diff --git a/cloud-function/.gitignore b/cloud-function/.gitignore
index 6b36db1..d542da2 100644
--- a/cloud-function/.gitignore
+++ b/cloud-function/.gitignore
@@ -4,4 +4,3 @@ node_modules
 *.log
 canonicals.json
 redirects.json
-src/**/*.js
diff --git a/cloud-function/package-lock.json b/cloud-function/package-lock.json
index 7fdf594..d448327 100644
--- a/cloud-function/package-lock.json
+++ b/cloud-function/package-lock.json
@@ -22,20 +22,18 @@
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-    "node_modules/cross-env": {
-      "version": "10.1.0",
-      "resolved": "https://registry.npmjs.org/cross-env/-/cross-env-10.1.0.tgz",
-      "integrity": "sha512-GsYosgnACZTADcmEyJctkJIoqAhHjttw7RsFrVoJNXbsWWqaq6Ym+7kZjq6mS45O0jij6vtiReppKQEtqWy6Dw==",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "@epic-web/invariant": "^1.0.0",
-        "cross-spawn": "^7.0.6"
-      },
-      "bin": {
-        "cross-env": "dist/bin/cross-env.js",
-        "cross-env-shell": "dist/bin/cross-env-shell.js"
-      },
-      "engines": {
-        "node": ">=20"
-      }
-    },
-    "node_modules/cross-spawn": {
-      "version": "7.0.6",
-      "resolved": "https://registry.npmjs.org/cross-spawn/-/cross-spawn-7.0.6.tgz",
-      "integrity": "sha512-uV2QOWP2nWzsy2aMp8aRibhi9dlzF5Hgh5SHaB9OiTGEyDTiJJyx0uy51QXdyWbtAHNua4XJzUKca3OzKUd3vA==",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "path-key": "^3.1.0",
-        "shebang-command": "^2.0.0",
-        "which": "^2.0.1"
-      },
-      "engines": {
-        "node": ">= 8"
-      }
-    },
     "node_modules/data-view-buffer": {
       "version": "1.0.2",
       "resolved": "https://registry.npmjs.org/data-view-buffer/-/data-view-buffer-1.0.2.tgz",
@@ -2052,16 +1688,6 @@
         "npm": "1.2.8000 || >= 1.4.16"
       }
     },
-    "node_modules/diff": {
-      "version": "4.0.2",
-      "resolved": "https://registry.npmjs.org/diff/-/diff-4.0.2.tgz",
-      "integrity": "sha512-58lmxKSA4BNyLz+HHMUzlOEpg09FV+ev6ZMe3vJihgdxzgcwZ8VoEEPmALCZG9LmqfVoNMMKpttIYTVG6uDY7A==",
-      "dev": true,
-      "license": "BSD-3-Clause",
-      "engines": {
-        "node": ">=0.3.1"
-      }
-    },
     "node_modules/dotenv": {
       "version": "17.2.3",
       "resolved": "https://registry.npmjs.org/dotenv/-/dotenv-17.2.3.tgz",
@@ -3438,13 +3064,6 @@
       "integrity": "sha512-JNAzZcXrCt42VGLuYz0zfAzDfAvJWW6AfYlDBQyDV5DClI2m5sAmK+OIO7s59XfsRsWHp02jAJrRadPRGTt6SQ==",
       "license": "ISC"
     },
-    "node_modules/make-error": {
-      "version": "1.3.6",
-      "resolved": "https://registry.npmjs.org/make-error/-/make-error-1.3.6.tgz",
-      "integrity": "sha512-s8UhlNe7vPKomQhC1qFelMokr/Sc3AgNbso3n74mVPA5LTZwkB9NlXf4XPamLxJE8h0gh73rM94xvwRT2CVInw==",
-      "dev": true,
-      "license": "ISC"
-    },
     "node_modules/math-intrinsics": {
       "version": "1.1.0",
       "resolved": "https://registry.npmjs.org/math-intrinsics/-/math-intrinsics-1.1.0.tgz",
@@ -3990,16 +3609,6 @@
         "node": ">= 0.8"
       }
     },
-    "node_modules/path-key": {
-      "version": "3.1.1",
-      "resolved": "https://registry.npmjs.org/path-key/-/path-key-3.1.1.tgz",
-      "integrity": "sha512-ojmeN0qd+y0jszEtoY48r0Peq5dwMEkIlCOu6Q5f41lfkswXuKtYrhgoTpLnyIcHm24Uhqx+5Tqm2InSwLhE6Q==",
-      "dev": true,
-      "license": "MIT",
-      "engines": {
-        "node": ">=8"
-      }
-    },
     "node_modules/path-parse": {
       "version": "1.0.7",
       "resolved": "https://registry.npmjs.org/path-parse/-/path-parse-1.0.7.tgz",
@@ -4663,29 +4272,6 @@
       "integrity": "sha512-E5LDX7Wrp85Kil5bhZv46j8jOeboKq5JMmYM3gVGdGH8xFpPWXUMsNrlODCrkoxMEeNi/XZIwuRvY4XNwYMJpw==",
       "license": "ISC"
     },
-    "node_modules/shebang-command": {
-      "version": "2.0.0",
-      "resolved": "https://registry.npmjs.org/shebang-command/-/shebang-command-2.0.0.tgz",
-      "integrity": "sha512-kHxr2zZpYtdmrN1qDjrrX/Z1rR1kG8Dx+gkpK1G4eXmvXswmcE1hTWBWYUzlraYw1/yZp6YuDY77YtvbN0dmDA==",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "shebang-regex": "^3.0.0"
-      },
-      "engines": {
-        "node": ">=8"
-      }
-    },
-    "node_modules/shebang-regex": {
-      "version": "3.0.0",
-      "resolved": "https://registry.npmjs.org/shebang-regex/-/shebang-regex-3.0.0.tgz",
-      "integrity": "sha512-7++dFhtcx3353uBaq8DDR4NuxBetBzC7ZQOhmTQInHEd6bSrXdiEyzCvG07Z44UYdLShWUyXt5M/yhz8ekcb1A==",
-      "dev": true,
-      "license": "MIT",
-      "engines": {
-        "node": ">=8"
-      }
-    },
     "node_modules/shell-quote": {
       "version": "1.8.2",
       "resolved": "https://registry.npmjs.org/shell-quote/-/shell-quote-1.8.2.tgz",
@@ -4990,50 +4576,6 @@
         "utf8-byte-length": "^1.0.1"
       }
     },
-    "node_modules/ts-node": {
-      "version": "10.9.2",
-      "resolved": "https://registry.npmjs.org/ts-node/-/ts-node-10.9.2.tgz",
-      "integrity": "sha512-f0FFpIdcHgn8zcPSbf1dRevwt047YMnaiJM3u2w2RewrB+fob/zePZcrOyQoLMMO7aBIddLcQIEK5dYjkLnGrQ==",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "@cspotcode/source-map-support": "^0.8.0",
-        "@tsconfig/node10": "^1.0.7",
-        "@tsconfig/node12": "^1.0.7",
-        "@tsconfig/node14": "^1.0.0",
-        "@tsconfig/node16": "^1.0.2",
-        "acorn": "^8.4.1",
-        "acorn-walk": "^8.1.1",
-        "arg": "^4.1.0",
-        "create-require": "^1.1.0",
-        "diff": "^4.0.1",
-        "make-error": "^1.1.1",
-        "v8-compile-cache-lib": "^3.0.1",
-        "yn": "3.1.1"
-      },
-      "bin": {
-        "ts-node": "dist/bin.js",
-        "ts-node-cwd": "dist/bin-cwd.js",
-        "ts-node-esm": "dist/bin-esm.js",
-        "ts-node-script": "dist/bin-script.js",
-        "ts-node-transpile-only": "dist/bin-transpile.js",
-        "ts-script": "dist/bin-script-deprecated.js"
-      },
-      "peerDependencies": {
-        "@swc/core": ">=1.2.50",
-        "@swc/wasm": ">=1.2.50",
-        "@types/node": "*",
-        "typescript": ">=2.7"
-      },
-      "peerDependenciesMeta": {
-        "@swc/core": {
-          "optional": true
-        },
-        "@swc/wasm": {
-          "optional": true
-        }
-      }
-    },
     "node_modules/type-fest": {
       "version": "4.41.0",
       "resolved": "https://registry.npmjs.org/type-fest/-/type-fest-4.41.0.tgz",
@@ -5260,13 +4802,6 @@
         "uuid": "dist/bin/uuid"
       }
     },
-    "node_modules/v8-compile-cache-lib": {
-      "version": "3.0.1",
-      "resolved": "https://registry.npmjs.org/v8-compile-cache-lib/-/v8-compile-cache-lib-3.0.1.tgz",
-      "integrity": "sha512-wa7YjyUGfNZngI/vtK0UHAN+lgDCxBPCylVXGp0zu59Fz5aiGtNXaq3DhIov063MorB+VfufLh3JlF2KdTK3xg==",
-      "dev": true,
-      "license": "MIT"
-    },
     "node_modules/validate-npm-package-license": {
       "version": "3.0.4",
       "resolved": "https://registry.npmjs.org/validate-npm-package-license/-/validate-npm-package-license-3.0.4.tgz",
@@ -5328,22 +4863,6 @@
         "webidl-conversions": "^3.0.0"
       }
     },
-    "node_modules/which": {
-      "version": "2.0.2",
-      "resolved": "https://registry.npmjs.org/which/-/which-2.0.2.tgz",
-      "integrity": "sha512-BLI3Tl1TW3Pvl70l3yq3Y64i+awpwXqsGBYWkkqMtnbXgrMD+yj7rhW0kuEDxzJaYXGjEW5ogapKNMEKNMjibA==",
-      "dev": true,
-      "license": "ISC",
-      "dependencies": {
-        "isexe": "^2.0.0"
-      },
-      "bin": {
-        "node-which": "bin/node-which"
-      },
-      "engines": {
-        "node": ">= 8"
-      }
-    },
     "node_modules/which-boxed-primitive": {
       "version": "1.1.1",
       "resolved": "https://registry.npmjs.org/which-boxed-primitive/-/which-boxed-primitive-1.1.1.tgz",
@@ -5439,16 +4958,6 @@
       "engines": {
         "node": ">=0.4"
       }
-    },
-    "node_modules/yn": {
-      "version": "3.1.1",
-      "resolved": "https://registry.npmjs.org/yn/-/yn-3.1.1.tgz",
-      "integrity": "sha512-Ux4ygGWsu2c7isFWe8Yu1YluJmqVhxqK2cLXNQA5AcC3QfbGNpM7fu0Y8b/z16pXLnFxZYvWhd3fhBY9DLmC6Q==",
-      "dev": true,
-      "license": "MIT",
-      "engines": {
-        "node": ">=6"
-      }
     }
   }
 }
diff --git a/cloud-function/package.json b/cloud-function/package.json
index d86009c..94bf426 100644
--- a/cloud-function/package.json
+++ b/cloud-function/package.json
@@ -8,17 +8,16 @@
   "type": "module",
   "main": "src/index.js",
   "scripts": {
-    "build": "tsc -b",
-    "build-canonicals": "cross-env NODE_OPTIONS='--no-warnings=ExperimentalWarning --loader ts-node/esm' node src/build-canonicals.ts",
-    "build-redirects": "cross-env NODE_OPTIONS='--no-warnings=ExperimentalWarning --loader ts-node/esm' node src/build-redirects.ts",
-    "gcp-build": "npm run build",
-    "proxy": "cross-env NODE_OPTIONS='--no-warnings=ExperimentalWarning --loader ts-node/esm' node src/proxy.ts",
-    "server": "npm run build && functions-framework --target=mdnHandler --ignored-routes \"\"",
+    "build-canonicals": "node src/build-canonicals.js",
+    "build-redirects": "node src/build-redirects.js",
+    "check": "tsc --noEmit",
+    "proxy": "node src/proxy.js",
+    "server": "functions-framework --target=mdnHandler --ignored-routes \"\"",
     "server:watch": "nodemon --exec npm run server",
     "start": "nf start"
   },
   "nodemonConfig": {
-    "ext": "json,ts",
+    "ext": "json,js",
     "watch": [
       ".env",
       "src"
@@ -38,20 +37,18 @@
     "sanitize-filename": "^1.6.3"
   },
   "devDependencies": {
-    "@swc/core": "^1.14.0",
     "@types/accept-language-parser": "^1.5.8",
     "@types/cookie": "^1.0.0",
     "@types/cookie-parser": "^1.4.10",
+    "@types/express": "^4.17.21",
     "@types/he": "^1.2.3",
     "@types/http-proxy": "^1.17.17",
     "@types/http-server": "^0.12.4",
-    "cross-env": "^10.1.0",
     "foreman": "^3.0.1",
     "http-proxy": "^1.18.1",
     "http-server": "^14.1.1",
     "nodemon": "^3.1.10",
     "npm-run-all": "^4.1.5",
-    "ts-node": "^10.9.2",
     "typescript": "^5.9.3"
   },
   "engines": {
diff --git a/cloud-function/src/app.ts b/cloud-function/src/app.js
similarity index 93%
rename from cloud-function/src/app.ts
rename to cloud-function/src/app.js
index 87d027a..b1c999e 100644
--- a/cloud-function/src/app.ts
+++ b/cloud-function/src/app.js
@@ -1,6 +1,7 @@
+/** @import { Request, Response } from "express" */
+
 import cookieParser from "cookie-parser";
-import express, { Request, Response } from "express";
-import { Router } from "express";
+import express, { Router } from "express";
 
