diff --git a/web/shared/src/typing_status.ts b/web/shared/src/typing_status.ts
index 463e5eb243..941c6d6f60 100644
--- a/web/shared/src/typing_status.ts
+++ b/web/shared/src/typing_status.ts
@@ -55,19 +55,27 @@ function same_recipient(a: Recipient | null, b: Recipient | null): boolean {
 /** Exported only for tests. */
 export let state: TypingStatusState | null = null;
 
+export function rewire_state(value: typeof state): void {
+    state = value;
+}
+
 /** Exported only for tests. */
-export function stop_last_notification(worker: TypingStatusWorker): void {
+export let stop_last_notification = (worker: TypingStatusWorker): void => {
     assert(state !== null, "State object should not be null here.");
     clearTimeout(state.idle_timer);
     worker.notify_server_stop(state.current_recipient);
     state = null;
+};
+
+export function rewire_stop_last_notification(value: typeof stop_last_notification): void {
+    stop_last_notification = value;
 }
 
 /** Exported only for tests. */
-export function start_or_extend_idle_timer(
+export let start_or_extend_idle_timer = (
     worker: TypingStatusWorker,
     typing_stopped_wait_period: number,
-): ReturnType<typeof setTimeout> {
+): ReturnType<typeof setTimeout> => {
     function on_idle_timeout(): void {
         // We don't do any real error checking here, because
         // if we've been idle, we need to tell folks, and if
@@ -80,6 +88,10 @@ export function start_or_extend_idle_timer(
         clearTimeout(state.idle_timer);
     }
     return setTimeout(on_idle_timeout, typing_stopped_wait_period);
+};
+
+export function rewire_start_or_extend_idle_timer(value: typeof start_or_extend_idle_timer): void {
+    start_or_extend_idle_timer = value;
 }
 
 function set_next_start_time(current_time: number, typing_started_wait_period: number): void {
@@ -88,27 +100,35 @@ function set_next_start_time(current_time: number, typing_started_wait_period: n
 }
 
 // Exported for tests
-export function actually_ping_server(
+export let actually_ping_server = (
     worker: TypingStatusWorker,
     recipient: Recipient,
     current_time: number,
     typing_started_wait_period: number,
-): void {
+): void => {
     worker.notify_server_start(recipient);
     set_next_start_time(current_time, typing_started_wait_period);
+};
+
+export function rewire_actually_ping_server(value: typeof actually_ping_server): void {
+    actually_ping_server = value;
 }
 
 /** Exported only for tests. */
-export function maybe_ping_server(
+export let maybe_ping_server = (
     worker: TypingStatusWorker,
     recipient: Recipient,
     typing_started_wait_period: number,
-): void {
+): void => {
     assert(state !== null, "State object should not be null here.");
     const current_time = worker.get_current_time();
     if (current_time > state.next_send_start_time) {
         actually_ping_server(worker, recipient, current_time, typing_started_wait_period);
     }
+};
+
+export function rewire_maybe_ping_server(value: typeof maybe_ping_server): void {
+    maybe_ping_server = value;
 }
 
 /**
diff --git a/web/src/activity.ts b/web/src/activity.ts
index 7a2179303c..d2f119473b 100644
--- a/web/src/activity.ts
+++ b/web/src/activity.ts
@@ -114,7 +114,7 @@ export function compute_active_status(): ActivityState {
     return ActivityState.IDLE;
 }
 
-export function send_presence_to_server(redraw?: () => void): void {
+export let send_presence_to_server = (redraw?: () => void): void => {
     // Zulip has 2 data feeds coming from the server to the client:
     // The server_events data, and this presence feed.  Data from
     // server_events is nicely serialized, but if we've been offline
@@ -180,6 +180,10 @@ export function send_presence_to_server(redraw?: () => void): void {
             }
         },
     });
+};
+
+export function rewire_send_presence_to_server(value: typeof send_presence_to_server): void {
+    send_presence_to_server = value;
 }
 
 export function mark_client_active(): void {
diff --git a/web/src/activity_ui.ts b/web/src/activity_ui.ts
index 7148b8077f..87aa6ce2ac 100644
--- a/web/src/activity_ui.ts
+++ b/web/src/activity_ui.ts
@@ -59,7 +59,7 @@ export function clear_for_testing(): void {
     user_filter = undefined;
 }
 
-export function update_presence_indicators(): void {
+export let update_presence_indicators = (): void => {
     $("[data-presence-indicator-user-id]").each(function () {
         const user_id = Number.parseInt($(this).attr("data-presence-indicator-user-id") ?? "", 10);
         assert(!Number.isNaN(user_id));
@@ -68,6 +68,10 @@ export function update_presence_indicators(): void {
             .removeClass("user_circle_empty user_circle_green user_circle_idle")
             .addClass(user_circle_class);
     });
+};
+
+export function rewire_update_presence_indicators(value: typeof update_presence_indicators): void {
+    update_presence_indicators = value;
 }
 
 export function redraw_user(user_id: number): void {
@@ -115,7 +119,7 @@ export function render_empty_user_list_message_if_needed($container: JQuery): vo
     $container.append($(empty_list_widget_html));
 }
 
-export function build_user_sidebar(): number[] | undefined {
+export let build_user_sidebar = (): number[] | undefined => {
     if (realm.realm_presence_disabled) {
         return undefined;
     }
@@ -131,6 +135,10 @@ export function build_user_sidebar(): number[] | undefined {
     render_empty_user_list_message_if_needed(buddy_list.$other_users_list);
 
     return all_user_ids; // for testing
+};
+
+export function rewire_build_user_sidebar(value: typeof build_user_sidebar): void {
+    build_user_sidebar = value;
 }
 
 function do_update_users_for_search(): void {
diff --git a/web/src/alert_words_ui.ts b/web/src/alert_words_ui.ts
index 0398a68bf1..70b268aebc 100644
--- a/web/src/alert_words_ui.ts
+++ b/web/src/alert_words_ui.ts
@@ -12,7 +12,7 @@ import * as ui_report from "./ui_report";
 
 export let loaded = false;
 
-export function rerender_alert_words_ui(): void {
+export let rerender_alert_words_ui = (): void => {
     if (!loaded) {
         return;
     }
@@ -33,6 +33,10 @@ export function rerender_alert_words_ui(): void {
             ...ListWidget.generic_sort_functions("alphabetic", ["word"]),
         },
     });
+};
+
+export function rewire_rerender_alert_words_ui(value: typeof rerender_alert_words_ui): void {
+    rerender_alert_words_ui = value;
 }
 
 function update_alert_word_status(status_text: string, is_error: boolean): void {
diff --git a/web/src/all_messages_data.ts b/web/src/all_messages_data.ts
index e711bea910..d63641c466 100644
--- a/web/src/all_messages_data.ts
+++ b/web/src/all_messages_data.ts
@@ -1,7 +1,11 @@
 import {Filter} from "./filter";
 import {MessageListData} from "./message_list_data";
 
-export const all_messages_data = new MessageListData({
+export let all_messages_data = new MessageListData({
     excludes_muted_topics: false,
     filter: new Filter([]),
 });
+
+export function rewire_all_messages_data(value: typeof all_messages_data): void {
+    all_messages_data = value;
+}
diff --git a/web/src/bootstrap_typeahead.ts b/web/src/bootstrap_typeahead.ts
index f4525adc75..e99ae6b8ba 100644
--- a/web/src/bootstrap_typeahead.ts
+++ b/web/src/bootstrap_typeahead.ts
@@ -186,7 +186,11 @@ export function defaultSorter(items: string[], query: string): string[] {
     return [...beginswith, ...caseSensitive, ...caseInsensitive];
 }
 
-export const MAX_ITEMS = 50;
+export let MAX_ITEMS = 50;
+
+export function rewire_MAX_ITEMS(value: typeof MAX_ITEMS): void {
+    MAX_ITEMS = value;
+}
 
 /* TYPEAHEAD PUBLIC CLASS DEFINITION
  * ================================= */
diff --git a/web/src/browser_history.ts b/web/src/browser_history.ts
index 506b33014b..6ae141957e 100644
--- a/web/src/browser_history.ts
+++ b/web/src/browser_history.ts
@@ -58,7 +58,7 @@ export function save_old_hash(): boolean {
     return was_internal_change;
 }
 
-export function update(new_hash: string): void {
+export let update = (new_hash: string): void => {
     const old_hash = window.location.hash;
 
     if (!new_hash.startsWith("#")) {
@@ -78,6 +78,10 @@ export function update(new_hash: string): void {
     state.old_hash = old_hash;
     state.is_internal_change = true;
     window.location.hash = new_hash;
+};
+
+export function rewire_update(value: typeof update): void {
+    update = value;
 }
 
 export function exit_overlay(): void {
diff --git a/web/src/buddy_data.ts b/web/src/buddy_data.ts
index 02b2475096..637f293651 100644
--- a/web/src/buddy_data.ts
+++ b/web/src/buddy_data.ts
@@ -27,8 +27,19 @@ import * as util from "./util";
 
 */
 
-export const max_size_before_shrinking = 600;
-export const max_channel_size_to_show_all_subscribers = 75;
+export let max_size_before_shrinking = 600;
+
+export function rewire_max_size_before_shrinking(value: typeof max_size_before_shrinking): void {
+    max_size_before_shrinking = value;
+}
+
+export let max_channel_size_to_show_all_subscribers = 75;
+
+export function rewire_max_channel_size_to_show_all_subscribers(
+    value: typeof max_channel_size_to_show_all_subscribers,
+): void {
+    max_channel_size_to_show_all_subscribers = value;
+}
 
 let is_searching_users = false;
 
@@ -71,11 +82,11 @@ export function level(user_id: number): number {
     }
 }
 
-export function user_matches_narrow(
+export let user_matches_narrow = (
     user_id: number,
     pm_ids: Set<number>,
     stream_id?: number | null,
-): boolean {
+): boolean => {
     if (stream_id) {
         return stream_data.is_user_subscribed(stream_id, user_id);
     }
@@ -83,6 +94,10 @@ export function user_matches_narrow(
         return pm_ids.has(user_id) || people.is_my_user_id(user_id);
     }
     return false;
+};
+
+export function rewire_user_matches_narrow(value: typeof user_matches_narrow): void {
+    user_matches_narrow = value;
 }
 
 export function compare_function(
diff --git a/web/src/compose.js b/web/src/compose.js
index 773d0c9907..d78e143a31 100644
--- a/web/src/compose.js
+++ b/web/src/compose.js
@@ -170,7 +170,7 @@ export function send_message_success(request, data) {
     }
 }
 
