diff --git a/frontend_tests/node_tests/stream_data.js b/frontend_tests/node_tests/stream_data.js
index 00621705e8..695dc4eee1 100644
--- a/frontend_tests/node_tests/stream_data.js
+++ b/frontend_tests/node_tests/stream_data.js
@@ -923,10 +923,10 @@ run_test("initialize", () => {
     initialize();
     assert(!stream_data.is_filtering_inactives());
 
-    const stream_names = stream_data.get_streams_for_admin().map((elem) => elem.name);
-    assert(stream_names.includes("subscriptions"));
-    assert(stream_names.includes("unsubscribed"));
-    assert(stream_names.includes("never_subscribed"));
+    const stream_names = new Set(stream_data.get_streams_for_admin().map((elem) => elem.name));
+    assert(stream_names.has("subscriptions"));
+    assert(stream_names.has("unsubscribed"));
+    assert(stream_names.has("never_subscribed"));
     assert.equal(stream_data.get_notifications_stream(), "");
 
     // Simulate a private stream the user isn't subscribed to
diff --git a/static/js/hotkey.js b/static/js/hotkey.js
index 4bbd03b948..50e500c02c 100644
--- a/static/js/hotkey.js
+++ b/static/js/hotkey.js
@@ -8,7 +8,7 @@ function do_narrow_action(action) {
 }
 
 // For message actions and user profile menu.
-const menu_dropdown_hotkeys = ["down_arrow", "up_arrow", "vim_up", "vim_down", "enter"];
+const menu_dropdown_hotkeys = new Set(["down_arrow", "up_arrow", "vim_up", "vim_down", "enter"]);
 
 // Note that multiple keys can map to the same event_name, which
 // we'll do in cases where they have the exact same semantics.
@@ -573,7 +573,7 @@ exports.process_hotkey = function (e, hotkey) {
         }
     }
 
-    if (menu_dropdown_hotkeys.includes(event_name)) {
+    if (menu_dropdown_hotkeys.has(event_name)) {
         if (popovers.actions_popped()) {
             popovers.actions_menu_handle_keyboard(event_name);
             return true;
diff --git a/static/js/message_fetch.js b/static/js/message_fetch.js
index c69536ea74..480cc0f048 100644
--- a/static/js/message_fetch.js
+++ b/static/js/message_fetch.js
@@ -130,8 +130,8 @@ function get_messages_success(data, opts) {
 // or convert the emails string to user IDs directly into the Filter code
 // because doing so breaks the app in various modules that expect emails string.
 function handle_operators_supporting_id_based_api(data) {
-    const operators_supporting_ids = ["pm-with"];
-    const operators_supporting_id = ["sender", "group-pm-with", "stream"];
+    const operators_supporting_ids = new Set(["pm-with"]);
+    const operators_supporting_id = new Set(["sender", "group-pm-with", "stream"]);
 
     if (data.narrow === undefined) {
         return data;
@@ -139,11 +139,11 @@ function handle_operators_supporting_id_based_api(data) {
 
     data.narrow = JSON.parse(data.narrow);
     data.narrow = data.narrow.map((filter) => {
-        if (operators_supporting_ids.includes(filter.operator)) {
+        if (operators_supporting_ids.has(filter.operator)) {
             filter.operand = people.emails_strings_to_user_ids_array(filter.operand);
         }
 
-        if (operators_supporting_id.includes(filter.operator)) {
+        if (operators_supporting_id.has(filter.operator)) {
             if (filter.operator === "stream") {
                 const stream_id = stream_data.get_stream_id(filter.operand);
                 if (stream_id !== undefined) {
diff --git a/static/js/portico/tabbed-instructions.js b/static/js/portico/tabbed-instructions.js
index f33c50abfa..de6a003dc4 100644
--- a/static/js/portico/tabbed-instructions.js
+++ b/static/js/portico/tabbed-instructions.js
@@ -19,7 +19,7 @@ export function detect_user_os() {
 
 export function activate_correct_tab($codeSection) {
     const user_os = detect_user_os();
-    const desktop_os = ["mac", "linux", "windows"];
+    const desktop_os = new Set(["mac", "linux", "windows"]);
     const $li = $codeSection.find("ul.nav li");
     const $blocks = $codeSection.find(".blocks div");
 
@@ -30,7 +30,7 @@ export function activate_correct_tab($codeSection) {
             $(this).addClass("active");
         }
 
-        if (desktop_os.includes(user_os) && language === "desktop-web") {
+        if (desktop_os.has(user_os) && language === "desktop-web") {
             $(this).addClass("active");
         }
     });
@@ -42,7 +42,7 @@ export function activate_correct_tab($codeSection) {
             $(this).addClass("active");
         }
 
-        if (desktop_os.includes(user_os) && language === "desktop-web") {
+        if (desktop_os.has(user_os) && language === "desktop-web") {
             $(this).addClass("active");
         }
     });
diff --git a/static/js/rtl.js b/static/js/rtl.js
index 0da1a13a75..4ce5609724 100644
--- a/static/js/rtl.js
+++ b/static/js/rtl.js
@@ -36,9 +36,9 @@ function convert_from_raw(digits, part_length, raw) {
 }
 
 /** Isolate initiator characters. */
-const i_chars = [0x2066, 0x2067, 0x2068];
+const i_chars = new Set([0x2066, 0x2067, 0x2068]);
 /** Pop directional isolate character. */
-const pdi_chars = [0x2069];
+const pdi_chars = new Set([0x2069]);
 /** The digits that are used for base conversions from base 92. */
 const digits =
     '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!"#$%&()*+,-./:;<=>?@[]^_`{|}~';
@@ -86,10 +86,10 @@ const lr_ranges = convert_from_raw(
  * @returns {'I' | 'PDI' | 'R' | 'L' | 'Other'}
  */
 function get_bidi_class(ch) {
-    if (i_chars.includes(ch)) {
+    if (i_chars.has(ch)) {
         return "I"; // LRI, RLI, FSI
     }
-    if (pdi_chars.includes(ch)) {
+    if (pdi_chars.has(ch)) {
         return "PDI";
     }
     let i = util.lower_bound(rl_ranges, ch);