 @@ const assert = require("node:assert/strict");
 
 const url_template_lib = require("url-template");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const markdown = zrequire("markdown");
 const linkifiers = zrequire("linkifiers");
diff --git a/web/tests/message_edit.test.js b/web/tests/message_edit.test.cjs
similarity index 99%
rename from web/tests/message_edit.test.js
rename to web/tests/message_edit.test.cjs
index 2006923614..600ab4946f 100644
--- a/web/tests/message_edit.test.js
+++ b/web/tests/message_edit.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const message_edit = zrequire("message_edit");
 const people = zrequire("people");
diff --git a/web/tests/message_events.test.js b/web/tests/message_events.test.cjs
similarity index 96%
rename from web/tests/message_events.test.js
rename to web/tests/message_events.test.cjs
index fae7d28d65..cab4aac724 100644
--- a/web/tests/message_events.test.js
+++ b/web/tests/message_events.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const message_edit = mock_esm("../src/message_edit");
 const message_lists = mock_esm("../src/message_lists");
diff --git a/web/tests/message_flags.test.js b/web/tests/message_flags.test.cjs
similarity index 99%
rename from web/tests/message_flags.test.js
rename to web/tests/message_flags.test.cjs
index 919a28c1ed..edeb7599d4 100644
--- a/web/tests/message_flags.test.js
+++ b/web/tests/message_flags.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, set_global, with_overrides, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {mock_esm, set_global, with_overrides, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const channel = mock_esm("../src/channel");
 const message_live_update = mock_esm("../src/message_live_update");
diff --git a/web/tests/message_list.test.js b/web/tests/message_list.test.cjs
similarity index 98%
rename from web/tests/message_list.test.js
rename to web/tests/message_list.test.cjs
index a92da40f3f..f826061f3b 100644
--- a/web/tests/message_list.test.js
+++ b/web/tests/message_list.test.cjs
@@ -2,11 +2,11 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {make_stub} = require("./lib/stub.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {make_stub} = require("./lib/stub.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 // These unit tests for web/src/message_list.ts emphasize the model-ish
 // aspects of the MessageList class.  We have to stub out a few functions
diff --git a/web/tests/message_list_data.test.js b/web/tests/message_list_data.test.cjs
similarity index 98%
rename from web/tests/message_list_data.test.js
rename to web/tests/message_list_data.test.cjs
index 0bec24d531..e21a6fceea 100644
--- a/web/tests/message_list_data.test.js
+++ b/web/tests/message_list_data.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 const user_topics = zrequire("user_topics");
 const muted_users = zrequire("muted_users");
diff --git a/web/tests/message_list_view.test.js b/web/tests/message_list_view.test.cjs
similarity index 99%
rename from web/tests/message_list_view.test.js
rename to web/tests/message_list_view.test.cjs
index cfdb2adf78..9b26d3773e 100644
--- a/web/tests/message_list_view.test.js
+++ b/web/tests/message_list_view.test.cjs
@@ -4,9 +4,9 @@ const assert = require("node:assert/strict");
 
 const _ = require("lodash");
 
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 set_global("document", "document-stub");
 
diff --git a/web/tests/message_store.test.js b/web/tests/message_store.test.cjs
similarity index 99%
rename from web/tests/message_store.test.js
rename to web/tests/message_store.test.cjs
index a36b66d329..3eb77ae311 100644
--- a/web/tests/message_store.test.js
+++ b/web/tests/message_store.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 mock_esm("../src/settings_data", {
     user_can_access_all_other_users: () => true,
diff --git a/web/tests/message_view.test.js b/web/tests/message_view.test.cjs
similarity index 99%
rename from web/tests/message_view.test.js
rename to web/tests/message_view.test.cjs
index 6a62490498..92de3c4ab6 100644
--- a/web/tests/message_view.test.js
+++ b/web/tests/message_view.test.cjs
@@ -2,11 +2,11 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const $ = require("./lib/zjquery.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const $ = require("./lib/zjquery.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 const hash_util = zrequire("hash_util");
 const compose_state = zrequire("compose_state");
diff --git a/web/tests/muted_users.test.js b/web/tests/muted_users.test.cjs
similarity index 96%
rename from web/tests/muted_users.test.js
rename to web/tests/muted_users.test.cjs
index 3b7f1bcada..c724249f42 100644
--- a/web/tests/muted_users.test.js
+++ b/web/tests/muted_users.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const muted_users = zrequire("muted_users");
 const {initialize_user_settings} = zrequire("user_settings");
diff --git a/web/tests/narrow_local.test.js b/web/tests/narrow_local.test.cjs
similarity index 99%
rename from web/tests/narrow_local.test.js
rename to web/tests/narrow_local.test.cjs
index 47fb6812cf..c4d6bd2883 100644
--- a/web/tests/narrow_local.test.js
+++ b/web/tests/narrow_local.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const all_messages_data = zrequire("../src/all_messages_data");
 
diff --git a/web/tests/narrow_state.test.js b/web/tests/narrow_state.test.cjs
similarity index 98%
rename from web/tests/narrow_state.test.js
rename to web/tests/narrow_state.test.cjs
index 2725240134..9c99fcf55b 100644
--- a/web/tests/narrow_state.test.js
+++ b/web/tests/narrow_state.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 const people = zrequire("people");
 const {Filter} = zrequire("../src/filter");
diff --git a/web/tests/narrow_unread.test.js b/web/tests/narrow_unread.test.cjs
similarity index 98%
rename from web/tests/narrow_unread.test.js
rename to web/tests/narrow_unread.test.cjs
index 4e02256e1f..d760a74dde 100644
--- a/web/tests/narrow_unread.test.js
+++ b/web/tests/narrow_unread.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 mock_esm("../src/user_topics", {
     is_topic_muted: () => false,
diff --git a/web/tests/navbar_alerts.test.js b/web/tests/navbar_alerts.test.cjs
similarity index 96%
rename from web/tests/navbar_alerts.test.js
rename to web/tests/navbar_alerts.test.cjs
index f016f259a6..8a286cfd05 100644
--- a/web/tests/navbar_alerts.test.js
+++ b/web/tests/navbar_alerts.test.cjs
@@ -4,9 +4,9 @@ const assert = require("node:assert/strict");
 