-export function send_message(request = create_message_object()) {
+export let send_message = (request = create_message_object()) => {
     compose_state.set_recipient_edited_manually(false);
     compose_state.set_is_content_unedited_restored_draft(false);
     if (request.type === "private") {
@@ -269,6 +269,10 @@ export function send_message(request = create_message_object()) {
         // taking a longtime to send.
         setTimeout(() => echo.display_slow_send_loading_spinner(message), 5000);
     }
+};
+
+export function rewire_send_message(value) {
+    send_message = value;
 }
 
 export function enter_with_preview_open(ctrl_pressed = false) {
@@ -287,7 +291,7 @@ export function enter_with_preview_open(ctrl_pressed = false) {
 // Common entrypoint for asking the server to send the message
 // currently drafted in the compose box, including for scheduled
 // messages.
-export function finish(scheduling_message = false) {
+export let finish = (scheduling_message = false) => {
     if (compose_ui.compose_spinner_visible) {
         // Avoid sending a message twice in parallel in races where
         // the user clicks the `Send` button very quickly twice or
@@ -326,6 +330,10 @@ export function finish(scheduling_message = false) {
     }
     do_post_send_tasks();
     return true;
+};
+
+export function rewire_finish(value) {
+    finish = value;
 }
 
 export function do_post_send_tasks() {
diff --git a/web/src/compose_actions.ts b/web/src/compose_actions.ts
index 611ac475b5..9738ccf9b1 100644
--- a/web/src/compose_actions.ts
+++ b/web/src/compose_actions.ts
@@ -70,8 +70,12 @@ function call_hooks(hooks: ComposeHook[]): void {
     }
 }
 
-export function blur_compose_inputs(): void {
+export let blur_compose_inputs = (): void => {
     $(".message_comp").find("input, textarea, button, #private_message_recipient").trigger("blur");
+};
+
+export function rewire_blur_compose_inputs(value: typeof blur_compose_inputs): void {
+    blur_compose_inputs = value;
 }
 
 function hide_box(): void {
@@ -109,8 +113,12 @@ function show_compose_box(opts: ComposeActionsOpts): void {
     compose_ui.set_focus(opts_by_message_type);
 }
 
-export function clear_textarea(): void {
+export let clear_textarea = (): void => {
     $("#compose").find("input[type=text], textarea").val("");
+};
+
+export function rewire_clear_textarea(value: typeof clear_textarea): void {
+    clear_textarea = value;
 }
 
 function clear_box(): void {
@@ -135,7 +143,7 @@ function clear_box(): void {
 }
 
 let autosize_callback_opts: ComposeActionsStartOpts;
-export function autosize_message_content(opts: ComposeActionsStartOpts): void {
+export let autosize_message_content = (opts: ComposeActionsStartOpts): void => {
     if (!compose_ui.is_expanded()) {
         autosize_callback_opts = opts;
         let has_resized_once = false;
@@ -157,15 +165,23 @@ export function autosize_message_content(opts: ComposeActionsStartOpts): void {
             });
         autosize($("textarea#compose-textarea"));
     }
+};
+
+export function rewire_autosize_message_content(value: typeof autosize_message_content): void {
+    autosize_message_content = value;
 }
 
-export function expand_compose_box(): void {
+export let expand_compose_box = (): void => {
     $("#compose_close").attr("data-tooltip-template-id", "compose_close_tooltip_template");
     $("#compose_controls").hide();
     $(".message_comp").show();
+};
+
+export function rewire_expand_compose_box(value: typeof expand_compose_box): void {
+    expand_compose_box = value;
 }
 
-export function complete_starting_tasks(opts: ComposeActionsOpts): void {
+export let complete_starting_tasks = (opts: ComposeActionsOpts): void => {
     // This is sort of a kitchen sink function, and it's called only
     // by compose.start() for now.  Having this as a separate function
     // makes testing a bit easier.
@@ -182,6 +198,10 @@ export function complete_starting_tasks(opts: ComposeActionsOpts): void {
     if (!narrow_state.narrowed_by_reply()) {
         compose_notifications.maybe_show_one_time_interleaved_view_messages_fading_banner();
     }
+};
+
+export function rewire_complete_starting_tasks(value: typeof complete_starting_tasks): void {
+    complete_starting_tasks = value;
 }
 
 export function maybe_scroll_up_selected_message(opts: ComposeActionsStartOpts): void {
@@ -246,7 +266,7 @@ function same_recipient_as_before(opts: ComposeActionsOpts): boolean {
     );
 }
 
-export function start(raw_opts: ComposeActionsStartOpts): void {
+export let start = (raw_opts: ComposeActionsStartOpts): void => {
     if (page_params.is_spectator) {
         spectators.login_to_access();
         return;
@@ -390,9 +410,13 @@ export function start(raw_opts: ComposeActionsStartOpts): void {
     resize.reset_compose_message_max_height();
 
     complete_starting_tasks(opts);
+};
+
+export function rewire_start(value: typeof start): void {
+    start = value;
 }
 
-export function cancel(): void {
+export let cancel = (): void => {
     // As user closes the compose box, restore the compose box max height
     if (compose_ui.is_expanded()) {
         compose_ui.make_compose_box_original_size();
@@ -420,6 +444,10 @@ export function cancel(): void {
     compose_state.set_message_type(undefined);
     compose_pm_pill.clear();
     $(document).trigger("compose_canceled.zulip");
+};
+
+export function rewire_cancel(value: typeof cancel): void {
+    cancel = value;
 }
 
 export function on_show_navigation_view(): void {
@@ -440,7 +468,7 @@ export function on_show_navigation_view(): void {
     cancel();
 }
 
-export function on_topic_narrow(): void {
+export let on_topic_narrow = (): void => {
     if (!compose_state.composing()) {
         // If our compose box is closed, then just
         // leave it closed, assuming that the user is
@@ -491,6 +519,10 @@ export function on_topic_narrow(): void {
     compose_fade.update_message_list();
     drafts.update_compose_draft_count();
     $("textarea#compose-textarea").trigger("focus");
+};
+
+export function rewire_on_topic_narrow(value: typeof on_topic_narrow): void {
+    on_topic_narrow = value;
 }
 
 // TODO/typescript: Fill this in when converting narrow.js to typescripot.
diff --git a/web/src/compose_banner.ts b/web/src/compose_banner.ts
index c39293e9c2..fba50340aa 100644
--- a/web/src/compose_banner.ts
+++ b/web/src/compose_banner.ts
@@ -91,10 +91,10 @@ export function update_or_append_banner(
     }
 }
 
-export function clear_message_sent_banners(
+export let clear_message_sent_banners = (
     include_unmute_banner = true,
     skip_automatic_new_visibility_policy_banner = false,
-): void {
+): void => {
     for (const classname of Object.values(MESSAGE_SENT_CLASSNAMES)) {
         if (
             skip_automatic_new_visibility_policy_banner &&
@@ -115,6 +115,10 @@ export function clear_message_sent_banners(
         clear_unmute_topic_notifications();
     }
     scroll_to_message_banner_message_id = null;
+};
+
+export function rewire_clear_message_sent_banners(value: typeof clear_message_sent_banners): void {
+    clear_message_sent_banners = value;
 }
 
 // TODO: Replace with compose_ui.hide_compose_spinner() when it is converted to ts.
diff --git a/web/src/compose_closed_ui.ts b/web/src/compose_closed_ui.ts
index 9949d59666..891755a847 100644
--- a/web/src/compose_closed_ui.ts
+++ b/web/src/compose_closed_ui.ts
@@ -61,7 +61,7 @@ export function get_recipient_label(message?: ComposeClosedMessage): string {
 }
 
 // Exported for tests
-export function update_reply_button_state(disable = false): void {
+export let update_reply_button_state = (disable = false): void => {
     $(".compose_reply_button").attr("disabled", disable ? "disabled" : null);
     if (disable) {
         $("#compose_buttons .compose-reply-button-wrapper").attr(
@@ -81,6 +81,10 @@ export function update_reply_button_state(disable = false): void {
             "selected_conversation",
         );
     }
+};
+
+export function rewire_update_reply_button_state(value: typeof update_reply_button_state): void {
+    update_reply_button_state = value;
 }
 
 function update_buttons(disable_reply?: boolean): void {
diff --git a/web/src/compose_pm_pill.ts b/web/src/compose_pm_pill.ts
index 8868014f71..8977c607e7 100644
--- a/web/src/compose_pm_pill.ts
+++ b/web/src/compose_pm_pill.ts
@@ -63,10 +63,14 @@ export function set_from_typeahead(person: User): void {
     });
 }
 
-export function set_from_emails(value: string): void {
+export let set_from_emails = (value: string): void => {
     // value is something like "alice@example.com,bob@example.com"
     clear();
     widget.appendValue(value);
+};
+
+export function rewire_set_from_emails(value: typeof set_from_emails): void {
+    set_from_emails = value;
 }
 
 export function get_user_ids(): number[] {
@@ -84,11 +88,15 @@ export function get_user_ids_string(): string {
     return user_ids_string;
 }
 
-export function get_emails(): string {
+export let get_emails = (): string => {
     // return something like "alice@example.com,bob@example.com"
     const user_ids = get_user_ids();
     const emails = user_ids.map((id) => people.get_by_user_id(id).email).join(",");
     return emails;
+};
+
+export function rewire_get_emails(value: typeof get_emails): void {
+    get_emails = value;
 }
 
 export function filter_taken_users(persons: User[]): User[] {
diff --git a/web/src/compose_recipient.ts b/web/src/compose_recipient.ts
index 7f5b61e2e1..8a47c5cdd8 100644
--- a/web/src/compose_recipient.ts
+++ b/web/src/compose_recipient.ts
@@ -62,7 +62,7 @@ function composing_to_current_private_message_narrow(): boolean {
     );
 }
 
