diff --git a/tools/test-js-with-node b/tools/test-js-with-node
index c6a007c921..a21cb4bb48 100755
--- a/tools/test-js-with-node
+++ b/tools/test-js-with-node
@@ -144,7 +144,7 @@ EXEMPT_FILES = make_set(
         "web/src/message_scroll_state.ts",
         "web/src/message_util.ts",
         "web/src/message_view_header.js",
-        "web/src/message_viewport.js",
+        "web/src/message_viewport.ts",
         "web/src/messages_overlay_ui.ts",
         "web/src/modals.ts",
         "web/src/muted_users_ui.js",
diff --git a/web/src/message_viewport.js b/web/src/message_viewport.ts
similarity index 77%
rename from web/src/message_viewport.js
rename to web/src/message_viewport.ts
index 787ec88e57..beec3ba069 100644
--- a/web/src/message_viewport.js
+++ b/web/src/message_viewport.ts
@@ -1,30 +1,54 @@
 import $ from "jquery";
+import assert from "minimalistic-assert";
 
 import * as blueslip from "./blueslip";
 import * as message_lists from "./message_lists";
 import * as message_scroll_state from "./message_scroll_state";
+import type {Message} from "./message_store";
 import * as rows from "./rows";
 import * as util from "./util";
+import type {CachedValue} from "./util";
+
+type MessageViewportInfo = {
+    visible_top: number;
+    visible_bottom: number;
+    visible_height: number;
+};
+
+type DimenFunc = JQuery["height"] | JQuery["width"];
+type DimenWrapper = {
+    (): number;
+    (val: string | number): JQuery;
+};
 
 export const $scroll_container = $("html");
 
-let $jwindow;
-const dimensions = {};
+let $jwindow: JQuery<Window & typeof globalThis>;
+const dimensions: Record<string, CachedValue<number>> = {};
 let in_stoppable_autoscroll = false;
 
-function make_dimen_wrapper(dimen_name, dimen_func) {
+function make_dimen_wrapper(dimen_name: string, dimen_func: DimenFunc): DimenWrapper {
     dimensions[dimen_name] = new util.CachedValue({
         compute_value() {
-            return dimen_func();
+            return dimen_func() ?? 0;
         },
     });
-    return function viewport_dimension_wrapper(val) {
+
+    // Inner function overload for the case when dimen_func is () => number | undefined
+    function viewport_dimension_wrapper(): number;
+
+    // Inner function overload for the case when dimen_func is (val: string | number) => JQuery
+    function viewport_dimension_wrapper(val: string | number): JQuery;
+
+    function viewport_dimension_wrapper(val?: string | number): number | JQuery {
         if (val !== undefined) {
             dimensions[dimen_name].reset();
             return dimen_func(val);
         }
         return dimensions[dimen_name].get();
-    };
+    }
+
+    return viewport_dimension_wrapper;
 }
 
 export const height = make_dimen_wrapper("height", $.fn.height.bind($scroll_container));
@@ -33,23 +57,23 @@ export const width = make_dimen_wrapper("width", $.fn.width.bind($scroll_contain
 // TODO: This function lets us use the DOM API instead of jquery
 // (<10x faster) for condense.js, but we want to eventually do a
 // bigger of refactor `height` and `width` above to do the same.
-export function max_message_height() {
-    return document.querySelector("html").offsetHeight * 0.65;
+export function max_message_height(): number {
+    return document.querySelector("html")!.offsetHeight * 0.65;
 }
 
 // Includes both scroll and arrow events. Negative means scroll up,
 // positive means scroll down.
 export let last_movement_direction = 1;
 
-export function set_last_movement_direction(value) {
+export function set_last_movement_direction(value: number): void {
     last_movement_direction = value;
 }
 
-export function at_top() {
+export function at_top(): boolean {
     return scrollTop() <= 0;
 }
 
