diff --git a/web/shared/src/internal_url.js b/web/shared/src/internal_url.ts
similarity index 51%
rename from web/shared/src/internal_url.js
rename to web/shared/src/internal_url.ts
index 45c51485fb..d1d0cf978b 100644
--- a/web/shared/src/internal_url.js
+++ b/web/shared/src/internal_url.ts
@@ -1,3 +1,5 @@
+type MaybeGetStreamName = (id: number) => string | undefined;
+
 const hashReplacements = new Map([
     ["%", "."],
     ["(", ".28"],
@@ -8,11 +10,11 @@ const hashReplacements = new Map([
 // Some browsers zealously URI-decode the contents of
 // window.location.hash.  So we hide our URI-encoding
 // by replacing % with . (like MediaWiki).
-export function encodeHashComponent(str) {
-    return encodeURIComponent(str).replace(/[%().]/g, (matched) => hashReplacements.get(matched));
+export function encodeHashComponent(str: string): string {
+    return encodeURIComponent(str).replace(/[%().]/g, (matched) => hashReplacements.get(matched)!);
 }
 
-export function decodeHashComponent(str) {
+export function decodeHashComponent(str: string): string {
     // This fails for URLs containing
     // foo.foo or foo%foo due to our fault in special handling
     // of such characters when encoding. This can also,
@@ -21,8 +23,11 @@ export function decodeHashComponent(str) {
     return decodeURIComponent(str.replace(/\./g, "%"));
 }
 
-export function stream_id_to_slug(stream_id, maybe_get_stream_name) {
-    let name = maybe_get_stream_name(stream_id) || "unknown";
+export function stream_id_to_slug(
+    stream_id: number,
+    maybe_get_stream_name: MaybeGetStreamName,
+): string {
+    let name = maybe_get_stream_name(stream_id) ?? "unknown";
 
     // The name part of the URL doesn't really matter, so we try to
     // make it pretty.
@@ -31,10 +36,13 @@ export function stream_id_to_slug(stream_id, maybe_get_stream_name) {
     // browsers that don't have it.
     name = name.replace(/ /g, "-");
 
-    return stream_id + "-" + name;
+    return `${stream_id}-${name}`;
 }
 
-export function encode_stream_id(stream_id, maybe_get_stream_name) {
+export function encode_stream_id(
+    stream_id: number,
+    maybe_get_stream_name: MaybeGetStreamName,
+): string {
     // stream_id_to_slug appends the stream name, but it does not do the
     // URI encoding piece.
     const slug = stream_id_to_slug(stream_id, maybe_get_stream_name);
@@ -42,15 +50,20 @@ export function encode_stream_id(stream_id, maybe_get_stream_name) {
     return encodeHashComponent(slug);
 }
 
-export function by_stream_url(stream_id, maybe_get_stream_name) {
-    return "#narrow/stream/" + encode_stream_id(stream_id, maybe_get_stream_name);
+export function by_stream_url(
+    stream_id: number,
+    maybe_get_stream_name: MaybeGetStreamName,
+): string {
+    return `#narrow/stream/${encode_stream_id(stream_id, maybe_get_stream_name)}`;
 }
 
-export function by_stream_topic_url(stream_id, topic, maybe_get_stream_name) {
-    return (
-        "#narrow/stream/" +
-        encode_stream_id(stream_id, maybe_get_stream_name) +
-        "/topic/" +
-        encodeHashComponent(topic)
-    );
+export function by_stream_topic_url(
+    stream_id: number,
+    topic: string,
+    maybe_get_stream_name: MaybeGetStreamName,
+): string {
+    return `#narrow/stream/${encode_stream_id(
+        stream_id,
+        maybe_get_stream_name,
+    )}/topic/${encodeHashComponent(topic)}`;
 }