-export function update_narrow_to_recipient_visibility(): void {
+export let update_narrow_to_recipient_visibility = (): void => {
     const message_type = compose_state.get_message_type();
     if (message_type === "stream") {
         const stream_exists = Boolean(compose_state.stream_id());
@@ -87,6 +87,12 @@ export function update_narrow_to_recipient_visibility(): void {
         }
     }
     $(".conversation-arrow").toggleClass("narrow_to_compose_recipients", false);
+};
+
+export function rewire_update_narrow_to_recipient_visibility(
+    value: typeof update_narrow_to_recipient_visibility,
+): void {
+    update_narrow_to_recipient_visibility = value;
 }
 
 function update_fade(): void {
@@ -131,7 +137,7 @@ export function get_posting_policy_error_message(): string {
     return "";
 }
 
-export function check_posting_policy_for_compose_box(): void {
+export let check_posting_policy_for_compose_box = (): void => {
     const banner_text = get_posting_policy_error_message();
     if (banner_text === "") {
         compose_validate.set_recipient_disallowed(false);
@@ -147,6 +153,12 @@ export function check_posting_policy_for_compose_box(): void {
     } else {
         compose_banner.show_error_message(banner_text, banner_classname, $("#compose_banners"));
     }
+};
+
+export function rewire_check_posting_policy_for_compose_box(
+    value: typeof check_posting_policy_for_compose_box,
+): void {
+    check_posting_policy_for_compose_box = value;
 }
 
 function switch_message_type(message_type: MessageType): void {
@@ -207,7 +219,7 @@ export function update_compose_for_message_type(opts: ComposeTriggeredOptions):
     compose_banner.clear_uploads();
 }
 
-export function on_compose_select_recipient_update(): void {
+export let on_compose_select_recipient_update = (): void => {
     const prev_message_type = compose_state.get_message_type();
 
     let curr_message_type: MessageType = "stream";
@@ -226,6 +238,12 @@ export function on_compose_select_recipient_update(): void {
     }
 
     update_on_recipient_change();
+};
+
+export function rewire_on_compose_select_recipient_update(
+    value: typeof on_compose_select_recipient_update,
+): void {
+    on_compose_select_recipient_update = value;
 }
 
 export function possibly_update_stream_name_in_compose(stream_id: number): void {
@@ -327,7 +345,7 @@ export function initialize(): void {
     });
 }
 
-export function update_placeholder_text(): void {
+export let update_placeholder_text = (): void => {
     const $textarea: JQuery<HTMLTextAreaElement> = $("textarea#compose-textarea");
     // Change compose placeholder text only if compose box is open.
     if (!$textarea.is(":visible")) {
@@ -352,4 +370,8 @@ export function update_placeholder_text(): void {
 
     $textarea.attr("placeholder", placeholder);
     compose_ui.autosize_textarea($textarea);
+};
+
+export function rewire_update_placeholder_text(value: typeof update_placeholder_text): void {
+    update_placeholder_text = value;
 }
diff --git a/web/src/compose_reply.ts b/web/src/compose_reply.ts
index 3b3b2cad6c..aaa9a04f33 100644
--- a/web/src/compose_reply.ts
+++ b/web/src/compose_reply.ts
@@ -22,12 +22,12 @@ import * as recent_view_util from "./recent_view_util";
 import * as stream_data from "./stream_data";
 import * as unread_ops from "./unread_ops";
 
-export function respond_to_message(opts: {
+export let respond_to_message = (opts: {
     keep_composebox_empty?: boolean;
     message_id?: number;
     reply_type?: "personal";
     trigger?: string;
-}): void {
+}): void => {
     let message;
     let msg_type: "private" | "stream";
     if (recent_view_util.is_visible()) {
@@ -147,6 +147,10 @@ export function respond_to_message(opts: {
         is_reply: true,
         keep_composebox_empty: opts.keep_composebox_empty,
     });
+};
+
+export function rewire_respond_to_message(value: typeof respond_to_message): void {
+    respond_to_message = value;
 }
 
 export function reply_with_mention(opts: {
@@ -166,7 +170,9 @@ export function reply_with_mention(opts: {
     compose_ui.insert_syntax_and_focus(mention);
 }
 
-export function selection_within_message_id(selection = window.getSelection()): number | undefined {
+export let selection_within_message_id = (
+    selection = window.getSelection(),
+): number | undefined => {
     // Returns the message_id if the selection is entirely within a message,
     // otherwise returns undefined.
     assert(selection !== null);
@@ -178,6 +184,12 @@ export function selection_within_message_id(selection = window.getSelection()):
         return start_id;
     }
     return undefined;
+};
+
+export function rewire_selection_within_message_id(
+    value: typeof selection_within_message_id,
+): void {
+    selection_within_message_id = value;
 }
 
 function get_quote_target(opts: {message_id?: number; quote_content?: string}): {
diff --git a/web/src/compose_state.ts b/web/src/compose_state.ts
index c7ff192663..f0839e859b 100644
--- a/web/src/compose_state.ts
+++ b/web/src/compose_state.ts
@@ -106,12 +106,16 @@ export function stream_id(): number | undefined {
     return undefined;
 }
 
-export function stream_name(): string {
+export let stream_name = (): string => {
     const stream_id = selected_recipient_id;
     if (typeof stream_id === "number") {
         return sub_store.maybe_get_stream_name(stream_id) ?? "";
     }
     return "";
+};
+
+export function rewire_stream_name(value: typeof stream_name): void {
+    stream_name = value;
 }
 
 export function set_stream_id(stream_id: number | ""): void {
diff --git a/web/src/compose_ui.ts b/web/src/compose_ui.ts
index 9b90051424..0a59705ae4 100644
--- a/web/src/compose_ui.ts
+++ b/web/src/compose_ui.ts
@@ -70,6 +70,11 @@ const message_render_response_schema = z.object({
 });
 
 export let compose_spinner_visible = false;
+
+export function rewire_compose_spinner_visible(value: typeof compose_spinner_visible): void {
+    compose_spinner_visible = value;
+}
+
 export let shift_pressed = false; // true or false
 export let code_formatting_button_triggered = false; // true or false
 export let compose_textarea_typeahead: Typeahead<TypeaheadSuggestion> | undefined;
@@ -102,20 +107,24 @@ export function is_full_size(): boolean {
     return full_size_status;
 }
 
-export function autosize_textarea($textarea: JQuery<HTMLTextAreaElement>): void {
+export let autosize_textarea = ($textarea: JQuery<HTMLTextAreaElement>): void => {
     // Since this supports both compose and file upload, one must pass
     // in the text area to autosize.
     if (!is_expanded()) {
         autosize.update($textarea);
     }
+};
+
+export function rewire_autosize_textarea(value: typeof autosize_textarea): void {
+    autosize_textarea = value;
 }
 
-export function insert_and_scroll_into_view(
+export let insert_and_scroll_into_view = (
     content: string,
     $textarea: JQuery<HTMLTextAreaElement>,
     replace_all = false,
     replace_all_without_undo_support = false,
-): void {
+): void => {
     if (replace_all_without_undo_support) {
         // setFieldText is very slow and noticeable when inserting 10k+
         // characters of text like from a drafted response,
@@ -132,6 +141,12 @@ export function insert_and_scroll_into_view(
     $textarea.trigger("blur");
     $textarea.trigger("focus");
     autosize_textarea($textarea);
+};
+
+export function rewire_insert_and_scroll_into_view(
+    value: typeof insert_and_scroll_into_view,
+): void {
+    insert_and_scroll_into_view = value;
 }
 
 function get_focus_area(opts: ComposeTriggeredOptions): string {
@@ -168,7 +183,7 @@ export function set_focus(opts: ComposeTriggeredOptions): void {
     }
 }
 
-export function smart_insert_inline($textarea: JQuery<HTMLTextAreaElement>, syntax: string): void {
+export let smart_insert_inline = ($textarea: JQuery<HTMLTextAreaElement>, syntax: string): void => {
     function is_space(c: string | undefined): boolean {
         return c === " " || c === "\t" || c === "\n";
     }
@@ -200,6 +215,10 @@ export function smart_insert_inline($textarea: JQuery<HTMLTextAreaElement>, synt
     }
 
     insert_and_scroll_into_view(syntax, $textarea);
+};
+
+export function rewire_smart_insert_inline(value: typeof smart_insert_inline): void {
+    smart_insert_inline = value;
 }
 
 export function smart_insert_block(
@@ -252,12 +271,12 @@ export function smart_insert_block(
     insert_and_scroll_into_view(syntax, $textarea);
 }
 
-export function insert_syntax_and_focus(
+export let insert_syntax_and_focus = (
     syntax: string,
     $textarea = $<HTMLTextAreaElement>("textarea#compose-textarea"),
     mode = "inline",
     padding_newlines?: number,
-): void {
+): void => {
     // Generic helper for inserting syntax into the main compose box
     // where the cursor was and focusing the area.  Mostly a thin
     // wrapper around smart_insert_inline and smart_inline_block.
@@ -277,13 +296,17 @@ export function insert_syntax_and_focus(
     } else if (mode === "block") {
         smart_insert_block($textarea, syntax, padding_newlines);
     }
+};
+
+export function rewire_insert_syntax_and_focus(value: typeof insert_syntax_and_focus): void {
+    insert_syntax_and_focus = value;
 }
 
-export function replace_syntax(
+export let replace_syntax = (
     old_syntax: string,
     new_syntax: string,
     $textarea = $<HTMLTextAreaElement>("textarea#compose-textarea"),
-): boolean {
+): boolean => {
     // The following couple lines are needed to later restore the initial
     // logical position of the cursor after the replacement
     const prev_caret = $textarea.caret();
@@ -322,6 +345,10 @@ export function replace_syntax(
 
     // Return if anything was actually replaced.
     return old_text !== new_text;
+};
+
+export function rewire_replace_syntax(value: typeof replace_syntax): void {
+    replace_syntax = value;
 }
 
 export function compute_placeholder_text(opts: ComposePlaceholderOptions): string {
@@ -374,7 +401,7 @@ export function compute_placeholder_text(opts: ComposePlaceholderOptions): strin
     return DEFAULT_COMPOSE_PLACEHOLDER;
 }
 
