diff --git a/web/e2e-tests/lib/common.ts b/web/e2e-tests/lib/common.ts
index 724755b5d4..378fb6b35f 100644
--- a/web/e2e-tests/lib/common.ts
+++ b/web/e2e-tests/lib/common.ts
@@ -66,19 +66,17 @@ export const window_size = {
 };
 
 export async function ensure_browser(): Promise<Browser> {
-    if (browser === null) {
-        browser = await puppeteer.launch({
-            args: [
-                `--window-size=${window_size.width},${window_size.height}`,
-                "--no-sandbox",
-                "--disable-setuid-sandbox",
-            ],
-            // TODO: Change defaultViewport to 1280x1024 when puppeteer fixes the window size issue with firefox.
-            // Here is link to the issue that is tracking the above problem https://github.com/puppeteer/puppeteer/issues/6442.
-            defaultViewport: null,
-            headless: true,
-        });
-    }
+    browser ??= await puppeteer.launch({
+        args: [
+            `--window-size=${window_size.width},${window_size.height}`,
+            "--no-sandbox",
+            "--disable-setuid-sandbox",
+        ],
+        // TODO: Change defaultViewport to 1280x1024 when puppeteer fixes the window size issue with firefox.
+        // Here is link to the issue that is tracking the above problem https://github.com/puppeteer/puppeteer/issues/6442.
+        defaultViewport: null,
+        headless: true,
+    });
     return browser;
 }
 
@@ -256,14 +254,11 @@ export async function get_internal_email_from_name(
 
 export async function log_in(
     page: Page,
-    credentials: {username: string; password: string} | null = null,
+    credentials: {username: string; password: string} = test_credentials.default_user,
 ): Promise<void> {
     console.log("Logging in");
     await page.goto(realm_url + "login/");
     assert.equal(realm_url + "login/", page.url());
-    if (credentials === null) {
-        credentials = test_credentials.default_user;
-    }
     // fill login form
     const params = {
         username: credentials.username,
diff --git a/web/src/browser_history.ts b/web/src/browser_history.ts
index e509346c7d..23d83420a2 100644
--- a/web/src/browser_history.ts
+++ b/web/src/browser_history.ts
@@ -175,13 +175,10 @@ export function current_scroll_offset(): number | undefined {
 
 export function update_current_history_state_data(
     new_data: StateData,
-    url: string | undefined = undefined,
+    url: string = window.location.href,
 ): void {
     // The optional url parameter is for those rare situations where
     // we want to adjust the URL without adding a new history entry.
-    if (url === undefined) {
-        url = window.location.href;
-    }
     const current_state = state_data_schema.nullable().parse(window.history.state);
     const current_state_data = {
         narrow_pointer: current_state?.narrow_pointer,
diff --git a/web/src/compose_recipient.ts b/web/src/compose_recipient.ts
index 0d31ec005d..993ccd7d0e 100644
--- a/web/src/compose_recipient.ts
+++ b/web/src/compose_recipient.ts
@@ -344,13 +344,7 @@ export function update_topic_inputbox_on_mandatory_topics_change(): void {
     update_topic_displayed_text(compose_state.topic());
 }
 
-export function update_topic_displayed_text(
-    topic_name: string | undefined,
-    has_topic_focus = false,
-): void {
-    if (topic_name === undefined) {
-        topic_name = "";
-    }
+export function update_topic_displayed_text(topic_name = "", has_topic_focus = false): void {
     compose_state.topic(topic_name);
 
     // When topics are mandatory, no additional adjustments are needed.
diff --git a/web/src/copy_messages.ts b/web/src/copy_messages.ts
index 07c1af3d6b..b00283fa5f 100644
--- a/web/src/copy_messages.ts
+++ b/web/src/copy_messages.ts
@@ -285,11 +285,9 @@ export function analyze_selection(selection: Selection): {
             // Skip any selection sections that don't intersect a message.
             continue;
         }
-        if (start_id === undefined) {
-            // start_id is the Zulip message ID of the first message
-            // touched by the selection.
-            start_id = start_data[0];
-        }
+        // start_id is the Zulip message ID of the first message
+        // touched by the selection.
+        start_id ??= start_data[0];
 