 const {addDays} = require("date-fns");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 page_params.is_spectator = false;
 
diff --git a/web/tests/notifications.test.js b/web/tests/notifications.test.cjs
similarity index 98%
rename from web/tests/notifications.test.js
rename to web/tests/notifications.test.cjs
index 994a368770..1a925155c1 100644
--- a/web/tests/notifications.test.js
+++ b/web/tests/notifications.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 mock_esm("../src/electron_bridge");
 mock_esm("../src/spoilers", {hide_spoilers_in_notification() {}});
diff --git a/web/tests/password.test.js b/web/tests/password.test.cjs
similarity index 95%
rename from web/tests/password.test.js
rename to web/tests/password.test.cjs
index 431055d6ee..7a51d14441 100644
--- a/web/tests/password.test.js
+++ b/web/tests/password.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const {password_quality, password_warning} = zrequire("password_quality");
 
diff --git a/web/tests/peer_data.test.js b/web/tests/peer_data.test.cjs
similarity index 98%
rename from web/tests/peer_data.test.js
rename to web/tests/peer_data.test.cjs
index 5ca6c99e37..37969914a5 100644
--- a/web/tests/peer_data.test.js
+++ b/web/tests/peer_data.test.cjs
@@ -8,10 +8,10 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 const peer_data = zrequire("peer_data");
 const people = zrequire("people");
diff --git a/web/tests/people.test.js b/web/tests/people.test.cjs
similarity index 99%
rename from web/tests/people.test.js
rename to web/tests/people.test.cjs
index 72d45470be..dd13e68ed1 100644
--- a/web/tests/people.test.js
+++ b/web/tests/people.test.cjs
@@ -6,11 +6,11 @@ const {parseISO} = require("date-fns");
 const _ = require("lodash");
 const MockDate = require("mockdate");
 
-const {$t} = require("./lib/i18n.js");
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {$t} = require("./lib/i18n.cjs");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 const message_user_ids = mock_esm("../src/message_user_ids");
 const settings_data = mock_esm("../src/settings_data", {
diff --git a/web/tests/people_errors.test.js b/web/tests/people_errors.test.cjs
similarity index 95%
rename from web/tests/people_errors.test.js
rename to web/tests/people_errors.test.cjs
index e14edcb7ac..5dfc1387a1 100644
--- a/web/tests/people_errors.test.js
+++ b/web/tests/people_errors.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 const reload_state = mock_esm("../src/reload_state", {
     is_in_progress: () => false,
diff --git a/web/tests/pill_typeahead.test.js b/web/tests/pill_typeahead.test.cjs
similarity index 99%
rename from web/tests/pill_typeahead.test.js
rename to web/tests/pill_typeahead.test.cjs
index b759bc4772..e987462c6a 100644
--- a/web/tests/pill_typeahead.test.js
+++ b/web/tests/pill_typeahead.test.cjs
@@ -2,11 +2,11 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire, mock_esm} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const $ = require("./lib/zjquery.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {zrequire, mock_esm} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const $ = require("./lib/zjquery.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 const noop = function () {};
 
diff --git a/web/tests/pm_conversations.test.js b/web/tests/pm_conversations.test.cjs
similarity index 98%
rename from web/tests/pm_conversations.test.js
rename to web/tests/pm_conversations.test.cjs
index bcc27e3754..4002ffeed1 100644
--- a/web/tests/pm_conversations.test.js
+++ b/web/tests/pm_conversations.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const user_topics = zrequire("user_topics");
 const muted_users = zrequire("muted_users");
diff --git a/web/tests/pm_list.test.js b/web/tests/pm_list.test.cjs
similarity index 84%
rename from web/tests/pm_list.test.js
rename to web/tests/pm_list.test.cjs
index bab781edbc..1f8d3b593f 100644
--- a/web/tests/pm_list.test.js
+++ b/web/tests/pm_list.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const pm_list = zrequire("pm_list");
 
diff --git a/web/tests/pm_list_data.test.js b/web/tests/pm_list_data.test.cjs
similarity index 98%
rename from web/tests/pm_list_data.test.js
rename to web/tests/pm_list_data.test.cjs
index 4122a97d1f..82a2a0fd89 100644
--- a/web/tests/pm_list_data.test.js
+++ b/web/tests/pm_list_data.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 const unread = mock_esm("../src/unread");
 
diff --git a/web/tests/poll_widget.test.js b/web/tests/poll_widget.test.cjs
similarity index 98%
rename from web/tests/poll_widget.test.js
rename to web/tests/poll_widget.test.cjs
index 41f3b79b15..887b5f5594 100644
--- a/web/tests/poll_widget.test.js
+++ b/web/tests/poll_widget.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 mock_esm("../src/settings_data", {
     user_can_access_all_other_users: () => true,
diff --git a/web/tests/popover_menus_data.test.js b/web/tests/popover_menus_data.test.cjs
similarity index 97%
rename from web/tests/popover_menus_data.test.js
rename to web/tests/popover_menus_data.test.cjs
index 306f0ce98c..6e37fc7018 100644
--- a/web/tests/popover_menus_data.test.js
+++ b/web/tests/popover_menus_data.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 const {Filter} = zrequire("filter");
 const {MessageList} = zrequire("message_list");
diff --git a/web/tests/postprocess_content.test.js b/web/tests/postprocess_content.test.cjs
similarity index 98%
rename from web/tests/postprocess_content.test.js
rename to web/tests/postprocess_content.test.cjs
index 0974dd01ca..2b43c4226c 100644
--- a/web/tests/postprocess_content.test.js
+++ b/web/tests/postprocess_content.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const thumbnail = mock_esm("../src/thumbnail");
 