 import { ANY_ATTACHMENT_EXT } from "./internal/constants/index.js";
 
@@ -123,8 +124,12 @@ router.get(
 );
 router.all("*", notFound);
 
+/**
+ * Create the main MDN handler function for Google Cloud Functions
+ * @returns {(req: Request, res: Response) => Promise<void>} Express-compatible handler
+ */
 export function createHandler() {
-  return async (req: Request, res: Response) =>
+  return async (req, res) =>
     router(req, res, () => {
       /* noop */
     });
diff --git a/cloud-function/src/build-canonicals.ts b/cloud-function/src/build-canonicals.js
similarity index 94%
rename from cloud-function/src/build-canonicals.ts
rename to cloud-function/src/build-canonicals.js
index 3e8e772..48dd54d 100644
--- a/cloud-function/src/build-canonicals.ts
+++ b/cloud-function/src/build-canonicals.js
@@ -23,7 +23,8 @@ async function buildCanonicals() {
   const lines = content.split("\n");
   const pages = lines.filter((line) => line.startsWith("/"));
 
-  const siteMap: Record<string, string> = {};
+  /** @type {Record<string, string>} */
+  const siteMap = {};
   for (const page of pages) {
     siteMap[normalizePath(page)] = page;
   }
diff --git a/cloud-function/src/build-redirects.ts b/cloud-function/src/build-redirects.js
similarity index 96%
rename from cloud-function/src/build-redirects.ts
rename to cloud-function/src/build-redirects.js
index c13dffc..543b024 100644
--- a/cloud-function/src/build-redirects.ts
+++ b/cloud-function/src/build-redirects.js
@@ -15,7 +15,8 @@ dotenv.config({
 });
 
 function buildRedirectsMap() {
-  const redirectMap = new Map<string, string>();
+  /** @type {Map<string, string>} */
+  const redirectMap = new Map();
 
   ["CONTENT_ROOT", "CONTENT_TRANSLATED_ROOT"].forEach((envvar) => {
     if (!process.env[envvar]) {
diff --git a/cloud-function/src/canonicals.ts b/cloud-function/src/canonicals.js
similarity index 100%
rename from cloud-function/src/canonicals.ts
rename to cloud-function/src/canonicals.js
diff --git a/cloud-function/src/constants.ts b/cloud-function/src/constants.js
similarity index 100%
rename from cloud-function/src/constants.ts
rename to cloud-function/src/constants.js
diff --git a/cloud-function/src/env.ts b/cloud-function/src/env.js
similarity index 57%
rename from cloud-function/src/env.ts
rename to cloud-function/src/env.js
index 50e7252..3c11568 100644
--- a/cloud-function/src/env.ts
+++ b/cloud-function/src/env.js
@@ -1,8 +1,9 @@
+/** @import { Request } from "express" */
+
 import * as path from "node:path";
 import { cwd } from "node:process";
 
 import dotenv from "dotenv";
-import { Request } from "express";
 
 dotenv.config({
   path: path.join(cwd(), process.env["ENV_FILE"] || ".env"),
@@ -12,33 +13,47 @@ dotenv.config({
 export const LOCAL_CONTENT = "http://localhost:8100/";
 export const LOCAL_RUMBA = "http://localhost:8000/";
 
-export enum Origin {
-  main = "main",
-  liveSamples = "liveSamples",
-  play = "play",
-  unsafe = "unsafe",
-}
+/**
+ * @typedef {"main" | "liveSamples" | "play" | "unsafe"} OriginType
+ */
 
-export enum Source {
-  content = "content",
-  liveSamples = "liveSamples",
-  api = "rumba",
-  sharedAssets = "sharedAssets",
-}
+/**
+ * @typedef {"content" | "liveSamples" | "rumba" | "sharedAssets"} SourceType
+ */
+
+/** @type {{main: "main", liveSamples: "liveSamples", play: "play", unsafe: "unsafe"}} */
+export const Origin = Object.freeze({
+  main: "main",
+  liveSamples: "liveSamples",
+  play: "play",
+  unsafe: "unsafe",
+});
+
+/** @type {{content: "content", liveSamples: "liveSamples", api: "rumba", sharedAssets: "sharedAssets"}} */
+export const Source = Object.freeze({
+  content: "content",
+  liveSamples: "liveSamples",
+  api: "rumba",
+  sharedAssets: "sharedAssets",
+});
 
-export const ORIGIN_MAIN: string = process.env["ORIGIN_MAIN"] || "localhost";
-export const ORIGIN_LIVE_SAMPLES: string =
+export const ORIGIN_MAIN = process.env["ORIGIN_MAIN"] || "localhost";
+export const ORIGIN_LIVE_SAMPLES =
   process.env["ORIGIN_LIVE_SAMPLES"] || "localhost";
-export const ORIGIN_PLAY: string = process.env["ORIGIN_PLAY"] || "localhost";
+export const ORIGIN_PLAY = process.env["ORIGIN_PLAY"] || "localhost";
 
-export const SOURCE_CONTENT: string =
-  process.env["SOURCE_CONTENT"] || LOCAL_CONTENT;
-export const SOURCE_API: string =
+export const SOURCE_CONTENT = process.env["SOURCE_CONTENT"] || LOCAL_CONTENT;
+export const SOURCE_API =
   process.env["SOURCE_API"] || "https://developer.allizom.org/";
-export const SOURCE_SHARED_ASSETS: string =
+export const SOURCE_SHARED_ASSETS =
   process.env["SOURCE_SHARED_ASSETS"] || "https://mdn.github.io/shared-assets/";
 
-export function getOriginFromRequest(req: Request): Origin {
+/**
+ * Determine the origin type from a request
+ * @param {Request} req - Express request object
+ * @returns {OriginType} The origin type
+ */
+export function getOriginFromRequest(req) {
   if (
     req.hostname === ORIGIN_MAIN &&
     !req.path.includes("/_sample_.") &&
@@ -57,7 +72,12 @@ export function getOriginFromRequest(req: Request): Origin {
   }
 }
 
-export function sourceUri(source: Source): string {
+/**
+ * Get the URI for a given source
+ * @param {SourceType} source - Source type
+ * @returns {string} Source URI
+ */
+export function sourceUri(source) {
   switch (source) {
     case Source.content:
       return SOURCE_CONTENT;
@@ -71,8 +91,8 @@ export function sourceUri(source: Source): string {
 }
 
 // Origin trial.
-export const ORIGIN_TRIAL_TOKEN: string | undefined =
-  process.env["ORIGIN_TRIAL_TOKEN"];
+/** @type {string | undefined} */
+export const ORIGIN_TRIAL_TOKEN = process.env["ORIGIN_TRIAL_TOKEN"];
 
 // Kevel.
 export const KEVEL_SITE_ID = Number(process.env["KEVEL_SITE_ID"] ?? 0);
diff --git a/cloud-function/src/handlers/handle-stripe-plans.ts b/cloud-function/src/handlers/handle-stripe-plans.js
similarity index 73%
rename from cloud-function/src/handlers/handle-stripe-plans.ts
rename to cloud-function/src/handlers/handle-stripe-plans.js
index aa9b912..061adc6 100644
--- a/cloud-function/src/handlers/handle-stripe-plans.ts
+++ b/cloud-function/src/handlers/handle-stripe-plans.js
@@ -1,21 +1,36 @@
+/** @import { Request, Response } from "express" */
+
 import acceptLanguageParser from "accept-language-parser";
-import { Request, Response } from "express";
 
 import { ORIGIN_MAIN } from "../env.js";
 import { getRequestCountry } from "../utils.js";
 import stageLookup from "../stripe-plans/stage.js";
 import prodLookup from "../stripe-plans/prod.js";
 
