diff --git a/web/src/alert_words.js b/web/src/alert_words.ts
similarity index 75%
rename from web/src/alert_words.js
rename to web/src/alert_words.ts
index 81024c2b03..f60e6823f9 100644
--- a/web/src/alert_words.js
+++ b/web/src/alert_words.ts
@@ -1,16 +1,17 @@
 import _ from "lodash";
 
 import * as people from "./people";
+import type {Message} from "./types";
 
 // For simplicity, we use a list for our internal
 // data, since that matches what the server sends us.
-let my_alert_words = [];
+let my_alert_words: string[] = [];
 
-export function set_words(words) {
+export function set_words(words: string[]): void {
     my_alert_words = words;
 }
 
-export function get_word_list() {
+export function get_word_list(): {word: string}[] {
     // Returns a array of objects
     // (with each alert_word as value and 'word' as key to the object.)
     const words = [];
@@ -20,11 +21,11 @@ export function get_word_list() {
     return words;
 }
 
-export function has_alert_word(word) {
+export function has_alert_word(word: string): boolean {
     return my_alert_words.includes(word);
 }
 
-const alert_regex_replacements = new Map([
+const alert_regex_replacements = new Map<string, string>([
     ["&", "&amp;"],
     ["<", "&lt;"],
     [">", "&gt;"],
@@ -33,7 +34,7 @@ const alert_regex_replacements = new Map([
     ["'", "(?:'|&#39;)"],
 ]);
 
-export function process_message(message) {
+export function process_message(message: Message): void {
     // Parsing for alert words is expensive, so we rely on the host
     // to tell us there any alert words to even look for.
     if (!message.alerted) {
@@ -41,8 +42,9 @@ export function process_message(message) {
     }
 
     for (const word of my_alert_words) {
-        const clean = _.escapeRegExp(word).replaceAll(/["&'<>]/g, (c) =>
-            alert_regex_replacements.get(c),
+        const clean = _.escapeRegExp(word).replaceAll(
+            /["&'<>]/g,
+            (c) => alert_regex_replacements.get(c)!,
         );
         const before_punctuation = "\\s|^|>|[\\(\\\".,';\\[]";
         const after_punctuation = "(?=\\s)|$|<|[\\)\\\"\\?!:.,';\\]!]";
@@ -50,7 +52,14 @@ export function process_message(message) {
         const regex = new RegExp(`(${before_punctuation})(${clean})(${after_punctuation})`, "ig");
         message.content = message.content.replace(
             regex,
-            (match, before, word, after, offset, content) => {
+            (
+                match: string,
+                before: string,
+                word: string,
+                after: string,
+                offset: number,
+                content: string,
+            ) => {
                 // Logic for ensuring that we don't muck up rendered HTML.
                 const pre_match = content.slice(0, offset);
                 // We want to find the position of the `<` and `>` only in the
@@ -69,7 +78,7 @@ export function process_message(message) {
     }
 }
 
-export function notifies(message) {
+export function notifies(message: Message): boolean {
     // We exclude ourselves from notifications when we type one of our own
     // alert words into a message, just because that can be annoying for
     // certain types of workflows where everybody on your team, including
@@ -77,6 +86,6 @@ export function notifies(message) {
     return !people.is_current_user(message.sender_email) && message.alerted;
 }
 
-export const initialize = (params) => {
+export const initialize = (params: {alert_words: string[]}): void => {
     my_alert_words = params.alert_words;
 };