diff --git a/client/package-lock.json b/client/package-lock.json
index 6ce8a0cfbe..b90ef6d654 100644
--- a/client/package-lock.json
+++ b/client/package-lock.json
@@ -5013,6 +5013,12 @@
         "@types/responselike": "*"
       }
     },
+    "@types/chai": {
+      "version": "4.2.21",
+      "resolved": "https://registry.npmjs.org/@types/chai/-/chai-4.2.21.tgz",
+      "integrity": "sha512-yd+9qKmJxm496BOV9CMNaey8TWsikaZOwMRwPHQIjcOJM9oV+fi9ZMNw3JsVnbEEbo2gRTDnGEBv8pjyn67hNg==",
+      "dev": true
+    },
     "@types/common-tags": {
       "version": "1.8.1",
       "resolved": "https://registry.npmjs.org/@types/common-tags/-/common-tags-1.8.1.tgz",
diff --git a/client/package.json b/client/package.json
index c3fca36d48..27f3c9fc09 100644
--- a/client/package.json
+++ b/client/package.json
@@ -131,6 +131,7 @@
     "@codesee/tracker": "0.112.1",
     "@testing-library/jest-dom": "5.14.1",
     "@testing-library/react": "12.1.2",
+    "@types/chai": "^4.2.21",
     "@types/jest": "26.0.24",
     "@types/loadable__component": "5.13.4",
     "@types/lodash-es": "4.17.5",
diff --git a/client/src/client/workers/sass-compile.js b/client/src/client/workers/sass-compile.js
deleted file mode 100644
index c20cb38895..0000000000
--- a/client/src/client/workers/sass-compile.js
+++ /dev/null
@@ -1,27 +0,0 @@
-// work around for SASS error in Edge
-// https://github.com/medialize/sass.js/issues/96#issuecomment-424386171
-if (!self.crypto) {
-  self.crypto = {
-    getRandomValues: function (array) {
-      for (var i = 0, l = array.length; i < l; i++) {
-        array[i] = Math.floor(Math.random() * 256);
-      }
-      return array;
-    }
-  };
-}
-
-self.importScripts('/js/sass.sync.js');
-
-self.onmessage = e => {
-  const data = e.data;
-  self.Sass.compile(data, result => {
-    if (result.status === 0) {
-      self.postMessage(result.text);
-    } else {
-      self.postMessage({ type: 'error', data: { message: result.formatted } });
-    }
-  });
-};
-
-self.postMessage({ type: 'contentLoaded' });
diff --git a/client/src/client/workers/sass-compile.ts b/client/src/client/workers/sass-compile.ts
new file mode 100644
index 0000000000..181a20634b
--- /dev/null
+++ b/client/src/client/workers/sass-compile.ts
@@ -0,0 +1,40 @@
+/* eslint-disable import/unambiguous */
+// work around for SASS error in Edge
+// https://github.com/medialize/sass.js/issues/96#issuecomment-424386171
+interface WorkerWithSass extends Worker {
+  Sass: {
+    compile(data: unknown, callback: unknown): void;
+  };
+}
+
+const ctx: WorkerWithSass & typeof globalThis =
+  self as unknown as WorkerWithSass & typeof globalThis;
+
+if (!ctx.crypto) {
+  (ctx.crypto as unknown) = {
+    getRandomValues: function (array: number[]) {
+      for (let i = 0, l = array.length; i < l; i++) {
+        array[i] = Math.floor(Math.random() * 256);
+      }
+      return array;
+    }
+  };
+}
+
+ctx.importScripts('/js/sass.sync.js');
+
+ctx.onmessage = e => {
+  const data: unknown = e.data;
+  ctx.Sass.compile(data, (result: Record<string, unknown>) => {
+    if (result.status === 0) {
+      ctx.postMessage(result.text);
+    } else {
+      ctx.postMessage({
+        type: 'error',
+        data: { message: result.formatted }
+      });
+    }
+  });
+};
+
+ctx.postMessage({ type: 'contentLoaded' });
diff --git a/client/src/client/workers/test-evaluator.js b/client/src/client/workers/test-evaluator.ts
similarity index 63%
rename from client/src/client/workers/test-evaluator.js
rename to client/src/client/workers/test-evaluator.ts
index bc1189cbe7..13ea866e24 100644
--- a/client/src/client/workers/test-evaluator.js
+++ b/client/src/client/workers/test-evaluator.ts
@@ -6,13 +6,16 @@ import curriculumHelpers, {
 } from '../../utils/curriculum-helpers';
 import { format as __format } from '../../utils/format';
 
+const ctx: Worker & typeof globalThis = self as unknown as Worker &
+  typeof globalThis;
+
 const __utils = (() => {
   const MAX_LOGS_SIZE = 64 * 1024;
 
-  let logs = [];
+  let logs: string[] = [];
   function flushLogs() {
     if (logs.length) {
-      self.postMessage({
+      ctx.postMessage({
         type: 'LOG',
         data: logs.join('\n')
       });
@@ -20,8 +23,8 @@ const __utils = (() => {
     }
   }
 
-  const oldLog = self.console.log.bind(self.console);
-  function proxyLog(...args) {
+  const oldLog = ctx.console.log.bind(ctx.console);
+  function proxyLog(...args: string[]) {
     logs.push(args.map(arg => __format(arg)).join(' '));
     if (logs.join('\n').length > MAX_LOGS_SIZE) {
       flushLogs();
@@ -30,12 +33,12 @@ const __utils = (() => {
   }
 
   // unless data.type is truthy, this sends data out to the testRunner
-  function postResult(data) {
+  function postResult(data: unknown) {
     flushLogs();
-    self.postMessage(data);
+    ctx.postMessage(data);
   }
 