-interface PlanResult {
-  [name: string]: { monthlyPriceInCents: number; id: string };
-}
-interface Result {
-  country: string;
-  currency: string;
-  plans: PlanResult;
-}
+/**
+ * @typedef {Object} PlanDetails
+ * @property {number} monthlyPriceInCents
+ * @property {string} id
+ */
+
+/**
+ * @typedef {Object.<string, PlanDetails>} PlanResult
+ */
+
+/**
+ * @typedef {Object} Result
+ * @property {string} country
+ * @property {string} currency
+ * @property {PlanResult} plans
+ */
 
-export async function handleStripePlans(req: Request, res: Response) {
+/**
+ * Handles requests for Stripe subscription plans based on country and language
+ * @param {Request} req - Express request object
+ * @param {Response} res - Express response object
+ * @returns {Promise<Response>}
+ */
+export async function handleStripePlans(req, res) {
   const lookupData =
     ORIGIN_MAIN === "developer.mozilla.org" ? prodLookup : stageLookup;
 
@@ -50,7 +65,8 @@ export async function handleStripePlans(req: Request, res: Response) {
     return res.sendStatus(500).end();
   }
 
-  const planResult: PlanResult = {};
+  /** @type {PlanResult} */
+  const planResult = {};
   Object.entries(plans).forEach(([name, plan]) => {
     let monthlyPriceInCents;
     if (plan.recurring.interval === "year") {
@@ -61,11 +77,12 @@ export async function handleStripePlans(req: Request, res: Response) {
     planResult[name] = { monthlyPriceInCents, id: plan.price_id };
   });
 
+  /** @type {Result} */
   const result = {
     country: countryCode,
     currency: supportedCurrency.currency,
     plans: planResult,
-  } satisfies Result;
+  };
 
   return (
     res
diff --git a/cloud-function/src/handlers/proxy-api.ts b/cloud-function/src/handlers/proxy-api.js
similarity index 81%
rename from cloud-function/src/handlers/proxy-api.ts
rename to cloud-function/src/handlers/proxy-api.js
index 27ea952..a16418c 100644
--- a/cloud-function/src/handlers/proxy-api.ts
+++ b/cloud-function/src/handlers/proxy-api.js
@@ -3,6 +3,10 @@ import { createProxyMiddleware, fixRequestBody } from "http-proxy-middleware";
 import { Source, sourceUri } from "../env.js";
 import { PROXY_TIMEOUT } from "../constants.js";
 
+/**
+ * Proxy middleware for API requests
+ * Forwards requests to the backend API server
+ */
 export const proxyApi = createProxyMiddleware({
   target: sourceUri(Source.api),
   changeOrigin: true,
diff --git a/cloud-function/src/handlers/proxy-bsa.ts b/cloud-function/src/handlers/proxy-bsa.js
similarity index 91%
rename from cloud-function/src/handlers/proxy-bsa.ts
rename to cloud-function/src/handlers/proxy-bsa.js
index 7dc41e3..a6ffcca 100644
--- a/cloud-function/src/handlers/proxy-bsa.ts
+++ b/cloud-function/src/handlers/proxy-bsa.js
@@ -1,4 +1,4 @@
-import type { Request, Response } from "express";
+/** @import { Request, Response } from "express" */
 
 import { Coder } from "../internal/pong/index.js";
 import {
@@ -23,7 +23,14 @@ const handleViewed = createPong2ViewedHandler(coder);
 const plusLookup =
   ORIGIN_MAIN === "developer.mozilla.org" ? prodPlusLookup : stagePlusLookup;
 
-export async function proxyBSA(req: Request, res: Response) {
+/**
+ * Handles BSA (BuySellAds) advertising requests
+ * Supports ad retrieval, click tracking, view tracking, and image proxying
+ * @param {Request} req - Express request object
+ * @param {Response} res - Express response object
+ * @returns {Promise<Response | void>}
+ */
+export async function proxyBSA(req, res) {
   const countryCode = getRequestCountry(req);
 
   const plusAvailable = countryCode in plusLookup.countryToCurrency;
diff --git a/cloud-function/src/handlers/proxy-content-assets.ts b/cloud-function/src/handlers/proxy-content-assets.js
similarity index 91%
rename from cloud-function/src/handlers/proxy-content-assets.ts
rename to cloud-function/src/handlers/proxy-content-assets.js
index 5e48416..23d6941 100644
--- a/cloud-function/src/handlers/proxy-content-assets.ts
+++ b/cloud-function/src/handlers/proxy-content-assets.js
@@ -12,6 +12,10 @@ import { ACTIVE_LOCALES } from "../internal/constants/index.js";
 
 const target = sourceUri(Source.content);
 
+/**
+ * Proxy middleware for content assets (attachments, media, fonts)
+ * Falls back to en-US assets for non-English locales, then to production if needed
+ */
 export const proxyContentAssets = createProxyMiddleware({
   target,
   changeOrigin: true,
diff --git a/cloud-function/src/handlers/proxy-content.ts b/cloud-function/src/handlers/proxy-content.js
similarity index 81%
rename from cloud-function/src/handlers/proxy-content.ts
rename to cloud-function/src/handlers/proxy-content.js
index af5a54f..bbcec43 100644
--- a/cloud-function/src/handlers/proxy-content.ts
+++ b/cloud-function/src/handlers/proxy-content.js
@@ -10,10 +10,15 @@ import { Source, sourceUri, WILDCARD_ENABLED } from "../env.js";
 import { PROXY_TIMEOUT } from "../constants.js";
 import { isLiveSampleURL } from "../utils.js";
 
-const notFoundBufferCache: Record<string, Promise<ArrayBuffer>> = {};
+/** @type {Record<string, Promise<ArrayBuffer>>} */
+const notFoundBufferCache = {};
 
 const target = sourceUri(Source.content);
 
+/**
+ * Proxy middleware for content requests
+ * Handles MDN content with 404 fallback logic and wildcard subdomain support
+ */
 export const proxyContent = createProxyMiddleware({
   changeOrigin: true,
   autoRewrite: true,
@@ -65,10 +70,14 @@ export const proxyContent = createProxyMiddleware({
   },
 });
 
-async function get404ForLocale(
-  locale: string
-): Promise<Buffer<ArrayBufferLike> | string> {
-  let notFoundBuffer: Promise<ArrayBuffer>;
+/**
+ * Fetches the 404 page for a given locale with caching
+ * @param {string} locale - The locale code (e.g., "en-us")
+ * @returns {Promise<Buffer | string>} The 404 page content as a Buffer or fallback string
+ */
+async function get404ForLocale(locale) {
+  /** @type {Promise<ArrayBuffer>} */
+  let notFoundBuffer;
   if (notFoundBufferCache[locale]) {
     notFoundBuffer = notFoundBufferCache[locale];
   } else {
diff --git a/cloud-function/src/handlers/proxy-kevel.ts b/cloud-function/src/handlers/proxy-kevel.js
similarity index 88%
rename from cloud-function/src/handlers/proxy-kevel.ts
rename to cloud-function/src/handlers/proxy-kevel.js
index 4ea63b7..a84413b 100644
--- a/cloud-function/src/handlers/proxy-kevel.ts
+++ b/cloud-function/src/handlers/proxy-kevel.js
@@ -1,5 +1,6 @@
+/** @import { Request, Response } from "express" */
+
 import { Client } from "@adzerk/decision-sdk";
-import type { Request, Response } from "express";
 
 import { Coder } from "../internal/pong/index.js";
 import {
@@ -28,7 +29,14 @@ const handleViewed = createPongViewedHandler(coder);
 const plusLookup =
   ORIGIN_MAIN === "developer.mozilla.org" ? prodPlusLookup : stagePlusLookup;
 
-export async function proxyKevel(req: Request, res: Response) {
+/**
+ * Handles Kevel (formerly Adzerk) advertising requests
+ * Supports ad retrieval, click tracking, view tracking, and image proxying
+ * @param {Request} req - Express request object
+ * @param {Response} res - Express response object
+ * @returns {Promise<Response | void>}
+ */
+export async function proxyKevel(req, res) {
   const countryCode = getRequestCountry(req);
 
   const plusAvailable = countryCode in plusLookup.countryToCurrency;
diff --git a/cloud-function/src/handlers/proxy-pong.js b/cloud-function/src/handlers/proxy-pong.js
new file mode 100644
index 0000000..0644530
--- /dev/null
+++ b/cloud-function/src/handlers/proxy-pong.js
@@ -0,0 +1,18 @@
+/** @import { Request, Response } from "express" */
+
+import { BSA_ENABLED } from "../env.js";
+import { proxyBSA } from "./proxy-bsa.js";
+import { proxyKevel } from "./proxy-kevel.js";
+
+/**
+ * Routes pong/advertising requests to either BSA or Kevel handler
+ * @param {Request} req - Express request object
+ * @param {Response} res - Express response object
+ * @returns {Promise<Response | void>}
+ */
+export async function proxyPong(req, res) {
+  if (BSA_ENABLED) {
+    return proxyBSA(req, res);
+  }
+  return proxyKevel(req, res);
+}
diff --git a/cloud-function/src/handlers/proxy-pong.ts b/cloud-function/src/handlers/proxy-pong.ts
deleted file mode 100644
index e26a90a..0000000
--- a/cloud-function/src/handlers/proxy-pong.ts
+++ /dev/null
@@ -1,12 +0,0 @@
-import { BSA_ENABLED } from "../env.js";
-import { proxyBSA } from "./proxy-bsa.js";
-import { proxyKevel } from "./proxy-kevel.js";
-
-import type { Request, Response } from "express";
-
-export async function proxyPong(req: Request, res: Response) {
-  if (BSA_ENABLED) {
-    return proxyBSA(req, res);
-  }
-  return proxyKevel(req, res);
-}
diff --git a/cloud-function/src/handlers/proxy-shared-assets.ts b/cloud-function/src/handlers/proxy-shared-assets.js
similarity index 88%
rename from cloud-function/src/handlers/proxy-shared-assets.ts
rename to cloud-function/src/handlers/proxy-shared-assets.js
index 0cb03e3..0d937bb 100644
--- a/cloud-function/src/handlers/proxy-shared-assets.ts
+++ b/cloud-function/src/handlers/proxy-shared-assets.js
@@ -9,6 +9,10 @@ import { PROXY_TIMEOUT } from "../constants.js";
 
 const target = sourceUri(Source.sharedAssets);
 