diff --git a/web/tests/presence.test.js b/web/tests/presence.test.cjs
similarity index 98%
rename from web/tests/presence.test.js
rename to web/tests/presence.test.cjs
index ccd6018b93..1c38d93890 100644
--- a/web/tests/presence.test.js
+++ b/web/tests/presence.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 mock_esm("../src/settings_data", {
     user_can_access_all_other_users: () => true,
diff --git a/web/tests/reactions.test.js b/web/tests/reactions.test.cjs
similarity index 99%
rename from web/tests/reactions.test.js
rename to web/tests/reactions.test.cjs
index 89333d9f6c..527498412d 100644
--- a/web/tests/reactions.test.js
+++ b/web/tests/reactions.test.cjs
@@ -2,12 +2,12 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {make_stub} = require("./lib/stub.js");
-const {run_test, noop} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const $ = require("./lib/zjquery.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {make_stub} = require("./lib/stub.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const $ = require("./lib/zjquery.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 const alice_user_id = 5;
 
diff --git a/web/tests/realm_playground.test.js b/web/tests/realm_playground.test.cjs
similarity index 98%
rename from web/tests/realm_playground.test.js
rename to web/tests/realm_playground.test.cjs
index 5495e96001..f0a1706ab3 100644
--- a/web/tests/realm_playground.test.js
+++ b/web/tests/realm_playground.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const {$t} = zrequire("i18n");
 const pygments_data = zrequire("pygments_data");
diff --git a/web/tests/recent_senders.test.js b/web/tests/recent_senders.test.cjs
similarity index 98%
rename from web/tests/recent_senders.test.js
rename to web/tests/recent_senders.test.cjs
index a224e8dda4..02487d3735 100644
--- a/web/tests/recent_senders.test.js
+++ b/web/tests/recent_senders.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 let next_id = 0;
 const messages = new Map();
diff --git a/web/tests/recent_view.test.js b/web/tests/recent_view.test.cjs
similarity index 99%
rename from web/tests/recent_view.test.js
rename to web/tests/recent_view.test.cjs
index 1326156d27..91de5a3aae 100644
--- a/web/tests/recent_view.test.js
+++ b/web/tests/recent_view.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 window.scrollTo = noop;
 const test_url = () => "https://www.example.com";
@@ -62,7 +62,7 @@ const ListWidget = mock_esm("../src/list_widget", {
             opts.filter.predicate(item);
         }
         // Just for coverage, the mechanisms
-        // are tested in list_widget.test.js
+        // are tested in list_widget.test.cjs
         if (mapped_topic_values.length >= 2) {
             opts.sort_fields.stream_sort(mapped_topic_values[0], mapped_topic_values[1]);
             opts.sort_fields.stream_sort(mapped_topic_values[1], mapped_topic_values[0]);
diff --git a/web/tests/reload.test.js b/web/tests/reload.test.cjs
similarity index 90%
rename from web/tests/reload.test.js
rename to web/tests/reload.test.cjs
index 816dde8879..09861da1a5 100644
--- a/web/tests/reload.test.js
+++ b/web/tests/reload.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
 
 // override file-level function call in reload.ts
 window.addEventListener = noop;
diff --git a/web/tests/reload_state.test.js b/web/tests/reload_state.test.cjs
similarity index 86%
rename from web/tests/reload_state.test.js
rename to web/tests/reload_state.test.cjs
index 26082ca368..e10adae127 100644
--- a/web/tests/reload_state.test.js
+++ b/web/tests/reload_state.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const reload_state = zrequire("reload_state");
 
diff --git a/web/tests/rendered_markdown.test.js b/web/tests/rendered_markdown.test.cjs
similarity index 99%
rename from web/tests/rendered_markdown.test.js
rename to web/tests/rendered_markdown.test.cjs
index 3058e5b901..66b19adf13 100644
--- a/web/tests/rendered_markdown.test.js
+++ b/web/tests/rendered_markdown.test.cjs
@@ -2,11 +2,11 @@
 
 const assert = require("node:assert/strict");
 
-const {$t} = require("./lib/i18n.js");
-const {mock_cjs, mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const $ = require("./lib/zjquery.js");
+const {$t} = require("./lib/i18n.cjs");
+const {mock_cjs, mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 let clipboard_args;
 class Clipboard {
diff --git a/web/tests/resolved_topic.test.js b/web/tests/resolved_topic.test.cjs
similarity index 95%
rename from web/tests/resolved_topic.test.js
rename to web/tests/resolved_topic.test.cjs
index 809346cafe..5f6f15ab46 100644
--- a/web/tests/resolved_topic.test.js
+++ b/web/tests/resolved_topic.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const resolved_topic = zrequire("../shared/src/resolved_topic");
 
diff --git a/web/tests/rtl.test.js b/web/tests/rtl.test.cjs
similarity index 98%
rename from web/tests/rtl.test.js
rename to web/tests/rtl.test.cjs
index 421f03c209..fa442838a5 100644
--- a/web/tests/rtl.test.js
+++ b/web/tests/rtl.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const rtl = zrequire("rtl");
 
diff --git a/web/tests/saved_snippets.test.js b/web/tests/saved_snippets.test.cjs
similarity index 92%
rename from web/tests/saved_snippets.test.js
rename to web/tests/saved_snippets.test.cjs
index b430080902..da4c4af4af 100644
--- a/web/tests/saved_snippets.test.js
+++ b/web/tests/saved_snippets.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {$t} = require("./lib/i18n.js");
-const {set_global, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const {$t} = require("./lib/i18n.cjs");
+const {set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 set_global("page_params", {
     is_spectator: false,
diff --git a/web/tests/scheduled_messages.test.js b/web/tests/scheduled_messages.test.cjs
similarity index 98%
rename from web/tests/scheduled_messages.test.js
rename to web/tests/scheduled_messages.test.cjs
index a13ca16f11..6925d26ea0 100644
--- a/web/tests/scheduled_messages.test.js
+++ b/web/tests/scheduled_messages.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const scheduled_messages = zrequire("scheduled_messages");
 const compose_send_menu_popover = zrequire("compose_send_menu_popover");
diff --git a/web/tests/scroll_util.test.js b/web/tests/scroll_util.test.cjs
similarity index 96%
rename from web/tests/scroll_util.test.js
rename to web/tests/scroll_util.test.cjs
index c014ff3f9a..754ff30afa 100644
--- a/web/tests/scroll_util.test.js
+++ b/web/tests/scroll_util.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const scroll_util = zrequire("scroll_util");
 