-export function set_compose_box_top(set_top: boolean): void {
+export let set_compose_box_top = (set_top: boolean): void => {
     if (set_top) {
         // As `#compose` has `position: fixed` property, we cannot
         // make the compose-box to attain the correct height just by
@@ -386,6 +413,10 @@ export function set_compose_box_top(set_top: boolean): void {
     } else {
         $("#compose").css("top", "");
     }
+};
+
+export function rewire_set_compose_box_top(value: typeof set_compose_box_top): void {
+    set_compose_box_top = value;
 }
 
 export function make_compose_box_full_size(): void {
@@ -504,11 +535,11 @@ export function position_inside_code_block(content: string, position: number): b
     return [...code_blocks].some((code_block) => code_block?.textContent?.includes(unique_insert));
 }
 
-export function format_text(
+export let format_text = (
     $textarea: JQuery<HTMLTextAreaElement>,
     type: string,
     inserted_content = "",
-): void {
+): void => {
     const italic_syntax = "*";
     const bold_syntax = "**";
     const bold_and_italic_syntax = "***";
@@ -1156,6 +1187,10 @@ export function format_text(
             break;
         }
     }
+};
+
+export function rewire_format_text(value: typeof format_text): void {
+    format_text = value;
 }
 
 /* TODO: This functions don't belong in this module, as they have
diff --git a/web/src/compose_validate.ts b/web/src/compose_validate.ts
index 8a07bc762d..d7057cf141 100644
--- a/web/src/compose_validate.ts
+++ b/web/src/compose_validate.ts
@@ -476,7 +476,7 @@ function is_recipient_large_topic(): boolean {
 }
 
 // Exported for tests
-export function wildcard_mention_policy_authorizes_user(): boolean {
+export let wildcard_mention_policy_authorizes_user = (): boolean => {
     if (
         realm.realm_wildcard_mention_policy ===
         settings_config.wildcard_mention_policy_values.by_everyone.code
@@ -518,6 +518,12 @@ export function wildcard_mention_policy_authorizes_user(): boolean {
         return days >= realm.realm_waiting_period_threshold && !current_user.is_guest;
     }
     return !current_user.is_guest;
+};
+
+export function rewire_wildcard_mention_policy_authorizes_user(
+    value: typeof wildcard_mention_policy_authorizes_user,
+): void {
+    wildcard_mention_policy_authorizes_user = value;
 }
 
 export function stream_wildcard_mention_allowed(): boolean {
diff --git a/web/src/composebox_typeahead.ts b/web/src/composebox_typeahead.ts
index 4d99a1ca1f..703aff8acb 100644
--- a/web/src/composebox_typeahead.ts
+++ b/web/src/composebox_typeahead.ts
@@ -106,7 +106,11 @@ export type TypeaheadSuggestion =
     | SlashCommandSuggestion;
 
 // We export it to allow tests to mock it.
-export const max_num_items = MAX_ITEMS;
+export let max_num_items = MAX_ITEMS;
+
+export function rewire_max_num_items(value: typeof max_num_items): void {
+    max_num_items = value;
+}
 
 export let emoji_collection: Emoji[] = [];
 
@@ -389,9 +393,13 @@ function handle_keyup(e: JQuery.KeyUpEvent): void {
     }
 }
 
-export function split_at_cursor(query: string, $input: JQuery): [string, string] {
+export let split_at_cursor = (query: string, $input: JQuery): [string, string] => {
     const cursor = $input.caret();
     return [query.slice(0, cursor), query.slice(cursor)];
+};
+
+export function rewire_split_at_cursor(value: typeof split_at_cursor): void {
+    split_at_cursor = value;
 }
 
 export function tokenize_compose_str(s: string): string {
diff --git a/web/src/drafts.ts b/web/src/drafts.ts
index db5d87c25e..f6daa8e87b 100644
--- a/web/src/drafts.ts
+++ b/web/src/drafts.ts
@@ -22,9 +22,13 @@ import * as timerender from "./timerender";
 import * as ui_util from "./ui_util";
 import * as util from "./util";
 
-export function set_count(count: number): void {
+export let set_count = (count: number): void => {
     const $drafts_li = $(".top_left_drafts");
     ui_util.update_unread_count_in_dom($drafts_li, count);
+};
+
+export function rewire_set_count(value: typeof set_count): void {
+    set_count = value;
 }
 
 function getTimestamp(): number {
@@ -219,7 +223,7 @@ export const draft_model = (function () {
     };
 })();
 
-export function update_compose_draft_count(): void {
+export let update_compose_draft_count = (): void => {
     const $count_container = $(".compose-drafts-count-container");
     const $count_ele = $count_container.find(".compose-drafts-count");
     if (!compose_state.has_full_recipient()) {
@@ -235,11 +239,19 @@ export function update_compose_draft_count(): void {
         $count_ele.text("");
         $count_container.hide();
     }
+};
+
+export function rewire_update_compose_draft_count(value: typeof update_compose_draft_count): void {
+    update_compose_draft_count = value;
 }
 
-export function sync_count(): void {
+export let sync_count = (): void => {
     const drafts = draft_model.get();
     set_count(Object.keys(drafts).length);
+};
+
+export function rewire_sync_count(value: typeof sync_count): void {
+    sync_count = value;
 }
 
 export function delete_all_drafts(): void {
@@ -396,7 +408,7 @@ type UpdateDraftOptions = {
     is_sending_saving?: boolean;
 };
 
-export function update_draft(opts: UpdateDraftOptions = {}): string | undefined {
+export let update_draft = (opts: UpdateDraftOptions = {}): string | undefined => {
     const draft_id = compose_draft_id;
     const old_draft = draft_id === undefined ? undefined : draft_model.getDraft(draft_id);
 
@@ -439,6 +451,10 @@ export function update_draft(opts: UpdateDraftOptions = {}): string | undefined
     maybe_notify(no_notify);
 
     return new_draft_id;
+};
+
+export function rewire_update_draft(value: typeof update_draft): void {
+    update_draft = value;
 }
 
 export const DRAFT_LIFETIME = 30;
diff --git a/web/src/echo.ts b/web/src/echo.ts
index 695b1712dd..3726fd3429 100644
--- a/web/src/echo.ts
+++ b/web/src/echo.ts
@@ -288,14 +288,14 @@ export function is_slash_command(content: string): boolean {
     return !content.startsWith("/me") && content.startsWith("/");
 }
 
-export function try_deliver_locally(
+export let try_deliver_locally = (
     message_request: MessageRequest,
     insert_new_messages: (
         messages: LocalMessage[],
         send_by_this_client: boolean,
         deliver_locally: boolean,
     ) => Message[],
-): Message | undefined {
+): Message | undefined => {
     // Checks if the message request can be locally echoed, and if so,
     // adds a local echoed copy of the message to appropriate message lists.
     //
@@ -349,6 +349,10 @@ export function try_deliver_locally(
 
     const message = insert_local_message(message_request, local_id_float, insert_new_messages);
     return message;
+};
+
+export function rewire_try_deliver_locally(value: typeof try_deliver_locally): void {
+    try_deliver_locally = value;
 }
 
 export function edit_locally(message: Message, request: LocalEditRequest): Message {
@@ -432,7 +436,7 @@ export function edit_locally(message: Message, request: LocalEditRequest): Messa
     return message;
 }
 
-export function reify_message_id(local_id: string, server_id: number): void {
+export let reify_message_id = (local_id: string, server_id: number): void => {
     const message = echo_state.get_message_waiting_for_id(local_id);
     echo_state.remove_message_from_waiting_for_id(local_id);
 
@@ -463,6 +467,10 @@ export function reify_message_id(local_id: string, server_id: number): void {
             message_id: message.id,
         });
     }
+};
+
+export function rewire_reify_message_id(value: typeof reify_message_id): void {
+    reify_message_id = value;
 }
 
 export function update_message_lists({old_id, new_id}: {old_id: number; new_id: number}): void {
@@ -576,13 +584,17 @@ export function process_from_server(messages: ServerMessage[]): ServerMessage[]
     return non_echo_messages;
 }
 
-export function message_send_error(message_id: number, error_response: string): void {
+export let message_send_error = (message_id: number, error_response: string): void => {
     // Error sending message, show inline
     const message = message_store.get(message_id)!;
     message.failed_request = true;
     message.show_slow_send_spinner = false;
 
     show_message_failed(message_id, error_response);
+};
+
+export function rewire_message_send_error(value: typeof message_send_error): void {
+    message_send_error = value;
 }
 
 function abort_message(message: Message): void {
diff --git a/web/src/hotkey.js b/web/src/hotkey.js
index 735a9bbaf7..a7b751c2be 100644
--- a/web/src/hotkey.js
+++ b/web/src/hotkey.js
@@ -250,7 +250,7 @@ export function get_keypress_hotkey(e) {
     return keypress_mappings[e.which];
 }
 
-export function processing_text() {
+export let processing_text = () => {
     const $focused_elt = $(":focus");
     return (
         $focused_elt.is("input") ||
@@ -260,6 +260,10 @@ export function processing_text() {
         $focused_elt.attr("id") === "compose-send-button" ||
         $focused_elt.parents(".dropdown-list-container").length >= 1
     );
+};
+
+export function rewire_processing_text(value) {
+    processing_text = value;
 }
 
 export function in_content_editable_widget(e) {
diff --git a/web/src/message_events.js b/web/src/message_events.js
index 2c7b7e9e2d..c2c19ccbdb 100644
--- a/web/src/message_events.js
+++ b/web/src/message_events.js
@@ -103,11 +103,11 @@ export function update_current_view_for_topic_visibility() {
     return false;
 }
 
-export function update_views_filtered_on_message_property(
+export let update_views_filtered_on_message_property = (
     message_ids,
     property_term_type,
     property_value,
-) {
+) => {
     // NOTE: Call this function after updating the message property locally.
     assert(!property_term_type.includes("not-"));
 
@@ -212,6 +212,7 @@ export function update_views_filtered_on_message_property(
                     // can be used to update other message lists and
                     // cached message data structures as well.
                 },
+                // eslint-disable-next-line no-loop-func
                 success(data) {
                     // `messages_to_fetch` might already be cached locally when
                     // we reach here but `message_helper.process_new_message`
@@ -258,6 +259,10 @@ export function update_views_filtered_on_message_property(
             }
         }
     }
+};
+
+export function rewire_update_views_filtered_on_message_property(value) {
+    update_views_filtered_on_message_property = value;
 }
 
 export function insert_new_messages(messages, sent_by_this_client, deliver_locally) {
diff --git a/web/src/message_flags.ts b/web/src/message_flags.ts
index 0014a60c54..414463f249 100644
--- a/web/src/message_flags.ts
+++ b/web/src/message_flags.ts
@@ -16,7 +16,11 @@ export function send_flag_update_for_messages(msg_ids: number[], flag: string, o
         },
     });
 }
-export const _unread_batch_size = 1000;
+export let _unread_batch_size = 1000;
+
+export function rewire__unread_batch_size(value: typeof _unread_batch_size): void {
+    _unread_batch_size = value;
+}
 
 export const send_read = (function () {
     let queue: Message[] = [];
diff --git a/web/src/message_view.ts b/web/src/message_view.ts
index 406eef2af9..5b1626f841 100644
--- a/web/src/message_view.ts
+++ b/web/src/message_view.ts
@@ -351,7 +351,7 @@ type ShowMessageViewOpts = {
     show_more_topics?: boolean;
 };
 
-export function show(raw_terms: NarrowTerm[], show_opts: ShowMessageViewOpts): void {
+export let show = (raw_terms: NarrowTerm[], show_opts: ShowMessageViewOpts): void => {
     /* Main entry point for switching to a new view / message list.
 