+/**
+ * Proxy middleware for shared assets (interactive examples)
+ * Rewrites paths and sets appropriate cache headers
+ */
 export const proxySharedAssets = createProxyMiddleware({
   target,
   pathRewrite: {
diff --git a/cloud-function/src/handlers/proxy-telemetry.ts b/cloud-function/src/handlers/proxy-telemetry.js
similarity index 74%
rename from cloud-function/src/handlers/proxy-telemetry.ts
rename to cloud-function/src/handlers/proxy-telemetry.js
index aa87565..8f48969 100644
--- a/cloud-function/src/handlers/proxy-telemetry.ts
+++ b/cloud-function/src/handlers/proxy-telemetry.js
@@ -2,6 +2,10 @@ import { createProxyMiddleware, fixRequestBody } from "http-proxy-middleware";
 
 import { PROXY_TIMEOUT } from "../constants.js";
 
+/**
+ * Proxy middleware for Mozilla telemetry submissions
+ * Forwards telemetry data to Mozilla's telemetry ingestion service
+ */
 export const proxyTelemetry = createProxyMiddleware({
   target: "https://incoming.telemetry.mozilla.org",
   changeOrigin: true,
diff --git a/cloud-function/src/headers.ts b/cloud-function/src/headers.js
similarity index 62%
rename from cloud-function/src/headers.ts
rename to cloud-function/src/headers.js
index 14866ab..15d207f 100644
--- a/cloud-function/src/headers.ts
+++ b/cloud-function/src/headers.js
@@ -1,4 +1,4 @@
-import type { IncomingMessage, ServerResponse } from "node:http";
+/** @import { IncomingMessage, ServerResponse } from "node:http" */
 
 import { CSP_VALUE } from "./internal/constants/index.js";
 import { isLiveSampleURL } from "./utils.js";
@@ -11,11 +11,14 @@ const NO_CACHE_VALUE = "no-store";
 
 const HASHED_REGEX = /\.[a-f0-9]{8,32}\./;
 
-export function withContentResponseHeaders(
-  proxyRes: IncomingMessage,
-  req: IncomingMessage,
-  res: ServerResponse<IncomingMessage>
-): ServerResponse<IncomingMessage> {
+/**
+ * Add content response headers to a proxy response
+ * @param {IncomingMessage} proxyRes - Proxy response
+ * @param {IncomingMessage} req - Request object
+ * @param {ServerResponse<IncomingMessage>} res - Response object
+ * @returns {ServerResponse<IncomingMessage>} Response object with headers
+ */
+export function withContentResponseHeaders(proxyRes, req, res) {
   if (res.headersSent) {
     console.warn(
       `Cannot set content response headers. Headers already sent for: ${req.url}`
@@ -53,7 +56,13 @@ export function withContentResponseHeaders(
   return res;
 }
 
-function getCacheControl(statusCode: number, url: string) {
+/**
+ * Get cache control header value based on status code and URL
+ * @param {number} statusCode - HTTP status code
+ * @param {string} url - Request URL
+ * @returns {string | null} Cache control value
+ */
+function getCacheControl(statusCode, url) {
   if (
     statusCode === 404 ||
     url.endsWith("/service-worker.js") ||
@@ -74,23 +83,41 @@ function getCacheControl(statusCode: number, url: string) {
   return null;
 }
 
-function getCacheMaxAgeForUrl(url: string): number {
+/**
+ * Get cache max age for a URL
+ * @param {string} url - Request URL
+ * @returns {number} Max age in seconds
+ */
+function getCacheMaxAgeForUrl(url) {
   const isHashed = HASHED_REGEX.test(url);
   const maxAge = isHashed ? HASHED_MAX_AGE : DEFAULT_MAX_AGE;
 
   return maxAge;
 }
 
-function parseContentType(value: unknown): string {
+/**
+ * Parse content type from header value
+ * @param {unknown} value - Content type header value
+ * @returns {string} Parsed content type
+ */
+function parseContentType(value) {
   const firstValue = Array.isArray(value) ? (value[0] ?? "") : value;
 
   return typeof firstValue === "string" ? firstValue : "";
 }
 
+/**
+ * Set content response headers using a setter function
+ * @param {(name: string, value: string) => void} setHeader - Header setter function
+ * @param {object} options - Header options
+ * @param {boolean} [options.csp=true] - Whether to set CSP header
+ * @param {boolean} [options.xFrame=true] - Whether to set X-Frame-Options header
+ * @returns {void}
+ */
 export function setContentResponseHeaders(
-  setHeader: (name: string, value: string) => void,
-  { csp = true, xFrame = true }: { csp?: boolean; xFrame?: boolean }
-): void {
+  setHeader,
+  { csp = true, xFrame = true } = {}
+) {
   [
     ["Referrer-Policy", "strict-origin-when-cross-origin"],
     ["X-Content-Type-Options", "nosniff"],
diff --git a/cloud-function/src/index.ts b/cloud-function/src/index.js
similarity index 100%
rename from cloud-function/src/index.ts
rename to cloud-function/src/index.js
diff --git a/cloud-function/src/internal/constants/index.ts b/cloud-function/src/internal/constants/index.js
similarity index 95%
rename from cloud-function/src/internal/constants/index.ts
rename to cloud-function/src/internal/constants/index.js
index 98e6a2f..3b8aafc 100644
--- a/cloud-function/src/internal/constants/index.ts
+++ b/cloud-function/src/internal/constants/index.js
@@ -196,7 +196,11 @@ export const CSP_DIRECTIVES = {
   "worker-src": ["'self'"],
 };
 
-export const cspToString = (csp: Record<string, string[]>) =>
+/**
+ * @param {Record<string, string[]>} csp
+ * @returns {string}
+ */
+export const cspToString = (csp) =>
   Object.entries(csp)
     .map(([directive, values]) => `${directive} ${values.join(" ")};`)
     .join(" ");
@@ -224,7 +228,11 @@ export const ANY_ATTACHMENT_EXT = [
   ...VIDEO_EXT,
 ].sort();
 
-export function createRegExpFromExtensions(...extensions: string[]) {
+/**
+ * @param {...string} extensions
+ * @returns {RegExp}
+ */
+export function createRegExpFromExtensions(...extensions) {
   return new RegExp(`\\.(${extensions.join("|")})$`, "i");
 }
 
diff --git a/cloud-function/src/internal/constants/index.test.ts b/cloud-function/src/internal/constants/index.test.js
similarity index 100%
rename from cloud-function/src/internal/constants/index.test.ts
rename to cloud-function/src/internal/constants/index.test.js
diff --git a/cloud-function/src/internal/fundamental-redirects/index.ts b/cloud-function/src/internal/fundamental-redirects/index.js
similarity index 98%
rename from cloud-function/src/internal/fundamental-redirects/index.ts
rename to cloud-function/src/internal/fundamental-redirects/index.js
index 7f39960..9d0ac61 100644
--- a/cloud-function/src/internal/fundamental-redirects/index.ts
+++ b/cloud-function/src/internal/fundamental-redirects/index.js
@@ -10,6 +10,14 @@ const startRe = /^\^?\/?/;
 const startTemplate = /^\//;
 const LOCALE_PATTERN = "(?:[a-zA-Z]{2}|eng)(?:-[a-zA-Z]{2})?";
 
+/**
+ * @param {RegExp} pattern
+ * @param {string | Function} template
+ * @param {Object} [options={}]
+ * @param {boolean} [options.permanent]
+ * @param {boolean} [options.colonToSlash]
+ * @returns {Function}
+ */
 function redirect(pattern, template, options = {}) {
   return (path) => {
     const match = pattern.exec(path);
@@ -31,6 +39,13 @@ function redirect(pattern, template, options = {}) {
   };
 }
 
+/**
+ * @param {RegExp} pattern
+ * @param {string | Function} template
+ * @param {Object} [options={}]
+ * @param {boolean} [options.prependLocale=true]
+ * @returns {Function}
+ */
 function localeRedirect(
   pattern,
   template,
@@ -52,6 +67,12 @@ function localeRedirect(
   return redirect(patternWithLocale, _template, options);
 }
 
+/**
+ * @param {RegExp} pattern
+ * @param {string | Function} template
+ * @param {Object} [options={}]
+ * @returns {Function}
+ */
 function externalRedirect(pattern, template, options = {}) {
   return localeRedirect(pattern, template, {
     prependLocale: false,
@@ -831,8 +852,19 @@ const zoneRedirects = [
   ["Marketplace", "Mozilla/بازار", ["fa"]],
 ];
 
+/**
+ * @param {string} prefix
+ * @param {string} zoneRootPattern
+ * @returns {RegExp}
+ */
 const zonePatternFmt = (prefix, zoneRootPattern) =>
   new RegExp(`^${prefix}${zoneRootPattern}(?:\\/?|(?<subPath>[\\/$].+))$`, "i");
+
+/**
+ * @param {string} prefix
+ * @param {string} wikiSlug
+ * @returns {Function}
+ */
 const subPathFmt =
   (prefix, wikiSlug) =>
   ({ subPath = "" } = {}) =>
@@ -1206,6 +1238,10 @@ const REDIRECT_PATTERNS = [].concat(
 const STARTING_SLASH = /^\//;
 const ABSOLUTE_URL = /^https?:\/\/.*/;
 
+/**
+ * @param {string} path
+ * @returns {Object}
+ */
 export function resolveFundamental(path) {
   if (ABSOLUTE_URL.exec(path)) {
     return {};
diff --git a/cloud-function/src/internal/locale-utils/index.ts b/cloud-function/src/internal/locale-utils/index.js
similarity index 62%
rename from cloud-function/src/internal/locale-utils/index.ts
rename to cloud-function/src/internal/locale-utils/index.js
index ffc8e2a..d747f9b 100644
--- a/cloud-function/src/internal/locale-utils/index.ts
+++ b/cloud-function/src/internal/locale-utils/index.js
@@ -1,8 +1,8 @@
+/** @import { Request } from "express" */
+
 import { parse } from "cookie";
 import acceptLanguageParser from "accept-language-parser";
 
-import { Request } from "express";
-
 import {
   DEFAULT_LOCALE,
   VALID_LOCALES,
@@ -11,9 +11,13 @@ import {
 
 const VALID_LOCALES_LIST = [...VALID_LOCALES.values()];
 