diff --git a/web/tests/search.test.js b/web/tests/search.test.cjs
similarity index 99%
rename from web/tests/search.test.js
rename to web/tests/search.test.cjs
index 6450dc4dd7..9844f483d6 100644
--- a/web/tests/search.test.js
+++ b/web/tests/search.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const bootstrap_typeahead = mock_esm("../src/bootstrap_typeahead");
 const search_suggestion = mock_esm("../src/search_suggestion");
diff --git a/web/tests/search_suggestion.test.js b/web/tests/search_suggestion.test.cjs
similarity index 99%
rename from web/tests/search_suggestion.test.js
rename to web/tests/search_suggestion.test.cjs
index 31c0cc44ed..1adf03961f 100644
--- a/web/tests/search_suggestion.test.js
+++ b/web/tests/search_suggestion.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 const narrow_state = mock_esm("../src/narrow_state");
 const stream_topic_history_util = mock_esm("../src/stream_topic_history_util");
diff --git a/web/tests/server_events.test.js b/web/tests/server_events.test.cjs
similarity index 95%
rename from web/tests/server_events.test.js
rename to web/tests/server_events.test.cjs
index ab2fc1414d..62d31f825b 100644
--- a/web/tests/server_events.test.js
+++ b/web/tests/server_events.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 mock_esm("../src/loading", {
     destroy_indicator: noop,
diff --git a/web/tests/settings_bots.test.js b/web/tests/settings_bots.test.cjs
similarity index 96%
rename from web/tests/settings_bots.test.js
rename to web/tests/settings_bots.test.cjs
index 8d29f39398..f67f78efeb 100644
--- a/web/tests/settings_bots.test.js
+++ b/web/tests/settings_bots.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const bot_data_params = {
     realm_bots: [
diff --git a/web/tests/settings_config.test.js b/web/tests/settings_config.test.cjs
similarity index 98%
rename from web/tests/settings_config.test.js
rename to web/tests/settings_config.test.cjs
index 86d2422d5b..f7c01f8435 100644
--- a/web/tests/settings_config.test.js
+++ b/web/tests/settings_config.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const settings_config = zrequire("settings_config");
 const {set_realm} = zrequire("state_data");
diff --git a/web/tests/settings_data.test.js b/web/tests/settings_data.test.cjs
similarity index 99%
rename from web/tests/settings_data.test.js
rename to web/tests/settings_data.test.cjs
index a214ee54d2..a271e505a3 100644
--- a/web/tests/settings_data.test.js
+++ b/web/tests/settings_data.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, with_overrides, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {mock_esm, with_overrides, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 const settings_data = zrequire("settings_data");
 const settings_config = zrequire("settings_config");
diff --git a/web/tests/settings_emoji.test.js b/web/tests/settings_emoji.test.cjs
similarity index 85%
rename from web/tests/settings_emoji.test.js
rename to web/tests/settings_emoji.test.cjs
index 221a9e0c72..de9332dc4d 100644
--- a/web/tests/settings_emoji.test.js
+++ b/web/tests/settings_emoji.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const upload_widget = mock_esm("../src/upload_widget");
 const settings_emoji = zrequire("settings_emoji");
diff --git a/web/tests/settings_muted_users.test.js b/web/tests/settings_muted_users.test.cjs
similarity index 95%
rename from web/tests/settings_muted_users.test.js
rename to web/tests/settings_muted_users.test.cjs
index ed35b6e5b2..9d9aa24f9c 100644
--- a/web/tests/settings_muted_users.test.js
+++ b/web/tests/settings_muted_users.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const channel = mock_esm("../src/channel");
 const list_widget = mock_esm("../src/list_widget", {
diff --git a/web/tests/settings_org.test.js b/web/tests/settings_org.test.cjs
similarity index 99%
rename from web/tests/settings_org.test.js
rename to web/tests/settings_org.test.cjs
index 6223afbeee..fde57155c1 100644
--- a/web/tests/settings_org.test.js
+++ b/web/tests/settings_org.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {$t} = require("./lib/i18n.js");
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {$t} = require("./lib/i18n.cjs");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const realm_icon = mock_esm("../src/realm_icon");
 
diff --git a/web/tests/settings_profile_fields.test.js b/web/tests/settings_profile_fields.test.cjs
similarity index 98%
rename from web/tests/settings_profile_fields.test.js
rename to web/tests/settings_profile_fields.test.cjs
index 9eb9674190..0db62151c8 100644
--- a/web/tests/settings_profile_fields.test.js
+++ b/web/tests/settings_profile_fields.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, with_overrides, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, with_overrides, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const loading = mock_esm("../src/loading");
 
diff --git a/web/tests/settings_realm_domains.test.js b/web/tests/settings_realm_domains.test.cjs
similarity index 95%
rename from web/tests/settings_realm_domains.test.js
rename to web/tests/settings_realm_domains.test.cjs
index c38984b627..74804edb23 100644
--- a/web/tests/settings_realm_domains.test.js
+++ b/web/tests/settings_realm_domains.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const channel = mock_esm("../src/channel");
 mock_esm("../src/ui_report", {
diff --git a/web/tests/settings_user_topics.test.js b/web/tests/settings_user_topics.test.cjs
similarity index 95%
rename from web/tests/settings_user_topics.test.js
rename to web/tests/settings_user_topics.test.cjs
index f152815914..07bf75f552 100644
--- a/web/tests/settings_user_topics.test.js
+++ b/web/tests/settings_user_topics.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const list_widget = mock_esm("../src/list_widget", {
     generic_sort_functions: noop,
diff --git a/web/tests/spoilers.test.js b/web/tests/spoilers.test.cjs
similarity index 89%
rename from web/tests/spoilers.test.js
rename to web/tests/spoilers.test.cjs
index 9c8ec78da1..c725abfb1c 100644
--- a/web/tests/spoilers.test.js
+++ b/web/tests/spoilers.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const spoilers = zrequire("spoilers");
 