-export function message_viewport_info() {
+export function message_viewport_info(): MessageViewportInfo {
     // Return a structure that tells us details of the viewport
     // accounting for fixed elements like the top navbar.
     //
@@ -78,7 +102,7 @@ export function message_viewport_info() {
     };
 }
 
-export function at_bottom() {
+export function at_bottom(): boolean {
     const bottom = scrollTop() + height();
     const full_height = $scroll_container.prop("scrollHeight");
 
@@ -91,7 +115,7 @@ export function at_bottom() {
 
 // This differs from at_bottom in that it only requires the bottom message to
 // be visible, but you may be able to scroll down further.
-export function bottom_message_visible() {
+export function bottom_message_visible(): boolean {
     const $last_row = rows.last_visible();
     if ($last_row.length) {
         const message_bottom = $last_row[0].getBoundingClientRect().bottom;
@@ -101,11 +125,11 @@ export function bottom_message_visible() {
     return false;
 }
 
-export function is_below_visible_bottom(offset) {
-    return offset > scrollTop() + height() - $("#compose").height();
+export function is_below_visible_bottom(offset: number): boolean {
+    return offset > scrollTop() + height() - ($("#compose").height() ?? 0);
 }
 
-export function is_scrolled_up() {
+export function is_scrolled_up(): boolean {
     // Let's determine whether the user was already dealing
     // with messages off the screen, which can guide auto
     // scrolling decisions.
@@ -119,7 +143,7 @@ export function is_scrolled_up() {
     return offset > 0;
 }
 
-export function offset_from_bottom($last_row) {
+export function offset_from_bottom($last_row: JQuery): number {
     // A positive return value here means the last row is
     // below the bottom of the feed (i.e. obscured by the compose
     // box or even further below the bottom).
@@ -129,7 +153,12 @@ export function offset_from_bottom($last_row) {
     return message_bottom - info.visible_bottom;
 }
 
-export function set_message_position(message_top, message_height, viewport_info, ratio) {
+export function set_message_position(
+    message_top: number,
+    message_height: number,
+    viewport_info: MessageViewportInfo,
+    ratio: number,
+): void {
     // message_top = offset of the top of a message that you are positioning
     // message_height = height of the message that you are positioning
     // viewport_info = result of calling message_viewport.message_viewport_info
@@ -160,7 +189,12 @@ export function set_message_position(message_top, message_height, viewport_info,
     scrollTop(new_scroll_top);
 }
 
-function in_viewport_or_tall(rect, top_of_feed, bottom_of_feed, require_fully_visible) {
+function in_viewport_or_tall(
+    rect: DOMRect,
+    top_of_feed: number,
+    bottom_of_feed: number,
+    require_fully_visible: boolean,
+): boolean {
     if (require_fully_visible) {
         return (
             rect.top > top_of_feed && // Message top is in view and
@@ -171,14 +205,14 @@ function in_viewport_or_tall(rect, top_of_feed, bottom_of_feed, require_fully_vi
     return rect.bottom > top_of_feed && rect.top < bottom_of_feed;
 }
 
-function add_to_visible(
-    $candidates,
-    visible,
-    top_of_feed,
-    bottom_of_feed,
-    require_fully_visible,
-    row_to_id,
-) {
+function add_to_visible<T>(
+    $candidates: JQuery,
+    visible: T[],
+    top_of_feed: number,
+    bottom_of_feed: number,
+    require_fully_visible: boolean,
+    row_to_id: ($row: HTMLElement) => T,
+): void {
     for (const row of $candidates) {
         const row_rect = row.getBoundingClientRect();
         // Mark very tall messages as read once we've gotten past them
@@ -209,13 +243,13 @@ const bottom_of_feed = new util.CachedValue({
     },
 });
 
-function _visible_divs(
-    $selected_row,
-    row_min_height,
-    row_to_output,
-    div_class,
-    require_fully_visible,
-) {
+function _visible_divs<T>(
+    $selected_row: JQuery,
+    row_min_height: number,
+    row_to_output: ($row: HTMLElement) => T,
+    div_class: string,
+    require_fully_visible: boolean,
+): T[] {
     // Note that when using getBoundingClientRect() we are getting offsets
     // relative to the visible window, but when using jQuery's offset() we are
     // getting offsets relative to the full scrollable window. You can't try to
@@ -225,7 +259,7 @@ function _visible_divs(
 