-// From https://github.com/aws-samples/cloudfront-authorization-at-edge/blob/01c1bc843d478977005bde86f5834ce76c479eec/src/lambda-edge/shared/shared.ts#L216
-// but rewritten in JavaScript (from TypeScript).
-function extractCookiesFromHeaders(headers: { cookie?: any }) {
+/**
+ * From https://github.com/aws-samples/cloudfront-authorization-at-edge/blob/01c1bc843d478977005bde86f5834ce76c479eec/src/lambda-edge/shared/shared.ts#L216
+ * but rewritten in JavaScript (from TypeScript).
+ * @param {{ cookie?: any }} headers
+ * @returns {Record<string, string>}
+ */
+function extractCookiesFromHeaders(headers) {
   let value = headers["cookie"];
 
   // Cookies are present in the HTTP header "Cookie" that may be present multiple times.
@@ -34,19 +38,32 @@ function extractCookiesFromHeaders(headers: { cookie?: any }) {
 
   // eslint-disable-next-line unicorn/no-array-reduce
   const cookies = value.reduce(
-    (reduced: Record<string, string>, header: { value: any }) =>
+    /**
+     * @param {Record<string, string>} reduced
+     * @param {{ value: string }} header
+     */
+    (reduced, header) =>
       Object.assign(reduced, parse(header.value)),
-    {}
+    /** @type {Record<string, string>} */ ({})
   );
 
   return cookies;
 }
 
-function getCookie(request: Request, cookieKey: string) {
-  return extractCookiesFromHeaders(request.headers as any)[cookieKey];
+/**
+ * @param {Request} request
+ * @param {string} cookieKey
+ * @returns {string | undefined}
+ */
+function getCookie(request, cookieKey) {
+  return extractCookiesFromHeaders(request.headers)[cookieKey];
 }
 
-function getAcceptLanguage(request: Request) {
+/**
+ * @param {Request} request
+ * @returns {string | null}
+ */
+function getAcceptLanguage(request) {
   const acceptLangHeaders = request.headers["accept-language"];
 
   if (typeof acceptLangHeaders === "string") {
@@ -58,14 +75,19 @@ function getAcceptLanguage(request: Request) {
   return value;
 }
 
-export function getLocale(request: Request, fallback = DEFAULT_LOCALE) {
+/**
+ * @param {Request} request
+ * @param {string} [fallback]
+ * @returns {string}
+ */
+export function getLocale(request, fallback = DEFAULT_LOCALE) {
   // First try by cookie.
   const cookieLocale = getCookie(request, PREFERRED_LOCALE_COOKIE_NAME);
   if (
     cookieLocale && // If it's valid, stick to it.
     VALID_LOCALES.has(cookieLocale.toLowerCase())
   ) {
-    return VALID_LOCALES.get(cookieLocale.toLowerCase());
+    return /** @type {string} */ (VALID_LOCALES.get(cookieLocale.toLowerCase()));
   }
 
   // Each header in request.headers is always a list of objects.
@@ -76,6 +98,10 @@ export function getLocale(request: Request, fallback = DEFAULT_LOCALE) {
   return locale || fallback;
 }
 
-export function isValidLocale(locale: any) {
+/**
+ * @param {any} locale
+ * @returns {boolean}
+ */
+export function isValidLocale(locale) {
   return typeof locale === "string" && VALID_LOCALES.has(locale.toLowerCase());
 }
diff --git a/cloud-function/src/internal/play/index.ts b/cloud-function/src/internal/play/index.js
similarity index 98%
rename from cloud-function/src/internal/play/index.ts
rename to cloud-function/src/internal/play/index.js
index 073a012..b85ea83 100644
--- a/cloud-function/src/internal/play/index.ts
+++ b/cloud-function/src/internal/play/index.js
@@ -45,6 +45,7 @@ export function withRunnerResponseHeaders(req, res) {
 
 /**
  * @param {Record<string, string[]>} csp
+ * @returns {string}
  */
 function cspToString(csp) {
   return Object.entries(csp)
@@ -80,7 +81,8 @@ export const PLAYGROUND_UNSAFE_CSP_VALUE = cspToString({
 
 /**
  * @param {TemplateStringsArray} strings
- * @param  {any[]} args
+ * @param  {...any} args
+ * @returns {string}
  */
 function html(strings, ...args) {
   return strings
@@ -92,6 +94,7 @@ function html(strings, ...args) {
  * @param {State | null} state
  * @param {string} hrefWithCode
  * @param {string} searchWithState
+ * @returns {string}
  */
 export function renderWarning(state, hrefWithCode, searchWithState) {
   const { css, html: htmlCode, js } = state || { css: "", html: "", js: "" };
@@ -218,6 +221,7 @@ export function renderWarning(state, hrefWithCode, searchWithState) {
 
 /**
  * @param {State | null} [state=null]
+ * @returns {string}
  */
 export function renderHtml(state = null) {
   const {
@@ -531,6 +535,7 @@ export function renderHtml(state = null) {
 
 /**
  * @param {Theme | undefined} [theme]
+ * @returns {string}
  */
 function renderBlank(theme) {
   return html`
@@ -588,6 +593,7 @@ function renderThemeStyles(theme) {
 
 /**
  * @param {string | null} base64String
+ * @returns {Promise<{state: string | null, hash: string | null}>}
  *
  * This is the Node.js version of `client/src/playground/utils.ts`. Keep in sync!
  */
@@ -615,8 +621,8 @@ export async function decompressFromBase64(base64String) {
 const ORIGIN_PLAY_SUFFIX = `.${ORIGIN_PLAY}`;
 
 /**
- *
  * @param {string} hostname
+ * @returns {string}
  */
 function playSubdomain(hostname) {
   if (hostname.endsWith(ORIGIN_PLAY_SUFFIX)) {
@@ -627,6 +633,7 @@ function playSubdomain(hostname) {
 
 /**
  * @param {string} hostname
+ * @returns {boolean}
  */
 function isMDNHost(hostname) {
   return (
@@ -640,6 +647,7 @@ function isMDNHost(hostname) {
 /**
  * @param {express.Request} req
  * @param {express.Response} res
+ * @returns {Promise<any>}
  */
 export async function handleRunner(req, res) {
   const url = new URL(req.url, "https://example.com");
diff --git a/cloud-function/src/internal/pong/cc2ip.ts b/cloud-function/src/internal/pong/cc2ip.js
similarity index 98%
rename from cloud-function/src/internal/pong/cc2ip.ts
rename to cloud-function/src/internal/pong/cc2ip.js
index e1ef207..34a595f 100644
--- a/cloud-function/src/internal/pong/cc2ip.ts
+++ b/cloud-function/src/internal/pong/cc2ip.js
@@ -178,6 +178,10 @@ export const cc2ip = {
   ZW: "41.174.104.223",
 };
 
+/**
+ * @param {string} countryCode
+ * @returns {string}
+ */
 export default function anonymousIpByCC(countryCode) {
   return cc2ip[countryCode] ?? "10.10.10.10";
 }
diff --git a/cloud-function/src/internal/pong/coding.ts b/cloud-function/src/internal/pong/coding.js
similarity index 87%
rename from cloud-function/src/internal/pong/coding.ts
rename to cloud-function/src/internal/pong/coding.js
index 192b202..d5bc98b 100644
--- a/cloud-function/src/internal/pong/coding.ts
+++ b/cloud-function/src/internal/pong/coding.js
@@ -15,6 +15,10 @@ export class Coder {
   constructor(signSecret) {
     this.signSecret = signSecret;
   }
+  /**
+   * @param {string} [s=""]
+   * @returns {string}
+   */
   encodeAndSign(s = "") {
     const hmac = createHmac("sha256", this.signSecret);
     hmac.update(s);
@@ -23,6 +27,10 @@ export class Coder {
     )}`;
   }
 
+  /**
+   * @param {string} [tuple=""]
+   * @returns {string | null}
+   */
   decodeAndVerify(tuple = "") {
     if (tuple === null) {
       return null;
diff --git a/cloud-function/src/internal/pong/image.ts b/cloud-function/src/internal/pong/image.js
similarity index 69%
rename from cloud-function/src/internal/pong/image.ts
rename to cloud-function/src/internal/pong/image.js
index 2378860..8dbd4ab 100644
--- a/cloud-function/src/internal/pong/image.ts
+++ b/cloud-function/src/internal/pong/image.js
@@ -1,5 +1,9 @@
 // @ts-nocheck
 /* global fetch */
+/**
+ * @param {string} src
+ * @returns {Promise<{status: number, buf: ArrayBuffer, contentType: string | null}>}
+ */
 export async function fetchImage(src) {
   const res = await fetch(src);
   const status = res.status;
diff --git a/cloud-function/src/internal/pong/index.ts b/cloud-function/src/internal/pong/index.js
similarity index 100%
rename from cloud-function/src/internal/pong/index.ts
rename to cloud-function/src/internal/pong/index.js
diff --git a/cloud-function/src/internal/pong/pong.ts b/cloud-function/src/internal/pong/pong.js
similarity index 94%
rename from cloud-function/src/internal/pong/pong.ts
rename to cloud-function/src/internal/pong/pong.js
index 83e2162..40e8950 100644
--- a/cloud-function/src/internal/pong/pong.ts
+++ b/cloud-function/src/internal/pong/pong.js
@@ -13,6 +13,12 @@ const PLACEMENTS = {
 // Allow list for client sent keywords.
 const ALLOWED_KEYWORDS = [];
 
+/**
+ * @param {any} client
+ * @param {any} coder
+ * @param {any} env
+ * @returns {Function}
+ */
 export function createPongGetHandler(client, coder, env) {
   return async (body, countryCode, userAgent) => {
     const { keywords = [], pongs = null } = body;
@@ -92,6 +98,10 @@ export function createPongGetHandler(client, coder, env) {
   };
 }
 
+/**
+ * @param {any} coder
+ * @returns {Function}
+ */
 export function createPongClickHandler(coder) {
   return async (params) => {
     const click = coder.decodeAndVerify(params.get("code"));
@@ -103,6 +113,10 @@ export function createPongClickHandler(coder) {
   };
 }
 
+/**
+ * @param {any} coder
+ * @returns {Function}
+ */
 export function createPongViewedHandler(coder) {
   return async (params) => {
     const view = coder.decodeAndVerify(params.get("code"));
diff --git a/cloud-function/src/internal/pong/pong2.ts b/cloud-function/src/internal/pong/pong2.js
similarity index 97%
rename from cloud-function/src/internal/pong/pong2.ts
rename to cloud-function/src/internal/pong/pong2.js
index 7b93f63..51ee8f8 100644
--- a/cloud-function/src/internal/pong/pong2.ts
+++ b/cloud-function/src/internal/pong/pong2.js
@@ -2,6 +2,10 @@
 import he from "he";
 import anonymousIpByCC from "./cc2ip.js";
 