diff --git a/web/tests/starred_messages.test.js b/web/tests/starred_messages.test.cjs
similarity index 97%
rename from web/tests/starred_messages.test.js
rename to web/tests/starred_messages.test.cjs
index 9b23f62db8..770a58bc63 100644
--- a/web/tests/starred_messages.test.js
+++ b/web/tests/starred_messages.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, with_overrides, zrequire} = require("./lib/namespace.js");
-const {make_stub} = require("./lib/stub.js");
-const {run_test} = require("./lib/test.js");
+const {mock_esm, with_overrides, zrequire} = require("./lib/namespace.cjs");
+const {make_stub} = require("./lib/stub.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const left_sidebar_navigation_area = mock_esm("../src/left_sidebar_navigation_area", {
     update_starred_count() {},
diff --git a/web/tests/stream_create_subscribers_data.test.js b/web/tests/stream_create_subscribers_data.test.cjs
similarity index 98%
rename from web/tests/stream_create_subscribers_data.test.js
rename to web/tests/stream_create_subscribers_data.test.cjs
index 5660a0aa35..756b6ed1f5 100644
--- a/web/tests/stream_create_subscribers_data.test.js
+++ b/web/tests/stream_create_subscribers_data.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const people = zrequire("people");
 const {set_current_user} = zrequire("state_data");
diff --git a/web/tests/stream_data.test.js b/web/tests/stream_data.test.cjs
similarity index 99%
rename from web/tests/stream_data.test.js
rename to web/tests/stream_data.test.cjs
index 4b02a099a5..c528ead2d9 100644
--- a/web/tests/stream_data.test.js
+++ b/web/tests/stream_data.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 // TODO: Remove after we enable support for
 // web_public_streams in production.
diff --git a/web/tests/stream_edit.test.js b/web/tests/stream_edit.test.cjs
similarity index 95%
rename from web/tests/stream_edit.test.js
rename to web/tests/stream_edit.test.cjs
index d45955135e..f0501b2718 100644
--- a/web/tests/stream_edit.test.js
+++ b/web/tests/stream_edit.test.cjs
@@ -4,7 +4,7 @@ const assert = require("node:assert/strict");
 
 const {get_stream_email_address} = require("../src/stream_edit.ts");
 
-const {run_test} = require("./lib/test.js");
+const {run_test} = require("./lib/test.cjs");
 
 run_test("get_stream_email_address", () => {
     let address = "announce.747b04693224b5d2f0d409b66ccd3866@zulipdev.com";
diff --git a/web/tests/stream_events.test.js b/web/tests/stream_events.test.cjs
similarity index 98%
rename from web/tests/stream_events.test.js
rename to web/tests/stream_events.test.cjs
index cb9132349f..c6cb11062b 100644
--- a/web/tests/stream_events.test.js
+++ b/web/tests/stream_events.test.cjs
@@ -2,11 +2,11 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {make_stub} = require("./lib/stub.js");
-const {run_test, noop} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {make_stub} = require("./lib/stub.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const browser_history = mock_esm("../src/browser_history");
 const color_data = mock_esm("../src/color_data");
diff --git a/web/tests/stream_list.test.js b/web/tests/stream_list.test.cjs
similarity index 99%
rename from web/tests/stream_list.test.js
rename to web/tests/stream_list.test.cjs
index b9acb9f533..2ffb78575e 100644
--- a/web/tests/stream_list.test.js
+++ b/web/tests/stream_list.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
-const {page_params} = require("./lib/zpage_params.js");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
+const {page_params} = require("./lib/zpage_params.cjs");
 
 const people = zrequire("people");
 const {set_current_user} = zrequire("state_data");
diff --git a/web/tests/stream_list_sort.test.js b/web/tests/stream_list_sort.test.cjs
similarity index 99%
rename from web/tests/stream_list_sort.test.js
rename to web/tests/stream_list_sort.test.cjs
index 877d943f5c..ce9bff785a 100644
--- a/web/tests/stream_list_sort.test.js
+++ b/web/tests/stream_list_sort.test.cjs
@@ -4,8 +4,8 @@ const assert = require("node:assert/strict");
 
 const _ = require("lodash");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const people = zrequire("people");
 const stream_data = zrequire("stream_data");
diff --git a/web/tests/stream_pill.test.js b/web/tests/stream_pill.test.cjs
similarity index 97%
rename from web/tests/stream_pill.test.js
rename to web/tests/stream_pill.test.cjs
index 0d01040252..fcebee9eb6 100644
--- a/web/tests/stream_pill.test.js
+++ b/web/tests/stream_pill.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const peer_data = zrequire("peer_data");
 const people = zrequire("people");
diff --git a/web/tests/stream_search.test.js b/web/tests/stream_search.test.cjs
similarity index 96%
rename from web/tests/stream_search.test.js
rename to web/tests/stream_search.test.cjs
index 3e24eb4b7a..ec3ac77af1 100644
--- a/web/tests/stream_search.test.js
+++ b/web/tests/stream_search.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 // This tests the stream searching functionality which currently
 // lives in stream_list.ts.
diff --git a/web/tests/stream_settings_ui.test.js b/web/tests/stream_settings_ui.test.cjs
similarity index 99%
rename from web/tests/stream_settings_ui.test.js
rename to web/tests/stream_settings_ui.test.cjs
index 823e8122e0..651456ca3d 100644
--- a/web/tests/stream_settings_ui.test.js
+++ b/web/tests/stream_settings_ui.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const denmark_stream_id = 101;
 