         $endc = $(range.endContainer);
         $initial_end_tr = get_end_tr_from_endc($endc);
diff --git a/web/src/custom_profile_fields_ui.ts b/web/src/custom_profile_fields_ui.ts
index 3526511aea..4215e3d7ce 100644
--- a/web/src/custom_profile_fields_ui.ts
+++ b/web/src/custom_profile_fields_ui.ts
@@ -37,14 +37,14 @@ export function append_custom_profile_fields(element_id: string, user_id: number
     ]);
 
     for (const field of all_custom_fields) {
-        let field_value = people.get_custom_profile_data(user_id, field.id);
+        const field_value = people.get_custom_profile_data(user_id, field.id) ?? {
+            value: "",
+            rendered_value: "",
+        };
         const editable_by_user = current_user.is_admin || field.editable_by_user;
         const is_select_field = field.type === all_field_types.SELECT.id;
         const field_choices = [];
 
-        if (field_value === undefined || field_value === null) {
-            field_value = {value: "", rendered_value: ""};
-        }
         if (is_select_field) {
             const field_choice_dict = settings_components.select_field_data_schema.parse(
                 JSON.parse(field.field_data),
diff --git a/web/src/drafts.ts b/web/src/drafts.ts
index f3ef5b115b..69142625d4 100644
--- a/web/src/drafts.ts
+++ b/web/src/drafts.ts
@@ -145,9 +145,7 @@ export const draft_model = (function () {
             // since we expect bugged drafts will have either been run
             // through this code or been deleted by the previous
             // behavior of deleting them after 30 days.
-            if (draft.topic === undefined) {
-                draft.topic = "";
-            }
+            draft.topic ??= "";
 
             valid_drafts[draft_id] = {
                 ...draft,
diff --git a/web/src/filter.ts b/web/src/filter.ts
index 34e87620bf..5ca8b4e1a2 100644
--- a/web/src/filter.ts
+++ b/web/src/filter.ts
@@ -1008,9 +1008,7 @@ export class Filter {
     }
 
     predicate(): (message: Message) => boolean {
-        if (this._predicate === undefined) {
-            this._predicate = this._build_predicate();
-        }
+        this._predicate ??= this._build_predicate();
         return this._predicate;
     }
 
@@ -1154,9 +1152,7 @@ export class Filter {
     }
 
     can_mark_messages_read(): boolean {
-        if (this._can_mark_messages_read === undefined) {
-            this._can_mark_messages_read = this.calc_can_mark_messages_read();
-        }
+        this._can_mark_messages_read ??= this.calc_can_mark_messages_read();
         return this._can_mark_messages_read;
     }
 
diff --git a/web/src/giphy.ts b/web/src/giphy.ts
index ab0965568a..4143c4322d 100644
--- a/web/src/giphy.ts
+++ b/web/src/giphy.ts
@@ -61,9 +61,7 @@ async function renderGIPHYGrid(targetEl: HTMLElement): Promise<{remove: () => vo
     const {renderGrid} = await import(/* webpackChunkName: "giphy-sdk" */ "@giphy/js-components");
     const {GiphyFetch} = await import(/* webpackChunkName: "giphy-sdk" */ "@giphy/js-fetch-api");
 
-    if (giphy_fetch === undefined) {
-        giphy_fetch = new GiphyFetch(realm.giphy_api_key);
-    }
+    giphy_fetch ??= new GiphyFetch(realm.giphy_api_key);
 