+/**
+ * @param {string | number | any} hash
+ * @returns {string | undefined}
+ */
 function fixupColor(hash) {
   if (typeof hash !== "string" && typeof hash !== "number") {
     return undefined;
@@ -12,6 +16,12 @@ function fixupColor(hash) {
   }
 }
 
+/**
+ * @param {any} zoneKeys
+ * @param {any} coder
+ * @param {any} env
+ * @returns {Function}
+ */
 export function createPong2GetHandler(zoneKeys, coder, env) {
   return async (body, countryCode, userAgent) => {
     let { pongs = null } = body;
@@ -296,6 +306,10 @@ export function createPong2GetHandler(zoneKeys, coder, env) {
   };
 }
 
+/**
+ * @param {any} coder
+ * @returns {Function}
+ */
 export function createPong2ClickHandler(coder) {
   return async (params, countryCode, userAgent) => {
     const code = params.get("code");
@@ -331,6 +345,10 @@ export function createPong2ClickHandler(coder) {
   };
 }
 
+/**
+ * @param {any} coder
+ * @returns {Function}
+ */
 export function createPong2ViewedHandler(coder) {
   return async (params, countryCode, userAgent) => {
     const code = params.get("code");
@@ -357,6 +375,10 @@ export function createPong2ViewedHandler(coder) {
   };
 }
 
+/**
+ * @param {string} payload
+ * @returns {URL}
+ */
 function createURL(payload) {
   if (payload.startsWith("//")) {
     // BSA omitted 'https:' until May 2024.
diff --git a/cloud-function/src/internal/pong/validate-cc2ip.ts b/cloud-function/src/internal/pong/validate-cc2ip.js
similarity index 100%
rename from cloud-function/src/internal/pong/validate-cc2ip.ts
rename to cloud-function/src/internal/pong/validate-cc2ip.js
diff --git a/cloud-function/src/internal/slug-utils/index.ts b/cloud-function/src/internal/slug-utils/index.js
similarity index 67%
rename from cloud-function/src/internal/slug-utils/index.ts
rename to cloud-function/src/internal/slug-utils/index.js
index b86f897..6394a65 100644
--- a/cloud-function/src/internal/slug-utils/index.ts
+++ b/cloud-function/src/internal/slug-utils/index.js
@@ -1,6 +1,11 @@
 import sanitizeFilename from "sanitize-filename";
 
-export function slugToFolder(slug: string, joiner = "/") {
+/**
+ * @param {string} slug
+ * @param {string} [joiner="/"]
+ * @returns {string}
+ */
+export function slugToFolder(slug, joiner = "/") {
   return (
     slug
       // We have slugs with these special characters that would be
@@ -14,17 +19,25 @@ export function slugToFolder(slug: string, joiner = "/") {
 
       .toLowerCase()
       .split("/")
-      .map(input => sanitizeFilename(input))
+      .map((input) => sanitizeFilename(input))
       .join(joiner)
   );
 }
 
-export function decodePath(path: string) {
+/**
+ * @param {string} path
+ * @returns {string}
+ */
+export function decodePath(path) {
   const decoded = path.split("/").map((element) => decodeURIComponent(element)).join("/");
   return decoded;
 }
 
-export function encodePath(path: string) {
+/**
+ * @param {string} path
+ * @returns {string}
+ */
+export function encodePath(path) {
   const decoded = path.split("/").map((element) => encodeURIComponent(element)).join("/");
   return decoded;
 }
diff --git a/cloud-function/src/middlewares/lowercase-pathname.js b/cloud-function/src/middlewares/lowercase-pathname.js
new file mode 100644
index 0000000..fd8dfd8
--- /dev/null
+++ b/cloud-function/src/middlewares/lowercase-pathname.js
@@ -0,0 +1,21 @@
+/** @import { NextFunction, Request, Response } from "express" */
+
+/**
+ * Middleware that lowercases the pathname of the request URL.
+ * Also updates originalUrl to support http-proxy-middleware v2.
+ * @param {Request} req - Express request
+ * @param {Response} _res - Express response
+ * @param {NextFunction} next - Express next function
+ * @returns {Promise<void>}
+ */
+export async function lowercasePathname(req, _res, next) {
+  const urlParsed = new URL(req.url, `${req.protocol}://${req.headers.host}`);
+  if (urlParsed.pathname) {
+    urlParsed.pathname = urlParsed.pathname.toLowerCase();
+    req.url = urlParsed.toString();
+    // Workaround for http-proxy-middleware v2 using `req.originalUrl`.
+    // See: https://github.com/chimurai/http-proxy-middleware/pull/731
+    req.originalUrl = req.url;
+  }
+  next();
+}
diff --git a/cloud-function/src/middlewares/lowercase-pathname.ts b/cloud-function/src/middlewares/lowercase-pathname.ts
deleted file mode 100644
index 0d311b3..0000000
--- a/cloud-function/src/middlewares/lowercase-pathname.ts
+++ /dev/null
@@ -1,17 +0,0 @@
-import { NextFunction, Request, Response } from "express";
-
-export async function lowercasePathname(
-  req: Request,
-  _res: Response,
-  next: NextFunction
-) {
-  const urlParsed = new URL(req.url, `${req.protocol}://${req.headers.host}`);
-  if (urlParsed.pathname) {
-    urlParsed.pathname = urlParsed.pathname.toLowerCase();
-    req.url = urlParsed.toString();
-    // Workaround for http-proxy-middleware v2 using `req.originalUrl`.
-    // See: https://github.com/chimurai/http-proxy-middleware/pull/731
-    req.originalUrl = req.url;
-  }
-  next();
-}
diff --git a/cloud-function/src/middlewares/not-found.js b/cloud-function/src/middlewares/not-found.js
new file mode 100644
index 0000000..89732cb
--- /dev/null
+++ b/cloud-function/src/middlewares/not-found.js
@@ -0,0 +1,11 @@
+/** @import { Request, Response } from "express" */
+
+/**
+ * Middleware that sends a 404 Not Found response.
+ * @param {Request} _req - Express request
+ * @param {Response} res - Express response
+ * @returns {Promise<void>}
+ */
+export async function notFound(_req, res) {
+  res.sendStatus(404).end();
+}
diff --git a/cloud-function/src/middlewares/not-found.ts b/cloud-function/src/middlewares/not-found.ts
deleted file mode 100644
index 71f0f6e..0000000
--- a/cloud-function/src/middlewares/not-found.ts
+++ /dev/null
@@ -1,5 +0,0 @@
-import type { Request, Response } from "express";
-
-export async function notFound(_req: Request, res: Response) {
-  res.sendStatus(404).end();
-}
diff --git a/cloud-function/src/middlewares/redirect-enforce-trailing-slash.ts b/cloud-function/src/middlewares/redirect-enforce-trailing-slash.js
similarity index 53%
rename from cloud-function/src/middlewares/redirect-enforce-trailing-slash.ts
rename to cloud-function/src/middlewares/redirect-enforce-trailing-slash.js
index 32d1b82..cc0a39e 100644
--- a/cloud-function/src/middlewares/redirect-enforce-trailing-slash.ts
+++ b/cloud-function/src/middlewares/redirect-enforce-trailing-slash.js
@@ -1,13 +1,17 @@
-import { NextFunction, Request, Response } from "express";
+/** @import { NextFunction, Request, Response } from "express" */
 
 import { THIRTY_DAYS } from "../constants.js";
 import { isAsset, redirect } from "../utils.js";
 
-export async function redirectEnforceTrailingSlash(
-  req: Request,
-  res: Response,
-  next: NextFunction
-) {
+/**
+ * Middleware that enforces trailing slashes on non-asset URLs.
+ * Redirects URLs without trailing slashes to the same URL with a trailing slash.
+ * @param {Request} req - Express request
+ * @param {Response} res - Express response
+ * @param {NextFunction} next - Express next function
+ * @returns {Promise<void>}
+ */
+export async function redirectEnforceTrailingSlash(req, res, next) {
   const url = new URL(req.url, `${req.protocol}://${req.headers.host}`);
   const requestURI = url.pathname;
   const qs = url.search;
diff --git a/cloud-function/src/middlewares/redirect-fundamental.ts b/cloud-function/src/middlewares/redirect-fundamental.js
similarity index 68%
rename from cloud-function/src/middlewares/redirect-fundamental.ts
rename to cloud-function/src/middlewares/redirect-fundamental.js
index 5e43cab..1dd655d 100644
--- a/cloud-function/src/middlewares/redirect-fundamental.ts
+++ b/cloud-function/src/middlewares/redirect-fundamental.js
@@ -1,14 +1,18 @@
-import { NextFunction, Request, Response } from "express";
+/** @import { NextFunction, Request, Response } from "express" */
 
 import { THIRTY_DAYS } from "../constants.js";
 import { resolveFundamental } from "../internal/fundamental-redirects/index.js";
 import { redirect } from "../utils.js";
 
-export async function redirectFundamental(
-  req: Request,
-  res: Response,
-  next: NextFunction
-) {
+/**
+ * Middleware that handles fundamental redirects from the redirect mapping.
+ * Preserves query strings when redirecting.
+ * @param {Request} req - Express request
+ * @param {Response} res - Express response
+ * @param {NextFunction} next - Express next function
+ * @returns {Promise<void>}
+ */
+export async function redirectFundamental(req, res, next) {
   const url = new URL(req.url, `${req.protocol}://${req.headers.host}`);
   const requestURI = req.path;
 
diff --git a/cloud-function/src/middlewares/redirect-leading-slash.ts b/cloud-function/src/middlewares/redirect-leading-slash.js
similarity index 60%
rename from cloud-function/src/middlewares/redirect-leading-slash.ts
rename to cloud-function/src/middlewares/redirect-leading-slash.js
index 910c0ba..1a6c343 100644
--- a/cloud-function/src/middlewares/redirect-leading-slash.ts
+++ b/cloud-function/src/middlewares/redirect-leading-slash.js
@@ -1,4 +1,4 @@
-import { NextFunction, Request, Response } from "express";
+/** @import { NextFunction, Request, Response } from "express" */
 
 import { redirect } from "../utils.js";
 