     // We do this explicitly without merges and without recalculating
     // the feed bounds to keep this computation as cheap as possible.
-    const visible = [];
+    const visible: T[] = [];
     const $above_pointer = $selected_row
         .prevAll(`div.${CSS.escape(div_class)}`)
         .slice(0, num_neighbors);
@@ -260,7 +294,8 @@ function _visible_divs(
     return visible;
 }
 
-export function visible_groups(require_fully_visible) {
+export function visible_groups(require_fully_visible: boolean): HTMLElement[] {
+    assert(message_lists.current !== undefined);
     const $selected_row = message_lists.current.selected_row();
     if ($selected_row === undefined || $selected_row.length === 0) {
         return [];
@@ -268,29 +303,45 @@ export function visible_groups(require_fully_visible) {
 
     const $selected_group = rows.get_message_recipient_row($selected_row);
 
-    function get_row(row) {
+    function get_row(row: HTMLElement): HTMLElement {
         return row;
     }
 
     // Being simplistic about this, the smallest group is about 75 px high.
-    return _visible_divs($selected_group, 75, get_row, "recipient_row", require_fully_visible);
+    return _visible_divs<HTMLElement>(
+        $selected_group,
+        75,
+        get_row,
+        "recipient_row",
+        require_fully_visible,
+    );
 }
 
-export function visible_messages(require_fully_visible) {
+export function visible_messages(require_fully_visible: boolean): Message[] {
+    assert(message_lists.current !== undefined);
     const $selected_row = message_lists.current.selected_row();
 
-    function row_to_id(row) {
-        return message_lists.current.get(rows.id($(row)));
+    function row_to_id(row: HTMLElement): Message {
+        assert(message_lists.current !== undefined);
+        return message_lists.current.get(rows.id($(row))!)!;
     }
 
     // Being simplistic about this, the smallest message is 25 px high.
-    return _visible_divs($selected_row, 25, row_to_id, "message_row", require_fully_visible);
+    return _visible_divs<Message>(
+        $selected_row,
+        25,
+        row_to_id,
+        "message_row",
+        require_fully_visible,
+    );
 }
 
-export function scrollTop(target_scrollTop) {
+export function scrollTop(): number;
+export function scrollTop(target_scrollTop: number): JQuery;
+export function scrollTop(target_scrollTop?: number): JQuery | number {
     const orig_scrollTop = $scroll_container.scrollTop();
     if (target_scrollTop === undefined) {
-        return orig_scrollTop;
+        return orig_scrollTop ?? 0;
     }
     let $ret = $scroll_container.scrollTop(target_scrollTop);
     const new_scrollTop = $scroll_container.scrollTop();
@@ -327,13 +378,13 @@ export function scrollTop(target_scrollTop) {
     return $ret;
 }
 
-export function stop_auto_scrolling() {
+export function stop_auto_scrolling(): void {
     if (in_stoppable_autoscroll) {
         $scroll_container.stop();
     }
 }
 
-export function system_initiated_animate_scroll(scroll_amount) {
+export function system_initiated_animate_scroll(scroll_amount: number): void {
     message_scroll_state.set_update_selection_on_next_scroll(false);
     const viewport_offset = scrollTop();
     in_stoppable_autoscroll = true;
@@ -345,7 +396,7 @@ export function system_initiated_animate_scroll(scroll_amount) {
     });
 }
 
-export function user_initiated_animate_scroll(scroll_amount) {
+export function user_initiated_animate_scroll(scroll_amount: number): void {
     message_scroll_state.set_update_selection_on_next_scroll(false);
     in_stoppable_autoscroll = false; // defensive
 
@@ -356,7 +407,10 @@ export function user_initiated_animate_scroll(scroll_amount) {
     });
 }
 