     async function fetchGifs(offset: number): Promise<GifsResult> {
         assert(giphy_fetch !== undefined);
diff --git a/web/src/information_density.ts b/web/src/information_density.ts
index 2ca0879cc5..54a660f575 100644
--- a/web/src/information_density.ts
+++ b/web/src/information_density.ts
@@ -291,12 +291,8 @@ export function update_information_density_settings(
     $elem: JQuery,
     changed_property: "web_font_size_px" | "web_line_height_percent",
     for_settings_ui = false,
-    new_value?: number,
+    new_value: number = get_new_value_for_information_density_settings($elem, changed_property),
 ): number {
-    if (new_value === undefined) {
-        new_value = get_new_value_for_information_density_settings($elem, changed_property);
-    }
-
     user_settings[changed_property] = new_value;
     $elem.closest(".button-group").find(".current-value").val(new_value);
     if (for_settings_ui) {
diff --git a/web/src/invite.ts b/web/src/invite.ts
index 1a5f1bf2f8..81ca4e0edc 100644
--- a/web/src/invite.ts
+++ b/web/src/invite.ts
@@ -432,12 +432,11 @@ function open_invite_user_modal(e: JQuery.ClickEvent<Document, undefined>): void
             $(e.target).parent().remove();
         });
 
-        function toggle_invite_submit_button(selected_tab?: string): void {
-            if (selected_tab === undefined) {
-                selected_tab = $(".invite_users_option_tabs")
-                    .find(".selected")
-                    .attr("data-tab-key");
-            }
+        function toggle_invite_submit_button(
+            selected_tab: string | undefined = $(".invite_users_option_tabs")
+                .find(".selected")
+                .attr("data-tab-key"),
+        ): void {
             const valid_custom_time = util.validate_custom_time_input(
                 custom_expiration_time_input,
                 false,
diff --git a/web/src/lightbox.ts b/web/src/lightbox.ts
index 780ac20283..d444ee6e60 100644
--- a/web/src/lightbox.ts
+++ b/web/src/lightbox.ts
@@ -373,17 +373,13 @@ function display_video(payload: Media): void {
 }
 
 export function build_open_media_function(
-    on_close: (() => void) | undefined,
+    on_close = (): void => {
+        remove_video_players();
+        is_open = false;
+        assert(document.activeElement instanceof HTMLElement);
+        document.activeElement.blur();
+    },
 ): ($media: JQuery<HTMLMediaElement | HTMLImageElement>) => void {
-    if (on_close === undefined) {
-        on_close = function () {
-            remove_video_players();
-            is_open = false;
-            assert(document.activeElement instanceof HTMLElement);
-            document.activeElement.blur();
-        };
-    }
-
     return function ($media: JQuery<HTMLMediaElement | HTMLImageElement>): void {
         // This is used both for clicking on media in the messagelist, as well as clicking on images
         // in the media list under the lightbox when it is open.
diff --git a/web/src/message_events.ts b/web/src/message_events.ts
index c241841945..68124df279 100644
--- a/web/src/message_events.ts
+++ b/web/src/message_events.ts
@@ -449,12 +449,9 @@ export function update_messages(events: UpdateMessageEvent[]): void {
                     // Add message's edit_history in message dict
                     // For messages that are edited, edit_history needs to
                     // be added to message in frontend.
-                    if (anchor_message.edit_history === undefined) {
-                        anchor_message.edit_history = [];
-                    }
                     anchor_message.edit_history = [
                         edit_history_entry,
-                        ...anchor_message.edit_history,
+                        ...(anchor_message.edit_history ?? []),
                     ];
                 }
                 any_message_content_edited = true;
@@ -587,12 +584,9 @@ export function update_messages(events: UpdateMessageEvent[]): void {
                         edit_history_entry.topic = new_topic;
                         edit_history_entry.prev_topic = orig_topic;
                     }
-                    if (moved_message.edit_history === undefined) {
-                        moved_message.edit_history = [];
-                    }
                     moved_message.edit_history = [
                         edit_history_entry,
-                        ...moved_message.edit_history,
+                        ...(moved_message.edit_history ?? []),
                     ];
                 }
 
diff --git a/web/src/message_helper.ts b/web/src/message_helper.ts
index 744c6b25c3..2972ca095d 100644
--- a/web/src/message_helper.ts
+++ b/web/src/message_helper.ts
@@ -56,10 +56,7 @@ export function process_new_message(raw_message: RawMessage, deliver_locally = f
 
     let message: Message;
     if (message_with_booleans.type === "stream") {
-        let topic = message_with_booleans.topic;
-        if (topic === undefined) {
-            topic = message_with_booleans.subject;
-        }
+        const topic = message_with_booleans.topic ?? message_with_booleans.subject;
         assert(topic !== undefined);
 