        Supported parameters:
@@ -777,6 +777,10 @@ export function show(raw_terms: NarrowTerm[], show_opts: ShowMessageViewOpts): v
             resize.resize_stream_filters_container();
         });
     });
+};
+
+export function rewire_show(value: typeof show): void {
+    show = value;
 }
 
 function navigate_to_anchor_message(opts: {
diff --git a/web/src/narrow_state.ts b/web/src/narrow_state.ts
index 5158602731..04421dc4da 100644
--- a/web/src/narrow_state.ts
+++ b/web/src/narrow_state.ts
@@ -114,7 +114,7 @@ export function set_compose_defaults(): {
     return opts;
 }
 
-export function stream_id(current_filter: Filter | undefined = filter()): number | undefined {
+export let stream_id = (current_filter: Filter | undefined = filter()): number | undefined => {
     if (current_filter === undefined) {
         return undefined;
     }
@@ -123,6 +123,10 @@ export function stream_id(current_filter: Filter | undefined = filter()): number
         return Number.parseInt(stream_operands[0], 10);
     }
     return undefined;
+};
+
+export function rewire_stream_id(value: typeof stream_id): void {
+    stream_id = value;
 }
 
 export function stream_name(current_filter: Filter | undefined = filter()): string | undefined {
@@ -148,7 +152,7 @@ export function stream_sub(
     return stream_data.get_sub_by_id_string(stream_operands[0]);
 }
 
-export function topic(current_filter: Filter | undefined = filter()): string | undefined {
+export let topic = (current_filter: Filter | undefined = filter()): string | undefined => {
     if (current_filter === undefined) {
         return undefined;
     }
@@ -157,6 +161,10 @@ export function topic(current_filter: Filter | undefined = filter()): string | u
         return operands[0];
     }
     return undefined;
+};
+
+export function rewire_topic(value: typeof topic): void {
+    topic = value;
 }
 
 export function pm_ids_string(filter?: Filter): string | undefined {
@@ -173,10 +181,14 @@ export function pm_ids_string(filter?: Filter): string | undefined {
     return user_ids_string;
 }
 
-export function pm_ids_set(filter?: Filter): Set<number> {
+export let pm_ids_set = (filter?: Filter): Set<number> => {
     const ids_string = pm_ids_string(filter);
     const pm_ids_list = ids_string ? people.user_ids_string_to_ids_array(ids_string) : [];
     return new Set(pm_ids_list);
+};
+
+export function rewire_pm_ids_set(value: typeof pm_ids_set): void {
+    pm_ids_set = value;
 }
 
 export function pm_emails_string(
@@ -194,9 +206,9 @@ export function pm_emails_string(
     return operands[0];
 }
 
-export function get_first_unread_info(
+export let get_first_unread_info = (
     current_filter: Filter | undefined = filter(),
-): {flavor: "cannot_compute" | "not_found"} | {flavor: "found"; msg_id: number} {
+): {flavor: "cannot_compute" | "not_found"} | {flavor: "found"; msg_id: number} => {
     const cannot_compute_response: {flavor: "cannot_compute"} = {flavor: "cannot_compute"};
     if (current_filter === undefined) {
         // we don't yet support the all-messages view
@@ -231,11 +243,15 @@ export function get_first_unread_info(
         flavor: "found",
         msg_id,
     };
+};
+
+export function rewire_get_first_unread_info(value: typeof get_first_unread_info): void {
+    get_first_unread_info = value;
 }
 
-export function _possible_unread_message_ids(
+export let _possible_unread_message_ids = (
     current_filter: Filter | undefined = filter(),
-): number[] | undefined {
+): number[] | undefined => {
     // This function currently only returns valid results for
     // certain types of narrows, mostly left sidebar narrows.
     // For more complicated narrows we may return undefined.
@@ -318,6 +334,12 @@ export function _possible_unread_message_ids(
     }
 
     return undefined;
+};
+
+export function rewire__possible_unread_message_ids(
+    value: typeof _possible_unread_message_ids,
+): void {
+    _possible_unread_message_ids = value;
 }
 
 // Are we narrowed to direct messages: the direct message feed or a
diff --git a/web/src/peer_data.ts b/web/src/peer_data.ts
index 20b1fd8093..1e072e19f5 100644
--- a/web/src/peer_data.ts
+++ b/web/src/peer_data.ts
@@ -69,7 +69,7 @@ export function potential_subscribers(stream_id: number): User[] {
     return people.filter_all_users(is_potential_subscriber);
 }
 
-export function get_subscriber_count(stream_id: number, include_bots = true): number {
+export let get_subscriber_count = (stream_id: number, include_bots = true): number => {
     if (include_bots) {
         return get_user_set(stream_id).size;
     }
@@ -81,6 +81,10 @@ export function get_subscriber_count(stream_id: number, include_bots = true): nu
         }
     }
     return count;
+};
+
+export function rewire_get_subscriber_count(value: typeof get_subscriber_count): void {
+    get_subscriber_count = value;
 }
 
 export function get_subscribers(stream_id: number): number[] {
diff --git a/web/src/people.ts b/web/src/people.ts
index f388c13852..c06615b5bb 100644
--- a/web/src/people.ts
+++ b/web/src/people.ts
@@ -89,10 +89,14 @@ export function get_users_from_ids(user_ids: number[]): User[] {
 }
 
 // Use this function only when you are sure that user_id is valid.
-export function get_by_user_id(user_id: number): User {
+export let get_by_user_id = (user_id: number): User => {
     const person = people_by_user_id_dict.get(user_id);
     assert(person, `Unknown user_id in get_by_user_id: ${user_id}`);
     return person;
+};
+
+export function rewire_get_by_user_id(value: typeof get_by_user_id): void {
+    get_by_user_id = value;
 }
 
 // This is type unsafe version of get_by_user_id for the callers that expects undefined values.
@@ -123,7 +127,7 @@ export function validate_user_ids(user_ids: number[]): number[] {
     return good_ids;
 }
 
-export function get_by_email(email: string): User | undefined {
+export let get_by_email = (email: string): User | undefined => {
     const person = people_dict.get(email);
 
     if (!person) {
@@ -137,6 +141,10 @@ export function get_by_email(email: string): User | undefined {
     }
 
     return person;
+};
+
+export function rewire_get_by_email(value: typeof get_by_email): void {
+    get_by_email = value;
 }
 
 export function get_bot_owner_user(user: User & {is_bot: true}): User | undefined {
@@ -382,7 +390,7 @@ export function emails_strings_to_user_ids_string(emails_string: string): string
     return email_list_to_user_ids_string(emails);
 }
 
-export function email_list_to_user_ids_string(emails: string[]): string | undefined {
+export let email_list_to_user_ids_string = (emails: string[]): string | undefined => {
     let user_ids = util.try_parse_as_truthy(
         emails.map((email) => {
             const person = get_by_email(email);
@@ -398,6 +406,12 @@ export function email_list_to_user_ids_string(emails: string[]): string | undefi
     user_ids = sort_numerically(user_ids);
 
     return user_ids.join(",");
+};
+
+export function rewire_email_list_to_user_ids_string(
+    value: typeof email_list_to_user_ids_string,
+): void {
+    email_list_to_user_ids_string = value;
 }
 
 export function get_full_names_for_poll_option(user_ids: number[]): string {
@@ -1056,7 +1070,7 @@ export function get_bot_ids(): number[] {
     return bot_ids;
 }
 
-export function get_active_human_count(): number {
+export let get_active_human_count = (): number => {
     let count = 0;
     for (const person of active_user_dict.values()) {
         if (!person.is_bot) {
@@ -1064,6 +1078,10 @@ export function get_active_human_count(): number {
         }
     }
     return count;
+};
+
+export function rewire_get_active_human_count(value: typeof get_active_human_count): void {
+    get_active_human_count = value;
 }
 
 export function get_active_user_ids(): number[] {
diff --git a/web/src/pm_conversations.ts b/web/src/pm_conversations.ts
index 8340fc76a1..83617bb3e2 100644
--- a/web/src/pm_conversations.ts
+++ b/web/src/pm_conversations.ts
@@ -11,8 +11,12 @@ type PMConversation = {
 
 const partners = new Set<number>();
 
-export function set_partner(user_id: number): void {
+export let set_partner = (user_id: number): void => {
     partners.add(user_id);
+};
+
+export function rewire_set_partner(value: typeof set_partner): void {
+    set_partner = value;
 }
 
 export function is_partner(user_id: number): boolean {
diff --git a/web/src/pygments_data.ts b/web/src/pygments_data.ts
index 6a2a404324..c41b457702 100644
--- a/web/src/pygments_data.ts
+++ b/web/src/pygments_data.ts
@@ -1,6 +1,8 @@
 import generated_pygments_data from "../generated/pygments_data.json";
 
 type PygmentsLanguage = {priority: number; pretty_name: string};
-const langs: Record<string, PygmentsLanguage | undefined> = generated_pygments_data.langs;
+export let langs: Record<string, PygmentsLanguage | undefined> = generated_pygments_data.langs;
 
