diff --git a/package.json b/package.json
index 5ba07c6cf37..a733f95a5aa 100644
--- a/package.json
+++ b/package.json
@@ -11,7 +11,7 @@
     "build:dev": "yarn clean && git rev-parse HEAD > git-revision.txt && yarn build:compile && yarn build:types",
     "build:types": "tsc -p tsconfig-build.json --emitDeclarationOnly",
     "build:compile": "babel -d lib --verbose --extensions \".ts,.js\" src",
-    "build:compile-browser": "mkdirp dist && browserify -d src/browser-index.js -p [ tsify -p ./tsconfig-build.json ] -t [ babelify --sourceMaps=inline --presets [ @babel/preset-env @babel/preset-typescript ] ] | exorcist dist/browser-matrix.js.map > dist/browser-matrix.js",
+    "build:compile-browser": "mkdirp dist && browserify -d src/browser-index.ts -p [ tsify -p ./tsconfig-build.json ] -t [ babelify --sourceMaps=inline --presets [ @babel/preset-env @babel/preset-typescript ] ] | exorcist dist/browser-matrix.js.map > dist/browser-matrix.js",
     "build:minify-browser": "terser dist/browser-matrix.js --compress --mangle --source-map --output dist/browser-matrix.min.js",
     "gendoc": "jsdoc -c jsdoc.json -P package.json",
     "lint": "yarn lint:types && yarn lint:js",
@@ -32,7 +32,7 @@
   "main": "./src/index.ts",
   "browser": "./lib/browser-index.js",
   "matrix_src_main": "./src/index.ts",
-  "matrix_src_browser": "./src/browser-index.js",
+  "matrix_src_browser": "./src/browser-index.ts",
   "matrix_lib_main": "./lib/index.js",
   "matrix_lib_typings": "./lib/index.d.ts",
   "author": "matrix.org",
@@ -78,6 +78,7 @@
     "@types/bs58": "^4.0.1",
     "@types/jest": "^26.0.20",
     "@types/node": "12",
+    "@types/qs": "^6.9.7",
     "@types/request": "^2.48.5",
     "@typescript-eslint/eslint-plugin": "^4.17.0",
     "@typescript-eslint/parser": "^4.17.0",
diff --git a/src/browser-index.js b/src/browser-index.ts
similarity index 69%
rename from src/browser-index.js
rename to src/browser-index.ts
index dfa3f4d68b2..625de982531 100644
--- a/src/browser-index.js
+++ b/src/browser-index.ts
@@ -16,9 +16,23 @@ limitations under the License.
 
 import * as matrixcs from "./matrix";
 import request from "browser-request";
-import queryString from "qs";
+import queryString, { IStringifyOptions } from "qs";
 
-matrixcs.request(function(opts, fn) {
+interface RequestOptions {
+    qs: string | Record<string | number | symbol, string>;
+    qsStringifyOptions: IStringifyOptions;
+}
+
+enum IndexedDBStoreName {
+    crypto = "matrix-js-sdk:crypto"
+}
+
+type GlobalObject = NodeJS.Global & typeof globalThis & Record<"matrixcs", typeof matrixcs>;
+
+matrixcs.request(function(
+    opts: RequestOptions,
+    fn: (er: Error, response: any, body: any) => any,
+) {
     // We manually fix the query string for browser-request because
     // it doesn't correctly handle cases like ?via=one&via=two. Instead
     // we mimic `request`'s query string interface to make it all work
@@ -31,24 +45,23 @@ matrixcs.request(function(opts, fn) {
 
 // just *accessing* indexedDB throws an exception in firefox with
 // indexeddb disabled.
-let indexedDB;
+let indexedDB: IDBFactory;
 try {
     indexedDB = global.indexedDB;
 } catch (e) {}
 
 // if our browser (appears to) support indexeddb, use an indexeddb crypto store.
 if (indexedDB) {
-    matrixcs.setCryptoStoreFactory(
-        function() {
-            return new matrixcs.IndexedDBCryptoStore(
-                indexedDB, "matrix-js-sdk:crypto",
-            );
-        },
-    );
+    matrixcs.setCryptoStoreFactory(function() {
+        return new matrixcs.IndexedDBCryptoStore(
+            indexedDB,
+            IndexedDBStoreName.crypto,
+        );
+    });
 }
 
 // We export 3 things to make browserify happy as well as downstream projects.
 // It's awkward, but required.
 export * from "./matrix";
 export default matrixcs; // keep export for browserify package deps
-global.matrixcs = matrixcs;
+(global as GlobalObject).matrixcs = matrixcs;
diff --git a/yarn.lock b/yarn.lock
index b57bc4f6ac9..e5502df6909 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -1483,6 +1483,11 @@
   resolved "https://registry.yarnpkg.com/@types/prettier/-/prettier-2.4.1.tgz#e1303048d5389563e130f5bdd89d37a99acb75eb"
   integrity sha512-Fo79ojj3vdEZOHg3wR9ksAMRz4P3S5fDB5e/YWZiFnyFQI1WY2Vftu9XoXVVtJfxB7Bpce/QTqWSSntkz2Znrw==
 
+"@types/qs@^6.9.7":
+  version "6.9.7"
+  resolved "https://registry.yarnpkg.com/@types/qs/-/qs-6.9.7.tgz#63bb7d067db107cc1e457c303bc25d511febf6cb"
+  integrity sha512-FGa1F62FT09qcrueBA6qYTrJPVDzah9a+493+o2PCXsesWHIn27G98TsSMs3WPNbZIEj4+VJf6saSFpvD+3Zsw==
+
 "@types/request@^2.48.5":
   version "2.48.7"
   resolved "https://registry.yarnpkg.com/@types/request/-/request-2.48.7.tgz#a962d11a26e0d71d9a9913d96bb806dc4d4c2f19"