diff --git a/web/tests/stream_topic_history.test.js b/web/tests/stream_topic_history.test.cjs
similarity index 99%
rename from web/tests/stream_topic_history.test.js
rename to web/tests/stream_topic_history.test.cjs
index ee6d176861..9452a31751 100644
--- a/web/tests/stream_topic_history.test.js
+++ b/web/tests/stream_topic_history.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
 
 const channel = mock_esm("../src/channel");
 const message_util = mock_esm("../src/message_util");
diff --git a/web/tests/submessage.test.js b/web/tests/submessage.test.cjs
similarity index 95%
rename from web/tests/submessage.test.js
rename to web/tests/submessage.test.cjs
index 263a12280b..88f282b7e4 100644
--- a/web/tests/submessage.test.js
+++ b/web/tests/submessage.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 const channel = mock_esm("../src/channel");
 const message_store = mock_esm("../src/message_store");
diff --git a/web/tests/support.test.js b/web/tests/support.test.cjs
similarity index 91%
rename from web/tests/support.test.js
rename to web/tests/support.test.cjs
index 7994a097ac..b73e7aaea6 100644
--- a/web/tests/support.test.js
+++ b/web/tests/support.test.cjs
@@ -6,9 +6,9 @@ const path = require("node:path");
 
 const {JSDOM} = require("jsdom");
 
-const {mock_cjs, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_cjs, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const template = fs.readFileSync(
     path.resolve(__dirname, "../../templates/corporate/support/realm_details.html"),
diff --git a/web/tests/templates.test.js b/web/tests/templates.test.cjs
similarity index 98%
rename from web/tests/templates.test.js
rename to web/tests/templates.test.cjs
index 4e837c0fc3..6efc63383a 100644
--- a/web/tests/templates.test.js
+++ b/web/tests/templates.test.cjs
@@ -2,7 +2,7 @@
 
 const assert = require("node:assert/strict");
 
-const {run_test} = require("./lib/test.js");
+const {run_test} = require("./lib/test.cjs");
 
 /*
     Note that the test runner automatically registers
diff --git a/web/tests/time_zone_util.test.js b/web/tests/time_zone_util.test.cjs
similarity index 97%
rename from web/tests/time_zone_util.test.js
rename to web/tests/time_zone_util.test.cjs
index 631b0ad790..b5a0579f87 100644
--- a/web/tests/time_zone_util.test.js
+++ b/web/tests/time_zone_util.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const {get_offset, start_of_day, is_same_day, difference_in_calendar_days} =
     zrequire("time_zone_util");
diff --git a/web/tests/timerender.test.js b/web/tests/timerender.test.cjs
similarity index 99%
rename from web/tests/timerender.test.js
rename to web/tests/timerender.test.cjs
index 8481cbf7f8..20dfe363b7 100644
--- a/web/tests/timerender.test.js
+++ b/web/tests/timerender.test.cjs
@@ -5,10 +5,10 @@ const assert = require("node:assert/strict");
 const {add} = require("date-fns");
 const MockDate = require("mockdate");
 
-const {$t} = require("./lib/i18n.js");
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {$t} = require("./lib/i18n.cjs");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const {initialize_user_settings} = zrequire("user_settings");
 
diff --git a/web/tests/topic_generator.test.js b/web/tests/topic_generator.test.cjs
similarity index 98%
rename from web/tests/topic_generator.test.js
rename to web/tests/topic_generator.test.cjs
index c81a0893a3..b76679002a 100644
--- a/web/tests/topic_generator.test.js
+++ b/web/tests/topic_generator.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const pm_conversations = mock_esm("../src/pm_conversations", {
     recent: {},
diff --git a/web/tests/topic_link_util.test.js b/web/tests/topic_link_util.test.cjs
similarity index 97%
rename from web/tests/topic_link_util.test.js
rename to web/tests/topic_link_util.test.cjs
index 605a8461dc..3b078ec030 100644
--- a/web/tests/topic_link_util.test.js
+++ b/web/tests/topic_link_util.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const topic_link_util = zrequire("topic_link_util");
 const stream_data = zrequire("stream_data");
diff --git a/web/tests/topic_list_data.test.js b/web/tests/topic_list_data.test.cjs
similarity index 99%
rename from web/tests/topic_list_data.test.js
rename to web/tests/topic_list_data.test.cjs
index 173ff2d3bd..c1afc0fd8f 100644
--- a/web/tests/topic_list_data.test.js
+++ b/web/tests/topic_list_data.test.cjs
@@ -4,8 +4,8 @@ const assert = require("node:assert/strict");
 
 const _ = require("lodash");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 mock_esm("../src/message_store", {
     get() {
diff --git a/web/tests/transmit.test.js b/web/tests/transmit.test.cjs
similarity index 97%
rename from web/tests/transmit.test.js
rename to web/tests/transmit.test.cjs
index 6520a0734a..a3ca623c82 100644
--- a/web/tests/transmit.test.js
+++ b/web/tests/transmit.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 const channel = mock_esm("../src/channel");
 const reload = mock_esm("../src/reload");
diff --git a/web/tests/typeahead.test.js b/web/tests/typeahead.test.cjs
similarity index 99%
rename from web/tests/typeahead.test.js
rename to web/tests/typeahead.test.cjs
index 24cfe2cf3e..83d492f7bf 100644
--- a/web/tests/typeahead.test.js
+++ b/web/tests/typeahead.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const typeahead = zrequire("../shared/src/typeahead");
 
diff --git a/web/tests/typeahead_helper.test.js b/web/tests/typeahead_helper.test.cjs
similarity index 99%
rename from web/tests/typeahead_helper.test.js
rename to web/tests/typeahead_helper.test.cjs
index abdd32ea5c..35940d3305 100644
--- a/web/tests/typeahead_helper.test.js
+++ b/web/tests/typeahead_helper.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const stream_topic_history = mock_esm("../src/stream_topic_history");
 