         // We add fully delivered messages to stream_topic_history,
diff --git a/web/src/message_list_data.ts b/web/src/message_list_data.ts
index de4f8fcb05..9bab2cd47c 100644
--- a/web/src/message_list_data.ts
+++ b/web/src/message_list_data.ts
@@ -206,9 +206,7 @@ export class MessageListData {
     }
     _get_predicate(): (message: Message) => boolean {
         // We cache this.
-        if (!this.predicate) {
-            this.predicate = this.filter.predicate();
-        }
+        this.predicate ??= this.filter.predicate();
         return this.predicate;
     }
 
diff --git a/web/src/message_view.ts b/web/src/message_view.ts
index 2c5e61d5eb..28d173a5e8 100644
--- a/web/src/message_view.ts
+++ b/web/src/message_view.ts
@@ -1202,10 +1202,7 @@ export function render_message_list_with_selected_message(opts: {
     const id_info = opts.id_info;
     const select_offset = opts.select_offset;
 
-    let msg_id = id_info.final_select_id;
-    if (msg_id === undefined) {
-        msg_id = message_lists.current.first_unread_message_id();
-    }
+    const msg_id = id_info.final_select_id ?? message_lists.current.first_unread_message_id();
     // There should be something since it's not visibly empty.
     assert(msg_id !== undefined);
 
diff --git a/web/src/people.ts b/web/src/people.ts
index 03e75e6bb3..ed41d15e61 100644
--- a/web/src/people.ts
+++ b/web/src/people.ts
@@ -1237,9 +1237,7 @@ export function build_termlet_matcher(termlet: string): (user: User) => boolean
         let full_name = user.full_name;
         // Only ignore diacritics if the query is plain ascii
         if (is_ascii) {
-            if (user.name_with_diacritics_removed === undefined) {
-                user.name_with_diacritics_removed = typeahead.remove_diacritics(full_name);
-            }
+            user.name_with_diacritics_removed ??= typeahead.remove_diacritics(full_name);
             full_name = user.name_with_diacritics_removed;
         }
         const names = full_name.toLowerCase().split(" ");
@@ -1576,11 +1574,7 @@ export function get_user_by_id_assert_valid(
         return get_by_user_id(user_id);
     }
 
-    let person = maybe_get_user_by_id(user_id, true);
-    if (person === undefined) {
-        person = add_inaccessible_user(user_id);
-    }
-    return person;
+    return maybe_get_user_by_id(user_id, true) ?? add_inaccessible_user(user_id);
 }
 
 function get_involved_people(message: MessageWithBooleans): DisplayRecipientUser[] {
diff --git a/web/src/portico/showroom.ts b/web/src/portico/showroom.ts
index e8ab5f0ba1..d4fda02e9c 100644
--- a/web/src/portico/showroom.ts
+++ b/web/src/portico/showroom.ts
@@ -1,5 +1,6 @@
 import Handlebars from "handlebars/runtime.js";
 import $ from "jquery";
+import assert from "minimalistic-assert";
 