-export {langs};
+export function rewire_langs(value: typeof langs): void {
+    langs = value;
+}
diff --git a/web/src/reactions.ts b/web/src/reactions.ts
index fa6df71a61..475b886051 100644
--- a/web/src/reactions.ts
+++ b/web/src/reactions.ts
@@ -249,10 +249,14 @@ export function get_reaction_sections(message_id: number): JQuery {
     return $rows.find(".message_reactions");
 }
 
-export function find_reaction(message_id: number, local_id: string): JQuery {
+export let find_reaction = (message_id: number, local_id: string): JQuery => {
     const $reaction_section = get_reaction_sections(message_id);
     const $reaction = $reaction_section.find(`[data-reaction-id='${CSS.escape(local_id)}']`);
     return $reaction;
+};
+
+export function rewire_find_reaction(value: typeof find_reaction): void {
+    find_reaction = value;
 }
 
 export function get_add_reaction_button(message_id: number): JQuery {
@@ -261,12 +265,16 @@ export function get_add_reaction_button(message_id: number): JQuery {
     return $add_button;
 }
 
-export function set_reaction_vote_text($reaction: JQuery, vote_text: string): void {
+export let set_reaction_vote_text = ($reaction: JQuery, vote_text: string): void => {
     const $count_element = $reaction.find(".message_reaction_count");
     $count_element.text(vote_text);
+};
+
+export function rewire_set_reaction_vote_text(value: typeof set_reaction_vote_text): void {
+    set_reaction_vote_text = value;
 }
 
-export function add_reaction(event: ReactionEvent): void {
+export let add_reaction = (event: ReactionEvent): void => {
     const message_id = event.message_id;
     const message = message_store.get(message_id);
 
@@ -311,13 +319,17 @@ export function add_reaction(event: ReactionEvent): void {
         message.clean_reactions.set(local_id, clean_reaction_object);
         insert_new_reaction(clean_reaction_object, message, user_id);
     }
+};
+
+export function rewire_add_reaction(value: typeof add_reaction): void {
+    add_reaction = value;
 }
 
-export function update_existing_reaction(
+export let update_existing_reaction = (
     clean_reaction_object: MessageCleanReaction,
     message: Message,
     acting_user_id: number,
-): void {
+): void => {
     // Our caller ensures that this message already has a reaction
     // for this emoji and sets up our user_list.  This function
     // simply updates the DOM.
@@ -335,13 +347,17 @@ export function update_existing_reaction(
     }
 
     update_vote_text_on_message(message);
+};
+
+export function rewire_update_existing_reaction(value: typeof update_existing_reaction): void {
+    update_existing_reaction = value;
 }
 
-export function insert_new_reaction(
+export let insert_new_reaction = (
     clean_reaction_object: MessageCleanReaction,
     message: Message,
     user_id: number,
-): void {
+): void => {
     // Our caller ensures we are the first user to react to this
     // message with this emoji. We then render the emoji/title/count
     // and insert it before the add button.
@@ -389,9 +405,13 @@ export function insert_new_reaction(
     }
 
     update_vote_text_on_message(message);
+};
+
+export function rewire_insert_new_reaction(value: typeof insert_new_reaction): void {
+    insert_new_reaction = value;
 }
 
-export function remove_reaction(event: ReactionEvent): void {
+export let remove_reaction = (event: ReactionEvent): void => {
     const message_id = event.message_id;
     const user_id = event.user_id;
     const message = message_store.get(message_id);
@@ -426,13 +446,17 @@ export function remove_reaction(event: ReactionEvent): void {
     update_user_fields(clean_reaction_object, should_display_reactors);
 
     remove_reaction_from_view(clean_reaction_object, message, user_id);
+};
+
+export function rewire_remove_reaction(value: typeof remove_reaction): void {
+    remove_reaction = value;
 }
 
-export function remove_reaction_from_view(
+export let remove_reaction_from_view = (
     clean_reaction_object: MessageCleanReaction,
     message: Message,
     user_id: number,
-): void {
+): void => {
     const local_id = get_local_reaction_id(clean_reaction_object);
     const $reaction = find_reaction(message.id, local_id);
     const reaction_count = clean_reaction_object.user_ids.length;
@@ -466,6 +490,10 @@ export function remove_reaction_from_view(
     }
 
     update_vote_text_on_message(message);
+};
+
+export function rewire_remove_reaction_from_view(value: typeof remove_reaction_from_view): void {
+    remove_reaction_from_view = value;
 }
 
 export function get_emojis_used_by_user_for_message_id(message_id: number): string[] {
@@ -687,7 +715,7 @@ function comma_separated_usernames(user_list: number[]): string {
     return comma_separated_usernames;
 }
 
-export function update_vote_text_on_message(message: Message): void {
+export let update_vote_text_on_message = (message: Message): void => {
     // Because whether we display a count or the names of reacting
     // users depends on total reactions on the message, we need to
     // recalculate this whenever adjusting reaction rendering on a
@@ -703,4 +731,10 @@ export function update_vote_text_on_message(message: Message): void {
         message_clean_reaction.vote_text = vote_text;
         set_reaction_vote_text(reaction_elem, vote_text);
     }
+};
+
+export function rewire_update_vote_text_on_message(
+    value: typeof update_vote_text_on_message,
+): void {
+    update_vote_text_on_message = value;
 }
diff --git a/web/src/recent_view_ui.ts b/web/src/recent_view_ui.ts
index 2616cdae15..0fa6f5608a 100644
--- a/web/src/recent_view_ui.ts
+++ b/web/src/recent_view_ui.ts
@@ -930,7 +930,7 @@ export function bulk_inplace_rerender(row_keys: string[]): void {
     setTimeout(revive_current_focus, 0);
 }
 
-export function inplace_rerender(topic_key: string, is_bulk_rerender?: boolean): boolean {
+export let inplace_rerender = (topic_key: string, is_bulk_rerender?: boolean): boolean => {
     if (!recent_view_util.is_visible()) {
         return false;
     }
@@ -990,6 +990,10 @@ export function inplace_rerender(topic_key: string, is_bulk_rerender?: boolean):
         setTimeout(revive_current_focus, 0);
     }
     return true;
+};
+
+export function rewire_inplace_rerender(value: typeof inplace_rerender): void {
+    inplace_rerender = value;
 }
 
 export function update_topic_visibility_policy(stream_id: number, topic: string): boolean {
diff --git a/web/src/search.ts b/web/src/search.ts
index 931e9c4f47..92dfb8b66a 100644
--- a/web/src/search.ts
+++ b/web/src/search.ts
@@ -17,6 +17,11 @@ import * as util from "./util";
 
 // Exported for unit testing
 export let is_using_input_method = false;
+
+export function rewire_is_using_input_method(value: typeof is_using_input_method): void {
+    is_using_input_method = value;
+}
+
 export let search_pill_widget: SearchPillWidget | null = null;
 let search_input_has_changed = false;
 
@@ -390,7 +395,7 @@ function reset_searchbox(clear = false): void {
 }
 
 // Exported for tests
-export function exit_search(opts: {keep_search_narrow_open: boolean}): void {
+export let exit_search = (opts: {keep_search_narrow_open: boolean}): void => {
     const filter = narrow_state.filter();
     if (!filter || filter.is_common_narrow()) {
         // for common narrows, we change the UI (and don't redirect)
@@ -404,13 +409,23 @@ export function exit_search(opts: {keep_search_narrow_open: boolean}): void {
     }
     $("#search_query").trigger("blur");
     $(".app").trigger("focus");
+};
+
+export function rewire_exit_search(value: typeof exit_search): void {
+    exit_search = value;
 }
 
-export function open_search_bar_and_close_narrow_description(clear = false): void {
+export let open_search_bar_and_close_narrow_description = (clear = false): void => {
     reset_searchbox(clear);
     $(".navbar-search").addClass("expanded");
     $("#message_view_header").addClass("hidden");
     popovers.hide_all();
+};
+
+export function rewire_open_search_bar_and_close_narrow_description(
+    value: typeof open_search_bar_and_close_narrow_description,
+): void {
+    open_search_bar_and_close_narrow_description = value;
 }
 
 export function close_search_bar_and_open_narrow_description(): void {
diff --git a/web/src/settings_components.ts b/web/src/settings_components.ts
index e44a7b354a..6bfe911c7e 100644
--- a/web/src/settings_components.ts
+++ b/web/src/settings_components.ts
@@ -631,10 +631,10 @@ export function get_input_type($input_elem: JQuery, input_type?: string): string
     return input_type;
 }
 
-export function get_input_element_value(
+export let get_input_element_value = (
     input_elem: HTMLElement,
     input_type?: string,
-): boolean | number | string | null | undefined | GroupSettingValue {
+): boolean | number | string | null | undefined | GroupSettingValue => {
     const $input_elem = $(input_elem);
     input_type = get_input_type($input_elem, input_type);
     let input_value;
@@ -691,6 +691,10 @@ export function get_input_element_value(
         default:
             return undefined;
     }
+};
+
+export function rewire_get_input_element_value(value: typeof get_input_element_value): void {
+    get_input_element_value = value;
 }
 
 export function set_input_element_value(
diff --git a/web/src/settings_org.ts b/web/src/settings_org.ts
index afdab19218..54bdbc0c6d 100644
--- a/web/src/settings_org.ts
+++ b/web/src/settings_org.ts
@@ -899,7 +899,7 @@ export function set_up_dropdown_widget_for_realm_group_settings(): void {
     }
 }
 
-export function init_dropdown_widgets(): void {
+export let init_dropdown_widgets = (): void => {
     const notification_stream_options = (): dropdown_widget.Option[] => {
         const streams = stream_settings_data.get_streams_for_settings_page();
         const options: dropdown_widget.Option[] = streams.map((stream) => ({
@@ -956,6 +956,10 @@ export function init_dropdown_widgets(): void {
     );
 
     set_up_dropdown_widget_for_realm_group_settings();
+};
+
+export function rewire_init_dropdown_widgets(value: typeof init_dropdown_widgets): void {
+    init_dropdown_widgets = value;
 }
 
 export function register_save_discard_widget_handlers(
@@ -1055,7 +1059,7 @@ export function register_save_discard_widget_handlers(
 }
 