diff --git a/web/tests/typing_data.test.js b/web/tests/typing_data.test.cjs
similarity index 98%
rename from web/tests/typing_data.test.js
rename to web/tests/typing_data.test.cjs
index b480c6aea7..365ba4fd10 100644
--- a/web/tests/typing_data.test.js
+++ b/web/tests/typing_data.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {set_global, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const muted_users = zrequire("muted_users");
 const typing_data = zrequire("typing_data");
diff --git a/web/tests/typing_events.test.js b/web/tests/typing_events.test.cjs
similarity index 96%
rename from web/tests/typing_events.test.js
rename to web/tests/typing_events.test.cjs
index f8dd6de28f..99008a3656 100644
--- a/web/tests/typing_events.test.js
+++ b/web/tests/typing_events.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const settings_data = mock_esm("../src/settings_data");
 
diff --git a/web/tests/typing_status.test.js b/web/tests/typing_status.test.cjs
similarity index 99%
rename from web/tests/typing_status.test.js
rename to web/tests/typing_status.test.cjs
index 960df3f3fc..d906e64af3 100644
--- a/web/tests/typing_status.test.js
+++ b/web/tests/typing_status.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const compose_pm_pill = mock_esm("../src/compose_pm_pill");
 const compose_state = mock_esm("../src/compose_state");
diff --git a/web/tests/ui_util.test.js b/web/tests/ui_util.test.cjs
similarity index 94%
rename from web/tests/ui_util.test.js
rename to web/tests/ui_util.test.cjs
index a9076458b5..f0e7fc7ff8 100644
--- a/web/tests/ui_util.test.js
+++ b/web/tests/ui_util.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const ui_util = zrequire("ui_util");
 
diff --git a/web/tests/unread.test.js b/web/tests/unread.test.cjs
similarity index 99%
rename from web/tests/unread.test.js
rename to web/tests/unread.test.cjs
index ac5d39fabf..6152337b0f 100644
--- a/web/tests/unread.test.js
+++ b/web/tests/unread.test.cjs
@@ -4,8 +4,8 @@ const assert = require("node:assert/strict");
 
 const _ = require("lodash");
 
-const {set_global, with_overrides, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {set_global, with_overrides, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 set_global("document", "document-stub");
 const {FoldDict} = zrequire("fold_dict");
diff --git a/web/tests/upload.test.js b/web/tests/upload.test.cjs
similarity index 99%
rename from web/tests/upload.test.js
rename to web/tests/upload.test.cjs
index ee1c31fc41..3bf5a84b9f 100644
--- a/web/tests/upload.test.js
+++ b/web/tests/upload.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 class ClipboardEvent {
     constructor({clipboardData}) {
diff --git a/web/tests/user_events.test.js b/web/tests/user_events.test.cjs
similarity index 98%
rename from web/tests/user_events.test.js
rename to web/tests/user_events.test.cjs
index d1ea196e2f..69936eda59 100644
--- a/web/tests/user_events.test.js
+++ b/web/tests/user_events.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const message_live_update = mock_esm("../src/message_live_update");
 const navbar_alerts = mock_esm("../src/navbar_alerts");
diff --git a/web/tests/user_group_pill.test.js b/web/tests/user_group_pill.test.cjs
similarity index 97%
rename from web/tests/user_group_pill.test.js
rename to web/tests/user_group_pill.test.cjs
index 32e0a53bf0..5a1869f96e 100644
--- a/web/tests/user_group_pill.test.js
+++ b/web/tests/user_group_pill.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const user_groups = zrequire("user_groups");
 const user_group_pill = zrequire("user_group_pill");
diff --git a/web/tests/user_groups.test.js b/web/tests/user_groups.test.cjs
similarity index 99%
rename from web/tests/user_groups.test.js
rename to web/tests/user_groups.test.cjs
index 79a344c4e0..d2e8772501 100644
--- a/web/tests/user_groups.test.js
+++ b/web/tests/user_groups.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 const user_groups = zrequire("user_groups");
 const {set_realm} = zrequire("state_data");
diff --git a/web/tests/user_pill.test.js b/web/tests/user_pill.test.cjs
similarity index 96%
rename from web/tests/user_pill.test.js
rename to web/tests/user_pill.test.cjs
index d47c81dcfe..ec83eaef7a 100644
--- a/web/tests/user_pill.test.js
+++ b/web/tests/user_pill.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 const people = zrequire("people");
 const user_pill = zrequire("user_pill");
diff --git a/web/tests/user_search.test.js b/web/tests/user_search.test.cjs
similarity index 98%
rename from web/tests/user_search.test.js
rename to web/tests/user_search.test.cjs
index 9b5966ed10..15eb0504c5 100644
--- a/web/tests/user_search.test.js
+++ b/web/tests/user_search.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {set_global, mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test, noop} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {set_global, mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test, noop} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const fake_buddy_list = {
     scroll_container_selector: "#whatever",
diff --git a/web/tests/user_status.test.js b/web/tests/user_status.test.cjs
similarity index 97%
rename from web/tests/user_status.test.js
rename to web/tests/user_status.test.cjs
index 37d44838fe..ebe0310b34 100644
--- a/web/tests/user_status.test.js
+++ b/web/tests/user_status.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const channel = mock_esm("../src/channel");
 
diff --git a/web/tests/user_topics.test.js b/web/tests/user_topics.test.cjs
similarity index 98%
rename from web/tests/user_topics.test.js
rename to web/tests/user_topics.test.cjs
index 66d169f106..fd9b5304bf 100644
--- a/web/tests/user_topics.test.js
+++ b/web/tests/user_topics.test.cjs
@@ -4,9 +4,9 @@ const assert = require("node:assert/strict");
 
 const {all_visibility_policies} = require("../src/user_topics.ts");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 const user_topics = zrequire("user_topics");
 const stream_data = zrequire("stream_data");
diff --git a/web/tests/user_topics_ui.test.js b/web/tests/user_topics_ui.test.cjs
similarity index 97%
rename from web/tests/user_topics_ui.test.js
rename to web/tests/user_topics_ui.test.cjs
index 00b8a504e1..395f502460 100644
--- a/web/tests/user_topics_ui.test.js
+++ b/web/tests/user_topics_ui.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const user_topics = zrequire("user_topics");
 const user_topics_ui = zrequire("user_topics_ui");
diff --git a/web/tests/util.test.js b/web/tests/util.test.cjs
similarity index 99%
rename from web/tests/util.test.js
rename to web/tests/util.test.cjs
index 420aee5f8b..fdc8876d76 100644
--- a/web/tests/util.test.js
+++ b/web/tests/util.test.cjs
@@ -5,8 +5,8 @@ const assert = require("node:assert/strict");
 const _ = require("lodash");
 const MockDate = require("mockdate");
 