-export function recenter_view($message, {from_scroll = false, force_center = false} = {}) {
+export function recenter_view(
+    $message: JQuery,
+    {from_scroll = false, force_center = false} = {},
+): void {
     // BarnOwl-style recentering: if the pointer is too high, move it to
     // the 1/2 marks. If the pointer is too low, move it to the 1/7 mark.
     // See keep_pointer_in_view() for related logic to keep the pointer onscreen.
@@ -394,7 +448,8 @@ export function recenter_view($message, {from_scroll = false, force_center = fal
     }
 }
 
-export function maybe_scroll_to_show_message_top() {
+export function maybe_scroll_to_show_message_top(): void {
+    assert(message_lists.current !== undefined);
     // Sets the top of the message to the top of the viewport.
     // Only applies if the top of the message is out of view above the visible area.
     const $selected_message = message_lists.current.selected_row();
@@ -406,13 +461,14 @@ export function maybe_scroll_to_show_message_top() {
     }
 }
 
-export function is_message_below_viewport($message_row) {
+export function is_message_below_viewport($message_row: JQuery): boolean {
     const info = message_viewport_info();
     const offset = $message_row.get_offset_to_window();
     return offset.top >= info.visible_bottom;
 }
 
-export function keep_pointer_in_view() {
+export function keep_pointer_in_view(): void {
+    assert(message_lists.current !== undefined);
     // See message_viewport.recenter_view() for related logic to keep the pointer onscreen.
     // This function mostly comes into place for mouse scrollers, and it
     // keeps the pointer in view.  For people who purely scroll with the
@@ -430,7 +486,7 @@ export function keep_pointer_in_view() {
     const top_threshold = info.visible_top + (1 / 10) * info.visible_height;
     const bottom_threshold = info.visible_top + (9 / 10) * info.visible_height;
 
-    function message_is_far_enough_down() {
+    function message_is_far_enough_down(): boolean {
         if (at_top()) {
             return true;
         }
@@ -455,11 +511,14 @@ export function keep_pointer_in_view() {
         return false;
     }
 
-    function message_is_far_enough_up() {
+    function message_is_far_enough_up(): boolean {
         return at_bottom() || $next_row.get_offset_to_window().top <= bottom_threshold;
     }
 
-    function adjust(in_view, get_next_row) {
+    function adjust(
+        in_view: () => boolean,
+        get_next_row: ($message_row: JQuery) => JQuery,
+    ): boolean {
         // return true only if we make an actual adjustment, so
         // that we know to short circuit the other direction
         if (in_view()) {
@@ -479,10 +538,11 @@ export function keep_pointer_in_view() {
         adjust(message_is_far_enough_up, rows.prev_visible);
     }
 
-    message_lists.current.select_id(rows.id($next_row), {from_scroll: true});
+    message_lists.current.select_id(rows.id($next_row)!, {from_scroll: true});
 }
 
-export function scroll_to_selected() {
+export function scroll_to_selected(): void {
+    assert(message_lists.current !== undefined);
     const $selected_row = message_lists.current.selected_row();
     if ($selected_row && $selected_row.length !== 0) {
         recenter_view($selected_row);
@@ -491,11 +551,11 @@ export function scroll_to_selected() {
 
 export let scroll_to_selected_planned = false;
 
-export function plan_scroll_to_selected() {
+export function plan_scroll_to_selected(): void {
     scroll_to_selected_planned = true;
 }
 
-export function maybe_scroll_to_selected() {
+export function maybe_scroll_to_selected(): void {
     // If we have made a plan to scroll to the selected message but
     // deferred doing so, do so here.
     if (scroll_to_selected_planned) {
@@ -504,9 +564,8 @@ export function maybe_scroll_to_selected() {
     }
 }
 
-export function initialize() {
+export function initialize(): void {
     $jwindow = $(window);
-
     // This handler must be placed before all resize handlers in our application
     $jwindow.on("resize", () => {
         dimensions.height.reset();