-  function log(...msgs) {
+  function log(...msgs: Error[]) {
     if (msgs && msgs[0] && !(msgs[0] instanceof chai.AssertionError)) {
       // discards the stack trace via toString as it only useful to debug the
       // site, not a specific challenge.
@@ -43,8 +46,8 @@ const __utils = (() => {
     }
   }
 
-  const toggleProxyLogger = on => {
-    self.console.log = on ? proxyLog : oldLog;
+  const toggleProxyLogger = (on: unknown) => {
+    ctx.console.log = on ? proxyLog : oldLog;
   };
 
   return {
@@ -55,9 +58,25 @@ const __utils = (() => {
   };
 })();
 
+interface TestEvaluatorEvent extends MessageEvent {
+  data: {
+    code: {
+      contents: string;
+      editableContents: string;
+    };
+    removeComments: boolean;
+    firstTest: unknown;
+    testString: string;
+    build: string;
+    sources: {
+      [fileName: string]: unknown;
+    };
+  };
+}
+
 /* Run the test if there is one.  If not just evaluate the user code */
-self.onmessage = async e => {
-  /* eslint-disable no-unused-vars */
+ctx.onmessage = async (e: TestEvaluatorEvent) => {
+  /* eslint-disable @typescript-eslint/no-unused-vars */
   let code = (e.data?.code?.contents || '').slice();
   code = e.data?.removeComments ? removeJSComments(code) : code;
   let editableContents = (e.data?.code?.editableContents || '').slice();
@@ -68,13 +87,16 @@ self.onmessage = async e => {
   const assert = chai.assert;
   const __helpers = curriculumHelpers;
   // Fake Deep Equal dependency
-  const DeepEqual = (a, b) => JSON.stringify(a) === JSON.stringify(b);
+  const DeepEqual = (a: unknown, b: unknown) =>
+    JSON.stringify(a) === JSON.stringify(b);
 
   // Build errors should be reported, but only once:
   __utils.toggleProxyLogger(e.data.firstTest);
-  /* eslint-enable no-unused-vars */
+  /* eslint-enable @typescript-eslint/no-unused-vars */
   try {
     let testResult;
+    // This can be reassigned by the eval inside the try block, so it should be declared as a let
+    // eslint-disable-next-line prefer-const
     let __userCodeWasExecuted = false;
     /* eslint-disable no-eval */
     try {
@@ -86,7 +108,7 @@ self.onmessage = async e => {
 __utils.flushLogs();
 __userCodeWasExecuted = true;
 __utils.toggleProxyLogger(true);
-${e.data.testString}`);
+${e.data.testString}`) as unknown;
     } catch (err) {
       if (__userCodeWasExecuted) {
         // rethrow error, since test failed.
@@ -95,19 +117,21 @@ ${e.data.testString}`);
       // log build errors unless they're related to import/export/require (there
       // are challenges that use them and they should not trigger warnings)
       if (
-        err.name !== 'ReferenceError' ||
-        (err.message !== 'require is not defined' &&
-          err.message !== 'exports is not defined')
+        (err as Error).name !== 'ReferenceError' ||
+        ((err as Error).message !== 'require is not defined' &&
+          (err as Error).message !== 'exports is not defined')
       ) {
-        __utils.log(err);
+        __utils.log(err as Error);
       }
       // the tests may not require working code, so they are evaluated even if
       // the user code does not get executed.
-      testResult = eval(e.data.testString);
+      testResult = eval(e.data.testString) as unknown;
     }
     /* eslint-enable no-eval */
     if (typeof testResult === 'function') {
-      await testResult(fileName => __toString(e.data.sources[fileName]));
+      await testResult((fileName: string) =>
+        __toString(e.data.sources[fileName])
+      );
     }
     __utils.postResult({
       pass: true
@@ -117,15 +141,15 @@ ${e.data.testString}`);
     __utils.toggleProxyLogger(false);
     // Report execution errors in case user code has errors that are only
     // uncovered during testing.
-    __utils.log(err);
+    __utils.log(err as Error);
     // postResult flushes the logs and must be called after logging is finished.
     __utils.postResult({
       err: {
-        message: err.message,
-        stack: err.stack
+        message: (err as Error).message,
+        stack: (err as Error).stack
       }
     });
   }
 };
 
-self.postMessage({ type: 'contentLoaded' });
+ctx.postMessage({ type: 'contentLoaded' });
diff --git a/client/tsconfig.json b/client/tsconfig.json
index 689bc83179..7dff098671 100644
--- a/client/tsconfig.json
+++ b/client/tsconfig.json
@@ -1,6 +1,11 @@
 {
   "include": ["./i18n/**/*", "./plugins/**/*", "./src/**/*", "./utils/**/*"],
   "compilerOptions": {
+    "lib": [
+      "WebWorker",
+      "DOM",
+      "DOM.Iterable"
+    ],
     "target": "es2020",
     "module": "es2020",
     "moduleResolution": "node",
diff --git a/client/webpack-workers.js b/client/webpack-workers.js
index a4e7f70792..4e7cc53375 100644
--- a/client/webpack-workers.js
+++ b/client/webpack-workers.js
@@ -12,8 +12,8 @@ module.exports = (env = {}) => {
     mode: __DEV__ ? 'development' : 'production',
     entry: {
       'frame-runner': './src/client/frame-runner.js',
-      'sass-compile': './src/client/workers/sass-compile.js',
-      'test-evaluator': './src/client/workers/test-evaluator.js'
+      'sass-compile': './src/client/workers/sass-compile.ts',
+      'test-evaluator': './src/client/workers/test-evaluator.ts'
     },
     devtool: __DEV__ ? 'inline-source-map' : 'source-map',
     output: {