 // Exported for tests
-export function initialize_group_setting_widgets(): void {
+export let initialize_group_setting_widgets = (): void => {
     const realm_group_permission_settings = Object.entries(
         realm.server_supported_permission_settings.realm,
     );
@@ -1080,6 +1084,12 @@ export function initialize_group_setting_widgets(): void {
 
     enable_or_disable_group_permission_settings();
     check_disable_direct_message_initiator_group_widget();
+};
+
+export function rewire_initialize_group_setting_widgets(
+    value: typeof initialize_group_setting_widgets,
+): void {
+    initialize_group_setting_widgets = value;
 }
 
 export function build_page(): void {
diff --git a/web/src/stream_data.ts b/web/src/stream_data.ts
index 682ae78e0e..3539276f8b 100644
--- a/web/src/stream_data.ts
+++ b/web/src/stream_data.ts
@@ -217,7 +217,7 @@ export function get_stream_name_from_id(stream_id: number): string {
     return get_sub_by_id(stream_id)?.name ?? "";
 }
 
-export function get_sub_by_name(name: string): StreamSubscription | undefined {
+export let get_sub_by_name = (name: string): StreamSubscription | undefined => {
     // Note: Only use this function for situations where
     // you are comfortable with a user dealing with an
     // old name of a stream (from prior to a rename).
@@ -230,6 +230,10 @@ export function get_sub_by_name(name: string): StreamSubscription | undefined {
     }
 
     return sub_store.get(stream_id);
+};
+
+export function rewire_get_sub_by_name(value: typeof get_sub_by_name): void {
+    get_sub_by_name = value;
 }
 
 export function id_to_slug(stream_id: number): string {
@@ -449,7 +453,7 @@ export function canonicalized_name(stream_name: string): string {
     return stream_name.toString().toLowerCase();
 }
 
-export function get_color(stream_id: number | undefined): string {
+export let get_color = (stream_id: number | undefined): string => {
     if (stream_id === undefined) {
         return DEFAULT_COLOR;
     }
@@ -458,6 +462,10 @@ export function get_color(stream_id: number | undefined): string {
         return DEFAULT_COLOR;
     }
     return sub.color;
+};
+
+export function rewire_get_color(value: typeof get_color): void {
+    get_color = value;
 }
 
 export function is_muted(stream_id: number): boolean {
@@ -675,7 +683,7 @@ export function is_default_stream_id(stream_id: number): boolean {
     return default_stream_ids.has(stream_id);
 }
 
-export function is_user_subscribed(stream_id: number, user_id: number): boolean {
+export let is_user_subscribed = (stream_id: number, user_id: number): boolean => {
     const sub = sub_store.get(stream_id);
     if (sub === undefined || !can_view_subscribers(sub)) {
         // If we don't know about the stream, or we ourselves cannot access subscriber list,
@@ -691,6 +699,10 @@ export function is_user_subscribed(stream_id: number, user_id: number): boolean
     }
 
     return peer_data.is_user_subscribed(stream_id, user_id);
+};
+
+export function rewire_is_user_subscribed(value: typeof is_user_subscribed): void {
+    is_user_subscribed = value;
 }
 
 export function create_streams(streams: Stream[]): void {
diff --git a/web/src/stream_list.ts b/web/src/stream_list.ts
index e1fd2ac8a5..7a0035632d 100644
--- a/web/src/stream_list.ts
+++ b/web/src/stream_list.ts
@@ -41,6 +41,10 @@ let zoomed_in = false;
 
 export let stream_cursor: ListCursor<number>;
 
+export function rewire_stream_cursor(value: typeof stream_cursor): void {
+    stream_cursor = value;
+}
+
 let has_scrolled = false;
 
 export function is_zoomed_in(): boolean {
@@ -97,13 +101,13 @@ export function clear_topics(): void {
     zoomed_in = false;
 }
 
-export function update_count_in_dom(
+export let update_count_in_dom = (
     $stream_li: JQuery,
     stream_counts: StreamCountInfo,
     stream_has_any_unread_mention_messages: boolean,
     stream_has_any_unmuted_unread_mention: boolean,
     stream_has_only_muted_unread_mention: boolean,
-): void {
+): void => {
     // The subscription_block properly excludes the topic list,
     // and it also has sensitive margins related to whether the
     // count is there or not.
@@ -176,6 +180,10 @@ export function update_count_in_dom(
         $subscription_block.removeClass("has-only-muted-unreads");
         $subscription_block.removeClass("stream-with-count");
     }
+};
+
+export function rewire_update_count_in_dom(value: typeof update_count_in_dom): void {
+    update_count_in_dom = value;
 }
 
 class StreamSidebar {
@@ -567,7 +575,7 @@ function set_stream_unread_count(
     );
 }
 
-export function update_streams_sidebar(force_rerender = false): void {
+export let update_streams_sidebar = (force_rerender = false): void => {
     if (!force_rerender && is_zoomed_in()) {
         // We do our best to update topics that are displayed
         // in case user zoomed in. Streams list will be updated,
@@ -591,6 +599,10 @@ export function update_streams_sidebar(force_rerender = false): void {
     }
 
     update_stream_sidebar_for_narrow(filter);
+};
+
+export function rewire_update_streams_sidebar(value: typeof update_streams_sidebar): void {
+    update_streams_sidebar = value;
 }
 
 export function update_dom_with_unread_counts(counts: FullUnreadCountsData): void {
diff --git a/web/src/stream_list_sort.ts b/web/src/stream_list_sort.ts
index 2b3bad124a..36f9ed9618 100644
--- a/web/src/stream_list_sort.ts
+++ b/web/src/stream_list_sort.ts
@@ -58,7 +58,7 @@ export function is_filtering_inactives(): boolean {
     return filter_out_inactives;
 }
 
-export function has_recent_activity(sub: StreamSubscription): boolean {
+export let has_recent_activity = (sub: StreamSubscription): boolean => {
     if (!filter_out_inactives || sub.pin_to_top) {
         // If users don't want to filter inactive streams
         // to the bottom, we respect that setting and don't
@@ -70,6 +70,10 @@ export function has_recent_activity(sub: StreamSubscription): boolean {
         return true;
     }
     return stream_topic_history.stream_has_topics(sub.stream_id) || sub.newly_subscribed;
+};
+
+export function rewire_has_recent_activity(value: typeof has_recent_activity): void {
+    has_recent_activity = value;
 }
 
 export function has_recent_activity_but_muted(sub: StreamSubscription): boolean {
diff --git a/web/src/stream_topic_history.ts b/web/src/stream_topic_history.ts
index 436160cfc1..288bb8ebe0 100644
--- a/web/src/stream_topic_history.ts
+++ b/web/src/stream_topic_history.ts
@@ -356,10 +356,14 @@ export function has_history_for(stream_id: number): boolean {
     return fetched_stream_ids.has(stream_id);
 }
 
-export function get_recent_topic_names(stream_id: number): string[] {
+export let get_recent_topic_names = (stream_id: number): string[] => {
     const history = find_or_create(stream_id);
 
     return history.get_recent_topic_names();
+};
+
+export function rewire_get_recent_topic_names(value: typeof get_recent_topic_names): void {
+    get_recent_topic_names = value;
 }
 
 export function get_max_message_id(stream_id: number): number {
diff --git a/web/src/sub_store.ts b/web/src/sub_store.ts
index 9479f702a7..ffb6d51573 100644
--- a/web/src/sub_store.ts
+++ b/web/src/sub_store.ts
@@ -33,8 +33,11 @@ export type StreamSubscription = z.infer<typeof stream_subscription_schema>;
 
 const subs_by_stream_id = new Map<number, StreamSubscription>();
 
-export function get(stream_id: number): StreamSubscription | undefined {
-    return subs_by_stream_id.get(stream_id);
+export let get = (stream_id: number): StreamSubscription | undefined =>
+    subs_by_stream_id.get(stream_id);
+
+export function rewire_get(value: typeof get): void {
+    get = value;
 }
 
 export function validate_stream_ids(stream_ids: number[]): number[] {
diff --git a/web/src/timerender.ts b/web/src/timerender.ts
index 162a001ebd..177bbd619f 100644
--- a/web/src/timerender.ts
+++ b/web/src/timerender.ts
@@ -175,7 +175,7 @@ export type TimeRender = {
     needs_update: boolean;
 };
 
-export function render_now(time: Date, today = new Date(), display_year?: boolean): TimeRender {
+export let render_now = (time: Date, today = new Date(), display_year?: boolean): TimeRender => {
     let time_str = "";
     let needs_update = false;
     // render formal time to be used for tippy tooltip
@@ -210,6 +210,10 @@ export function render_now(time: Date, today = new Date(), display_year?: boolea
         formal_time_str,
         needs_update,
     };
+};
+
+export function rewire_render_now(value: typeof render_now): void {
+    render_now = value;
 }
 
 // Relative time rendering for use in most screens like Recent conversations.
diff --git a/web/src/typeahead_helper.ts b/web/src/typeahead_helper.ts
index ef3fa6647a..2a12e1bed7 100644
--- a/web/src/typeahead_helper.ts
+++ b/web/src/typeahead_helper.ts
@@ -94,7 +94,7 @@ type StreamData = {
     subscribed: boolean;
 };
 
-export function render_typeahead_item(args: {
+export let render_typeahead_item = (args: {
     primary?: string | undefined;
     is_person?: boolean;
     img_src?: string;
@@ -105,7 +105,7 @@ export function render_typeahead_item(args: {
     is_user_group?: boolean;
     stream?: StreamData;
     emoji_code?: string | undefined;
-}): string {
+}): string => {
     const has_image = args.img_src !== undefined;
     const has_status = args.status_emoji_info !== undefined;
     const has_secondary = args.secondary !== undefined;
@@ -119,9 +119,13 @@ export function render_typeahead_item(args: {
         has_secondary_html,
         has_pronouns,
     });
+};
+
+export function rewire_render_typeahead_item(value: typeof render_typeahead_item): void {
+    render_typeahead_item = value;
 }
 
-export function render_person(person: UserPillData | UserOrMentionPillData): string {
+export let render_person = (person: UserPillData | UserOrMentionPillData): string => {
     if (person.type === "broadcast") {
         return render_typeahead_item({
             primary: person.user.special_item_text,
@@ -155,14 +159,21 @@ export function render_person(person: UserPillData | UserOrMentionPillData): str
     };
 
     return render_typeahead_item(typeahead_arguments);
+};
+
+export function rewire_render_person(value: typeof render_person): void {
+    render_person = value;
 }
 