-const {set_global, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
+const {set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
 
 const blueslip = zrequire("blueslip");
 const {initialize_user_settings} = zrequire("user_settings");
diff --git a/web/tests/vdom.test.js b/web/tests/vdom.test.cjs
similarity index 97%
rename from web/tests/vdom.test.js
rename to web/tests/vdom.test.cjs
index a9e9e82d3d..a8606091e0 100644
--- a/web/tests/vdom.test.js
+++ b/web/tests/vdom.test.cjs
@@ -2,9 +2,9 @@
 
 const assert = require("node:assert/strict");
 
-const {zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const {zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 const vdom = zrequire("vdom");
 
diff --git a/web/tests/watchdog.test.js b/web/tests/watchdog.test.cjs
similarity index 92%
rename from web/tests/watchdog.test.js
rename to web/tests/watchdog.test.cjs
index 075db73b3c..287928a7bf 100644
--- a/web/tests/watchdog.test.js
+++ b/web/tests/watchdog.test.cjs
@@ -4,9 +4,9 @@ const assert = require("node:assert/strict");
 
 const MockDate = require("mockdate");
 
-const {set_global, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const {set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 let time = 0;
 let checker;
diff --git a/web/tests/widgetize.test.js b/web/tests/widgetize.test.cjs
similarity index 97%
rename from web/tests/widgetize.test.js
rename to web/tests/widgetize.test.cjs
index b4cb568fa4..f5c6a04c49 100644
--- a/web/tests/widgetize.test.js
+++ b/web/tests/widgetize.test.cjs
@@ -2,10 +2,10 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, set_global, zrequire} = require("./lib/namespace.js");
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
-const $ = require("./lib/zjquery.js");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 const sample_events = [
     {
diff --git a/web/tests/zblueslip.test.js b/web/tests/zblueslip.test.cjs
similarity index 95%
rename from web/tests/zblueslip.test.js
rename to web/tests/zblueslip.test.cjs
index 10f58ffa2e..8e1460d405 100644
--- a/web/tests/zblueslip.test.js
+++ b/web/tests/zblueslip.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {run_test} = require("./lib/test.js");
-const blueslip = require("./lib/zblueslip.js");
+const {run_test} = require("./lib/test.cjs");
+const blueslip = require("./lib/zblueslip.cjs");
 
 /*
 
@@ -23,10 +23,10 @@ What is zblueslip?
 
 The code we are testing lives here:
 
-    https://github.com/zulip/zulip/blob/main/web/tests/lib/zblueslip.js
+    https://github.com/zulip/zulip/blob/main/web/tests/lib/zblueslip.cjs
 
 Read the following contents for an overview of how zblueslip works. Also take a
-look at `people_errors.test.js` for actual usage of this module.
+look at `people_errors.test.cjs` for actual usage of this module.
 */
 
 run_test("basics", () => {
diff --git a/web/tests/zjquery.test.js b/web/tests/zjquery.test.cjs
similarity index 98%
rename from web/tests/zjquery.test.js
rename to web/tests/zjquery.test.cjs
index d7226e7fd0..9e07e503d3 100644
--- a/web/tests/zjquery.test.js
+++ b/web/tests/zjquery.test.cjs
@@ -2,8 +2,8 @@
 
 const assert = require("node:assert/strict");
 
-const {run_test} = require("./lib/test.js");
-const $ = require("./lib/zjquery.js");
+const {run_test} = require("./lib/test.cjs");
+const $ = require("./lib/zjquery.cjs");
 
 /*
 
@@ -25,7 +25,7 @@ What is zjquery?
 
 The code we are testing lives here:
 
-    https://github.com/zulip/zulip/blob/main/web/tests/lib/zjquery.js
+    https://github.com/zulip/zulip/blob/main/web/tests/lib/zjquery.cjs
 
 */
 
diff --git a/zerver/lib/mdiff.py b/zerver/lib/mdiff.py
index 3cced977bf..23d2355f41 100644
--- a/zerver/lib/mdiff.py
+++ b/zerver/lib/mdiff.py
@@ -8,7 +8,7 @@ class DiffError(Exception):
 
 
 def diff_strings(output: str, expected_output: str) -> str:
-    mdiff_path = "web/tests/lib/mdiff.js"
+    mdiff_path = "web/tests/lib/mdiff.cjs"
     if not os.path.isfile(mdiff_path):  # nocoverage
         msg = "Cannot find mdiff for Markdown diff rendering"
         logging.error(msg)
diff --git a/zerver/tests/test_markdown.py b/zerver/tests/test_markdown.py
index 74684fe75b..bd3f3d27da 100644
--- a/zerver/tests/test_markdown.py
+++ b/zerver/tests/test_markdown.py
@@ -1621,7 +1621,7 @@ class MarkdownLinkifierTest(ZulipTestCase):
         )
 
     def test_topic_links_ordering_by_priority(self) -> None:
-        # The same test case is also implemented in web/tests/markdown_parse.test.js
+        # The same test case is also implemented in web/tests/markdown_parse.test.cjs
         realm = get_realm("zulip")
         self.check_add_linkifiers(
             [