@@ -14,11 +14,16 @@ import { redirect } from "../utils.js";
 // Prevent any pathnames that start with a double //.
 // This essentially means that a request for `GET /////anything` becomes
 // 302 with `Location: /anything`.
-export async function redirectLeadingSlash(
-  req: Request,
-  res: Response,
-  next: NextFunction
-) {
+
+/**
+ * Middleware that prevents security issues from multiple leading slashes.
+ * Normalizes pathnames that start with multiple slashes to a single slash.
+ * @param {Request} req - Express request
+ * @param {Response} res - Express response
+ * @param {NextFunction} next - Express next function
+ * @returns {Promise<void>}
+ */
+export async function redirectLeadingSlash(req, res, next) {
   const pathname = req.url;
   const normalizedPathname = normalizeLeadingSlash(pathname);
   if (pathname !== normalizedPathname) {
@@ -28,6 +33,11 @@ export async function redirectLeadingSlash(
   next();
 }
 
-function normalizeLeadingSlash(pathname: string): string {
+/**
+ * Normalizes multiple leading slashes to a single slash.
+ * @param {string} pathname - The pathname to normalize
+ * @returns {string} The normalized pathname
+ */
+function normalizeLeadingSlash(pathname) {
   return pathname.replace(/^(\/|%2f)+/i, "/");
 }
diff --git a/cloud-function/src/middlewares/redirect-locale.ts b/cloud-function/src/middlewares/redirect-locale.js
similarity index 81%
rename from cloud-function/src/middlewares/redirect-locale.ts
rename to cloud-function/src/middlewares/redirect-locale.js
index 999c268..f158c87 100644
--- a/cloud-function/src/middlewares/redirect-locale.ts
+++ b/cloud-function/src/middlewares/redirect-locale.js
@@ -1,4 +1,4 @@
-import { NextFunction, Request, Response } from "express";
+/** @import { NextFunction, Request, Response } from "express" */
 
 import { getLocale } from "../internal/locale-utils/index.js";
 import { VALID_LOCALES } from "../internal/constants/index.js";
@@ -7,11 +7,15 @@ import { redirect } from "../utils.js";
 const NEEDS_LOCALE =
   /^\/(?:blog|curriculum|docs|play|search|settings|plus)(?:$|\/)/;
 
-export async function redirectLocale(
-  req: Request,
-  res: Response,
-  next: NextFunction
-) {
+/**
+ * Middleware that handles locale-related redirects.
+ * Inserts missing locales and corrects locale casing.
+ * @param {Request} req - Express request
+ * @param {Response} res - Express response
+ * @param {NextFunction} next - Express next function
+ * @returns {Promise<void>}
+ */
+export async function redirectLocale(req, res, next) {
   const url = new URL(req.url, `${req.protocol}://${req.headers.host}`);
   const requestURI = url.pathname;
   const requestURILowerCase = requestURI.toLowerCase();
diff --git a/cloud-function/src/middlewares/redirect-moved-pages.ts b/cloud-function/src/middlewares/redirect-moved-pages.js
similarity index 72%
rename from cloud-function/src/middlewares/redirect-moved-pages.ts
rename to cloud-function/src/middlewares/redirect-moved-pages.js
index 284dd60..21d8128 100644
--- a/cloud-function/src/middlewares/redirect-moved-pages.ts
+++ b/cloud-function/src/middlewares/redirect-moved-pages.js
@@ -1,20 +1,25 @@
-import { createRequire } from "node:module";
+/** @import { NextFunction, Request, Response } from "express" */
 
-import { NextFunction, Request, Response } from "express";
+import { createRequire } from "node:module";
 
 import { decodePath } from "../internal/slug-utils/index.js";
 import { THIRTY_DAYS } from "../constants.js";
 import { redirect } from "../utils.js";
 
 const require = createRequire(import.meta.url);
+/** @type {Record<string, string>} */
 const REDIRECTS = require("../../redirects.json");
 const REDIRECT_SUFFIXES = ["/index.json", "/bcd.json", ""];
 
-export async function redirectMovedPages(
-  req: Request,
-  res: Response,
-  next: NextFunction
-) {
+/**
+ * Middleware that handles redirects for moved pages.
+ * Uses the redirects.json mapping generated from _redirects.txt files.
+ * @param {Request} req - Express request
+ * @param {Response} res - Express response
+ * @param {NextFunction} next - Express next function
+ * @returns {Promise<void>}
+ */
+export async function redirectMovedPages(req, res, next) {
   // Important: The requestURI may be URI-encoded.
   // Example:
   // - Encoded: /zh-TW/docs/AJAX:%E4%B8%8A%E6%89%8B%E7%AF%87
diff --git a/cloud-function/src/middlewares/redirect-non-canonicals.ts b/cloud-function/src/middlewares/redirect-non-canonicals.js
similarity index 64%
rename from cloud-function/src/middlewares/redirect-non-canonicals.ts
rename to cloud-function/src/middlewares/redirect-non-canonicals.js
index 5739098..0d287f6 100644
--- a/cloud-function/src/middlewares/redirect-non-canonicals.ts
+++ b/cloud-function/src/middlewares/redirect-non-canonicals.js
@@ -1,4 +1,4 @@
-import { NextFunction, Request, Response } from "express";
+/** @import { NextFunction, Request, Response } from "express" */
 
 import { THIRTY_DAYS } from "../constants.js";
 import { normalizePath, redirect } from "../utils.js";
@@ -6,11 +6,15 @@ import { CANONICALS } from "../canonicals.js";
 
 const REDIRECT_SUFFIXES = ["/index.json", "/bcd.json", ""];
 
-export async function redirectNonCanonicals(
-  req: Request,
-  res: Response,
-  next: NextFunction
-) {
+/**
+ * Middleware that redirects non-canonical URLs to their canonical versions.
+ * Handles URLs with different suffixes (index.json, bcd.json, or none).
+ * @param {Request} req - Express request
+ * @param {Response} res - Express response
+ * @param {NextFunction} next - Express next function
+ * @returns {Promise<void>}
+ */
+export async function redirectNonCanonicals(req, res, next) {
   const parsedUrl = new URL(req.url, `${req.protocol}://${req.headers.host}/`);
   const { pathname } = parsedUrl;
 
@@ -44,7 +48,13 @@ export async function redirectNonCanonicals(
   next();
 }
 
-function joinPath(a: string, b: string) {
+/**
+ * Joins two path segments, handling overlapping slashes.
+ * @param {string} a - First path segment
+ * @param {string} b - Second path segment
+ * @returns {string} The joined path
+ */
+function joinPath(a, b) {
   if (a.endsWith("/") && b.startsWith("/")) {
     return a + b.slice(1);
   } else {
diff --git a/cloud-function/src/middlewares/redirect-preferred-locale.ts b/cloud-function/src/middlewares/redirect-preferred-locale.js
similarity index 62%
rename from cloud-function/src/middlewares/redirect-preferred-locale.ts
rename to cloud-function/src/middlewares/redirect-preferred-locale.js
index 40a2360..2b81dd1 100644
--- a/cloud-function/src/middlewares/redirect-preferred-locale.ts
+++ b/cloud-function/src/middlewares/redirect-preferred-locale.js
@@ -1,12 +1,17 @@
-import { NextFunction, Request, Response } from "express";
+/** @import { NextFunction, Request, Response } from "express" */
+
 import { normalizePath, redirect } from "../utils.js";
 import { CANONICALS } from "../canonicals.js";
 
-export async function redirectPreferredLocale(
-  req: Request,
-  res: Response,
-  next: NextFunction
-) {
+/**
+ * Middleware that redirects to the user's preferred locale if available.
+ * Checks the 'preferredlocale' cookie and redirects if the content exists in that locale.
+ * @param {Request} req - Express request
+ * @param {Response} res - Express response
+ * @param {NextFunction} next - Express next function
+ * @returns {Promise<void>}
+ */
+export async function redirectPreferredLocale(req, res, next) {
   // Check 1: Does the user prefer a locale and has redirect enabled?
 
   const preferredLocale = req.cookies["preferredlocale"];
@@ -39,7 +44,12 @@ export async function redirectPreferredLocale(
   next();
 }
 
-function localeAndSlugOf(url: URL): [string, string] {
+/**
+ * Extracts the locale and slug from a URL.
+ * @param {URL} url - The URL to parse
+ * @returns {[string, string]} A tuple of [locale, slug]
+ */
+function localeAndSlugOf(url) {
   const locale = url.pathname.split("/").at(1) || "";
   const slug = url.pathname.split("/").slice(2).join("/");
 
diff --git a/cloud-function/src/middlewares/redirect-trailing-slash.ts b/cloud-function/src/middlewares/redirect-trailing-slash.js
similarity index 85%
rename from cloud-function/src/middlewares/redirect-trailing-slash.ts
rename to cloud-function/src/middlewares/redirect-trailing-slash.js
index f0a94b4..e2d1ece 100644
--- a/cloud-function/src/middlewares/redirect-trailing-slash.ts
+++ b/cloud-function/src/middlewares/redirect-trailing-slash.js
@@ -1,4 +1,4 @@
-import { NextFunction, Request, Response } from "express";
+/** @import { NextFunction, Request, Response } from "express" */
 
 import { THIRTY_DAYS } from "../constants.js";
 import { VALID_LOCALES } from "../internal/constants/index.js";
@@ -25,11 +25,16 @@ const LEGACY_URI_NEEDING_TRAILING_SLASH = new RegExp(
   )})?/(?:account|contribute|maintenance-mode|payments)/?$`
 );
 