-export function render_user_group(user_group: {name: string; description: string}): string {
-    return render_typeahead_item({
+export let render_user_group = (user_group: {name: string; description: string}): string =>
+    render_typeahead_item({
         primary: user_groups.get_display_group_name(user_group.name),
         secondary: user_group.description,
         is_user_group: true,
     });
+
+export function rewire_render_user_group(value: typeof render_user_group): void {
+    render_user_group = value;
 }
 
 export function render_person_or_user_group(
@@ -175,14 +186,17 @@ export function render_person_or_user_group(
     return render_person(item);
 }
 
-export function render_stream(stream: StreamData): string {
-    return render_typeahead_item({
+export let render_stream = (stream: StreamData): string =>
+    render_typeahead_item({
         secondary_html: stream.rendered_description,
         stream,
     });
+
+export function rewire_render_stream(value: typeof render_stream): void {
+    render_stream = value;
 }
 
-export function render_emoji(item: EmojiSuggestion): string {
+export let render_emoji = (item: EmojiSuggestion): string => {
     const args = {
         is_emoji: true,
         primary: item.emoji_name.replaceAll("_", " "),
@@ -198,6 +212,10 @@ export function render_emoji(item: EmojiSuggestion): string {
         ...args,
         emoji_code: item.emoji_code,
     });
+};
+
+export function rewire_render_emoji(value: typeof render_emoji): void {
+    render_emoji = value;
 }
 
 export function sorter<T>(query: string, objs: T[], get_item: (x: T) => string): T[] {
@@ -415,7 +433,7 @@ export function sort_languages(matches: LanguageSuggestion[], query: string): La
     }));
 }
 
-export function sort_recipients<UserType extends UserOrMentionPillData | UserPillData>({
+export let sort_recipients = <UserType extends UserOrMentionPillData | UserPillData>({
     users,
     query,
     current_stream_id,
@@ -429,7 +447,7 @@ export function sort_recipients<UserType extends UserOrMentionPillData | UserPil
     current_topic?: string | undefined;
     groups?: UserGroupPillData[];
     max_num_items?: number | undefined;
-}): (UserType | UserGroupPillData)[] {
+}): (UserType | UserGroupPillData)[] => {
     function sort_relevance(items: UserType[]): UserType[] {
         return sort_people_for_relevance(items, current_stream_id, current_topic);
     }
@@ -556,6 +574,10 @@ export function sort_recipients<UserType extends UserOrMentionPillData | UserPil
     // FirstName, which we don't want to artificially prioritize over the
     // the lone active user whose name is FirstName LastName.
     return recipients.slice(0, max_num_items);
+};
+
+export function rewire_sort_recipients(value: typeof sort_recipients): void {
+    sort_recipients = value;
 }
 
 export function compare_setting_options(
@@ -618,7 +640,7 @@ export function compare_setting_options(
     return 1;
 }
 
-export function sort_group_setting_options({
+export let sort_group_setting_options = ({
     users,
     query,
     groups,
@@ -628,7 +650,7 @@ export function sort_group_setting_options({
     query: string;
     groups: UserGroupPillData[];
     target_group: UserGroup | undefined;
-}): (UserPillData | UserGroupPillData)[] {
+}): (UserPillData | UserGroupPillData)[] => {
     function sort_group_setting_items(
         objs: (UserPillData | UserGroupPillData)[],
     ): (UserPillData | UserGroupPillData)[] {
@@ -703,6 +725,10 @@ export function sort_group_setting_options({
     }
 
     return setting_options.slice(0, MAX_ITEMS);
+};
+
+export function rewire_sort_group_setting_options(value: typeof sort_group_setting_options): void {
+    sort_group_setting_options = value;
 }
 
 type SlashCommand = {
@@ -781,7 +807,7 @@ function compare_by_name(stream_a: StreamSubscription, stream_b: StreamSubscript
     return util.strcmp(stream_a.name, stream_b.name);
 }
 
-export function sort_streams(matches: StreamPillData[], query: string): StreamPillData[] {
+export let sort_streams = (matches: StreamPillData[], query: string): StreamPillData[] => {
     const name_results = typeahead.triage(query, matches, (x) => x.name, compare_by_activity);
     const desc_results = typeahead.triage(
         query,
@@ -791,11 +817,19 @@ export function sort_streams(matches: StreamPillData[], query: string): StreamPi
     );
 
     return [...name_results.matches, ...desc_results.matches, ...desc_results.rest];
+};
+
+export function rewire_sort_streams(value: typeof sort_streams): void {
+    sort_streams = value;
 }
 
-export function sort_streams_by_name(matches: StreamPillData[], query: string): StreamPillData[] {
+export let sort_streams_by_name = (matches: StreamPillData[], query: string): StreamPillData[] => {
     const results = typeahead.triage(query, matches, (x) => x.name, compare_by_name);
     return [...results.matches, ...results.rest];
+};
+
+export function rewire_sort_streams_by_name(value: typeof sort_streams_by_name): void {
+    sort_streams_by_name = value;
 }
 
 export function query_matches_person(
diff --git a/web/src/ui_util.ts b/web/src/ui_util.ts
index bb71cbddf5..78574b67f7 100644
--- a/web/src/ui_util.ts
+++ b/web/src/ui_util.ts
@@ -72,10 +72,15 @@ export function is_user_said_paragraph($element: JQuery): boolean {
     return remaining_text.trim() === ":";
 }
 
-export function get_collapsible_status_array($elements: JQuery): boolean[] {
-    return [...$elements].map(
+export let get_collapsible_status_array = ($elements: JQuery): boolean[] =>
+    [...$elements].map(
         (element) => $(element).is("blockquote") || is_user_said_paragraph($(element)),
     );
+
+export function rewire_get_collapsible_status_array(
+    value: typeof get_collapsible_status_array,
+): void {
+    get_collapsible_status_array = value;
 }
 
 export function potentially_collapse_quotes($element: JQuery): boolean {
diff --git a/web/src/upload.ts b/web/src/upload.ts
index 06f49cc2ea..95ab4bf2fa 100644
--- a/web/src/upload.ts
+++ b/web/src/upload.ts
@@ -125,7 +125,7 @@ export function edit_config(row: number): Config {
     };
 }
 
-export function hide_upload_banner(uppy: Uppy, config: Config, file_id: string): void {
+export let hide_upload_banner = (uppy: Uppy, config: Config, file_id: string): void => {
     config.upload_banner(file_id).remove();
     if (uppy.getFiles().length === 0) {
         if (config.mode === "compose") {
@@ -134,6 +134,10 @@ export function hide_upload_banner(uppy: Uppy, config: Config, file_id: string):
             config.send_button().prop("disabled", false);
         }
     }
+};
+
+export function rewire_hide_upload_banner(value: typeof hide_upload_banner): void {
+    hide_upload_banner = value;
 }
 
 function add_upload_banner(
@@ -172,7 +176,7 @@ export function show_error_message(
     }
 }
 
-export function upload_files(uppy: Uppy, config: Config, files: File[] | FileList): void {
+export let upload_files = (uppy: Uppy, config: Config, files: File[] | FileList): void => {
     if (files.length === 0) {
         return;
     }
@@ -230,6 +234,7 @@ export function upload_files(uppy: Uppy, config: Config, files: File[] | FileLis
             file_id,
             true,
         );
+        // eslint-disable-next-line no-loop-func
         config.upload_banner_cancel_button(file_id).one("click", () => {
             compose_ui.replace_syntax(get_translated_status(file), "", config.textarea());
             compose_ui.autosize_textarea(config.textarea());
@@ -238,10 +243,15 @@ export function upload_files(uppy: Uppy, config: Config, files: File[] | FileLis
             uppy.removeFile(file_id);
             hide_upload_banner(uppy, config, file_id);
         });
+        // eslint-disable-next-line no-loop-func
         config.upload_banner_hide_button(file_id).one("click", () => {
             hide_upload_banner(uppy, config, file_id);
         });
     }
+};
+
+export function rewire_upload_files(value: typeof upload_files): void {
+    upload_files = value;
 }
 
 export function setup_upload(config: Config): Uppy {
diff --git a/web/src/user_topics.ts b/web/src/user_topics.ts
index 4d7575599d..d1f74fdce4 100644
--- a/web/src/user_topics.ts
+++ b/web/src/user_topics.ts
@@ -116,7 +116,7 @@ export function get_user_topics_for_visibility_policy(visibility_policy: number)
     return topics;
 }
 
-export function set_user_topic_visibility_policy(
+export let set_user_topic_visibility_policy = (
     stream_id: number,
     topic: string,
     visibility_policy: number,
@@ -125,7 +125,7 @@ export function set_user_topic_visibility_policy(
     $status_element?: JQuery,
     success_cb?: () => void,
     error_cb?: () => void,
-): void {
+): void => {
     const data = {
         stream_id,
         topic,
@@ -203,6 +203,12 @@ export function set_user_topic_visibility_policy(
             }
         },
     });
+};
+
+export function rewire_set_user_topic_visibility_policy(
+    value: typeof set_user_topic_visibility_policy,
+): void {
+    set_user_topic_visibility_policy = value;
 }
 
 export function set_visibility_policy_for_element($elt: JQuery, visibility_policy: number): void {
diff --git a/web/src/util.ts b/web/src/util.ts
index eaacb89808..4daa69ede6 100644
--- a/web/src/util.ts
+++ b/web/src/util.ts
@@ -315,7 +315,7 @@ export function get_time_from_date_muted(date_muted: number | undefined): number
     return date_muted * 1000;
 }
 
-export function call_function_periodically(callback: () => void, delay: number): void {
+export let call_function_periodically = (callback: () => void, delay: number): void => {
     // We previously used setInterval for this purpose, but
     // empirically observed that after unsuspend, Chrome can end
     // up trying to "catch up" by doing dozens of these requests
@@ -337,6 +337,10 @@ export function call_function_periodically(callback: () => void, delay: number):
         // exception.
         callback();
     }, delay);
+};
+
+export function rewire_call_function_periodically(value: typeof call_function_periodically): void {
+    call_function_periodically = value;
 }
 
 export function get_string_diff(string1: string, string2: string): [number, number, number] {