 import render_banner from "../../templates/components/banner.hbs";
 import {$t, $t_html} from "../i18n.ts";
@@ -404,7 +405,8 @@ $(window).on("load", () => {
                 return;
             }
             let label = $<HTMLInputElement>("input#primary_button_text").val();
-            if (!label) {
+            assert(label !== undefined);
+            if (label === "") {
                 label = "Primary Button";
             }
             const is_icon_enabled = $("#enable_primary_button_icon").prop("checked") === true;
@@ -498,7 +500,8 @@ $(window).on("load", () => {
                 return;
             }
             let label = $<HTMLInputElement>("input#quiet_button_text").val();
-            if (!label) {
+            assert(label !== undefined);
+            if (label === "") {
                 label = "Quiet Button";
             }
             const is_icon_enabled = $("#enable_quiet_button_icon").prop("checked") === true;
@@ -587,7 +590,8 @@ $(window).on("load", () => {
                 return;
             }
             let label = $<HTMLInputElement>("input#borderless_button_text").val();
-            if (!label) {
+            assert(label !== undefined);
+            if (label === "") {
                 label = "Borderless Button";
             }
             const is_icon_enabled = $("#enable_borderless_button_icon").prop("checked") === true;
diff --git a/web/src/postprocess_content.ts b/web/src/postprocess_content.ts
index 1ad0d7fa35..abdc242bd5 100644
--- a/web/src/postprocess_content.ts
+++ b/web/src/postprocess_content.ts
@@ -8,9 +8,7 @@ import * as util from "./util.ts";
 let inertDocument: Document | undefined;
 
 export function postprocess_content(html: string): string {
-    if (inertDocument === undefined) {
-        inertDocument = new DOMParser().parseFromString("", "text/html");
-    }
+    inertDocument ??= new DOMParser().parseFromString("", "text/html");
     const template = inertDocument.createElement("template");
     template.innerHTML = html;
 
diff --git a/web/src/recent_senders.ts b/web/src/recent_senders.ts
index 361a4cfb7d..f557e8b0c6 100644
--- a/web/src/recent_senders.ts
+++ b/web/src/recent_senders.ts
@@ -31,9 +31,7 @@ export class IdTracker {
     }
 
     max_id(): number {
-        if (this._cached_max_id === undefined) {
-            this._cached_max_id = _.max([...this.ids]);
-        }
+        this._cached_max_id ??= _.max([...this.ids]);
         return this._cached_max_id ?? -1;
     }
 
diff --git a/web/src/settings_notifications.ts b/web/src/settings_notifications.ts
index 40074322ad..d6b9c529ad 100644
--- a/web/src/settings_notifications.ts
+++ b/web/src/settings_notifications.ts
@@ -160,10 +160,7 @@ function stream_notification_setting_changed(target: HTMLInputElement): void {
 
     const $status_element = $(target).closest(".subsection-parent").find(".alert-notification");
     const setting = stream_specific_notification_settings_schema.keyof().parse(target.name);
-    if (sub[setting] === null) {
-        sub[setting] =
-            user_settings[settings_config.generalize_stream_notification_setting[setting]];
-    }
+    sub[setting] ??= user_settings[settings_config.generalize_stream_notification_setting[setting]];
     stream_settings_api.set_stream_property(
         sub,
         {property: setting, value: target.checked},
diff --git a/web/src/stream_data.ts b/web/src/stream_data.ts
index bcef05e319..77b2eecd2d 100644
--- a/web/src/stream_data.ts
+++ b/web/src/stream_data.ts
@@ -209,11 +209,7 @@ export function get_stream_id(name: string): number | 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).
-    let stream_id = stream_ids_by_name.get(name);
-    if (!stream_id) {
-        stream_id = stream_ids_by_old_names.get(name);
-    }
-    return stream_id;
+    return stream_ids_by_name.get(name) ?? stream_ids_by_old_names.get(name);
 }
 
 export function get_stream_name_from_id(stream_id: number): string {
@@ -224,10 +220,7 @@ 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).
-    let stream_id = stream_ids_by_name.get(name);
-    if (!stream_id) {
-        stream_id = stream_ids_by_old_names.get(name);
-    }
+    const stream_id = stream_ids_by_name.get(name) ?? stream_ids_by_old_names.get(name);
     if (!stream_id) {
         return undefined;
     }
diff --git a/web/src/submessage.ts b/web/src/submessage.ts
index a63212f06d..63c65af1f4 100644
--- a/web/src/submessage.ts
+++ b/web/src/submessage.ts
@@ -165,9 +165,7 @@ export function update_message(submsg: Submessage): void {
         return;
     }
 
-    if (message.submessages === undefined) {
-        message.submessages = [];
-    }
+    message.submessages ??= [];
 
     const existing = message.submessages.find((sm) => sm.id === submsg.id);