-export async function redirectTrailingSlash(
-  req: Request,
-  res: Response,
-  next: NextFunction
-) {
+/**
+ * Middleware that handles trailing slash redirects.
+ * Home pages require trailing slashes, all other pages should not have them.
+ * Special handling for locale home pages and legacy URIs.
+ * @param {Request} req - Express request
+ * @param {Response} res - Express response
+ * @param {NextFunction} next - Express next function
+ * @returns {Promise<void>}
+ */
+export async function redirectTrailingSlash(req, res, next) {
   const url = new URL(req.url, `${req.protocol}://${req.headers.host}`);
   let requestURI = url.pathname;
   const requestURILowerCase = requestURI.toLowerCase();
diff --git a/cloud-function/src/middlewares/require-origin.js b/cloud-function/src/middlewares/require-origin.js
new file mode 100644
index 0000000..f60eea1
--- /dev/null
+++ b/cloud-function/src/middlewares/require-origin.js
@@ -0,0 +1,26 @@
+/** @import { NextFunction, Request, Response } from "express" */
+/** @import { OriginType } from "../env.js" */
+
+import { WILDCARD_ENABLED, getOriginFromRequest } from "../env.js";
+
+/**
+ * Creates a middleware that requires the request to come from one of the expected origins.
+ * Returns 404 if the origin doesn't match (unless WILDCARD_ENABLED is true).
+ * @param {...OriginType} expectedOrigins - The allowed origin types
+ * @returns {(req: Request, res: Response, next: NextFunction) => void} Express middleware function
+ */
+export function requireOrigin(...expectedOrigins) {
+  return async (req, res, next) => {
+    if (WILDCARD_ENABLED) {
+      return next();
+    }
+
+    const actualOrigin = getOriginFromRequest(req);
+
+    if (expectedOrigins.includes(actualOrigin)) {
+      return next();
+    } else {
+      return res.sendStatus(404).end();
+    }
+  };
+}
diff --git a/cloud-function/src/middlewares/require-origin.ts b/cloud-function/src/middlewares/require-origin.ts
deleted file mode 100644
index 79dd79d..0000000
--- a/cloud-function/src/middlewares/require-origin.ts
+++ /dev/null
@@ -1,19 +0,0 @@
-import type { NextFunction, Request, Response } from "express";
-
-import { Origin, WILDCARD_ENABLED, getOriginFromRequest } from "../env.js";
-
-export function requireOrigin(...expectedOrigins: Origin[]) {
-  return async (req: Request, res: Response, next: NextFunction) => {
-    if (WILDCARD_ENABLED) {
-      return next();
-    }
-
-    const actualOrigin = getOriginFromRequest(req);
-
-    if (expectedOrigins.includes(actualOrigin)) {
-      return next();
-    } else {
-      return res.sendStatus(404).end();
-    }
-  };
-}
diff --git a/cloud-function/src/middlewares/resolve-index-html.ts b/cloud-function/src/middlewares/resolve-index-html.js
similarity index 57%
rename from cloud-function/src/middlewares/resolve-index-html.ts
rename to cloud-function/src/middlewares/resolve-index-html.js
index 5594d64..9a2e29b 100644
--- a/cloud-function/src/middlewares/resolve-index-html.ts
+++ b/cloud-function/src/middlewares/resolve-index-html.js
@@ -1,15 +1,20 @@
-import * as path from "node:path";
+/** @import { NextFunction, Request, Response } from "express" */
 
-import { NextFunction, Request, Response } from "express";
+import * as path from "node:path";
 
 import { slugToFolder } from "../internal/slug-utils/index.js";
 import { isAsset } from "../utils.js";
 
-export async function resolveIndexHTML(
-  req: Request,
-  _res: Response,
-  next: NextFunction
-) {
+/**
+ * Middleware that resolves document URLs to index.html paths.
+ * Converts URL slugs to folder paths and appends index.html for non-asset requests.
+ * Also updates originalUrl to support http-proxy-middleware v2.
+ * @param {Request} req - Express request
+ * @param {Response} _res - Express response
+ * @param {NextFunction} next - Express next function
+ * @returns {Promise<void>}
+ */
+export async function resolveIndexHTML(req, _res, next) {
   const urlParsed = new URL(req.url, `${req.protocol}://${req.headers.host}`);
   if (urlParsed.pathname) {
     let pathname = slugToFolder(urlParsed.pathname);
diff --git a/cloud-function/src/middlewares/stripForwardedHostHeaders.js b/cloud-function/src/middlewares/stripForwardedHostHeaders.js
new file mode 100644
index 0000000..b7bd749
--- /dev/null
+++ b/cloud-function/src/middlewares/stripForwardedHostHeaders.js
@@ -0,0 +1,19 @@
+/** @import { NextFunction, Request, Response } from "express" */
+
+// Don't strip other `X-Forwarded-*` headers.
+const HEADER_REGEXP = /^(x-forwarded-host|forwarded)$/i;
+
+/**
+ * Middleware that strips X-Forwarded-Host and Forwarded headers from requests.
+ * Other X-Forwarded-* headers are left intact.
+ * @param {Request} req - Express request
+ * @param {Response} _res - Express response
+ * @param {NextFunction} next - Express next function
+ * @returns {Promise<void>}
+ */
+export async function stripForwardedHostHeaders(req, _res, next) {
+  Object.keys(req.headers)
+    .filter((name) => HEADER_REGEXP.test(name))
+    .forEach((name) => delete req.headers[name]);
+  next();
+}
diff --git a/cloud-function/src/middlewares/stripForwardedHostHeaders.ts b/cloud-function/src/middlewares/stripForwardedHostHeaders.ts
deleted file mode 100644
index 3d15b26..0000000
--- a/cloud-function/src/middlewares/stripForwardedHostHeaders.ts
+++ /dev/null
@@ -1,15 +0,0 @@
-import { NextFunction, Request, Response } from "express";
-
-// Don't strip other `X-Forwarded-*` headers.
-const HEADER_REGEXP = /^(x-forwarded-host|forwarded)$/i;
-
-export async function stripForwardedHostHeaders(
-  req: Request,
-  _res: Response,
-  next: NextFunction
-) {
-  Object.keys(req.headers)
-    .filter((name) => HEADER_REGEXP.test(name))
-    .forEach((name) => delete req.headers[name]);
-  next();
-}
diff --git a/cloud-function/src/proxy.ts b/cloud-function/src/proxy.js
similarity index 100%
rename from cloud-function/src/proxy.ts
rename to cloud-function/src/proxy.js
diff --git a/cloud-function/src/stripe-plans/index.ts b/cloud-function/src/stripe-plans/index.d.ts
similarity index 100%
rename from cloud-function/src/stripe-plans/index.ts
rename to cloud-function/src/stripe-plans/index.d.ts
diff --git a/cloud-function/src/stripe-plans/prod.ts b/cloud-function/src/stripe-plans/prod.js
similarity index 99%
rename from cloud-function/src/stripe-plans/prod.ts
rename to cloud-function/src/stripe-plans/prod.js
index 6fa3d7a..9627991 100644
--- a/cloud-function/src/stripe-plans/prod.ts
+++ b/cloud-function/src/stripe-plans/prod.js
@@ -1,6 +1,7 @@
-import type { LookupData } from "./index.js";
+/** @import { LookupData } from "./index.d.ts" */
 
-const plans: LookupData = {
+/** @type {LookupData} */
+const plans = {
   countryToCurrency: {
     AS: {
       currency: "usd",
diff --git a/cloud-function/src/stripe-plans/stage.ts b/cloud-function/src/stripe-plans/stage.js
similarity index 99%
rename from cloud-function/src/stripe-plans/stage.ts
rename to cloud-function/src/stripe-plans/stage.js
index 1d329b7..db52195 100644
--- a/cloud-function/src/stripe-plans/stage.ts
+++ b/cloud-function/src/stripe-plans/stage.js
@@ -1,6 +1,7 @@
-import type { LookupData } from "./index.js";
+/** @import { LookupData } from "./index.d.ts" */
 
-const plans: LookupData = {
+/** @type {LookupData} */
+const plans = {
   countryToCurrency: {
     AS: {
       currency: "usd",
diff --git a/cloud-function/src/utils.ts b/cloud-function/src/utils.js
similarity index 58%
rename from cloud-function/src/utils.ts
rename to cloud-function/src/utils.js
index 893570e..cbc950a 100644
--- a/cloud-function/src/utils.ts
+++ b/cloud-function/src/utils.js
@@ -1,4 +1,4 @@
-import { Request, Response } from "express";
+/** @import { Request, Response } from "express" */
 
 import {
   ANY_ATTACHMENT_EXT,
@@ -7,7 +7,12 @@ import {
 
 import { DEFAULT_COUNTRY } from "./constants.js";
 
-export function getRequestCountry(req: Request): string {
+/**
+ * Get the country code from the request headers
+ * @param {Request} req - Express request object
+ * @returns {string} Two-letter country code
+ */
+export function getRequestCountry(req) {
   const value = req.headers["cloudfront-viewer-country"];
 
   if (typeof value === "string" && value !== "ZZ") {
@@ -17,11 +22,20 @@ export function getRequestCountry(req: Request): string {
   }
 }
 
+/**
+ * Send a redirect response with appropriate caching headers
+ * @param {Response} res - Express response object
+ * @param {string} location - Redirect location URL
+ * @param {object} options - Redirect options
+ * @param {number} [options.status=302] - HTTP status code
+ * @param {number} [options.cacheControlSeconds=0] - Cache duration in seconds
+ * @returns {void}
+ */
 export function redirect(
-  res: Response,
-  location: string,
+  res,
+  location,
   { status = 302, cacheControlSeconds = 0 } = {}
-): void {
+) {
   let cacheControlValue;
   if (cacheControlSeconds) {
     cacheControlValue = `max-age=${cacheControlSeconds},public`;
@@ -44,7 +58,12 @@ export function redirect(
   res.set("Cache-Control", cacheControlValue).redirect(status, newLocation);
 }
 
-export function isLiveSampleURL(url: string) {
+/**
+ * Check if a URL is a live sample URL
+ * @param {string} url - URL to check
+ * @returns {boolean} True if the URL contains '/_sample_.'
+ */
+export function isLiveSampleURL(url) {
   return url.includes("/_sample_.");
 }
 
@@ -56,10 +75,20 @@ const ANY_ATTACHMENT_REGEXP = createRegExpFromExtensions(
   ...TEXT_EXT
 );
 
-export function isAsset(url: string) {
+/**
+ * Check if a URL points to an asset file
+ * @param {string} url - URL to check
+ * @returns {boolean} True if the URL matches asset file extensions
+ */
+export function isAsset(url) {
   return ANY_ATTACHMENT_REGEXP.test(url);
 }
 
-export function normalizePath(path: string) {
+/**
+ * Normalize a path by lowercasing and removing trailing slash
+ * @param {string} path - Path to normalize
+ * @returns {string} Normalized path
+ */
+export function normalizePath(path) {
   return path.toLowerCase().replace(/\/$/, "");
 }
diff --git a/cloud-function/tsconfig.json b/cloud-function/tsconfig.json
index c96df50..82a3fba 100644
--- a/cloud-function/tsconfig.json
+++ b/cloud-function/tsconfig.json
@@ -1,6 +1,5 @@
 {
   "$schema": "https://json.schemastore.org/tsconfig",
-  "display": "Node 18 + ESM + Strictest",
   "compilerOptions": {
     "lib": ["es2022", "DOM"],
     "module": "NodeNext",
@@ -20,10 +19,9 @@
     "noUncheckedIndexedAccess": true,
     "noUnusedLocals": true,
     "noUnusedParameters": true,
-    "checkJs": true
-  },
-  "ts-node": {
-    "esm": true,
-    "swc": true
+    "checkJs": true,
+    "allowJs": true,
+    "maxNodeModuleJsDepth": 0,
+    "noEmit": true
   }
 }