diff --git a/client/src/client/workers/test-evaluator.js b/client/src/client/workers/test-evaluator.js
index f43ecba264..2478c52658 100644
--- a/client/src/client/workers/test-evaluator.js
+++ b/client/src/client/workers/test-evaluator.js
@@ -8,7 +8,7 @@ self.console.log = function proxyConsole(...args) {
   return oldLog(...args);
 };
 
-onmessage = async e => {
+self.onmessage = async e => {
   /* eslint-disable no-unused-vars */
   const { code = '' } = e.data;
   const assert = chai.assert;
@@ -16,8 +16,22 @@ onmessage = async e => {
   const DeepEqual = (a, b) => JSON.stringify(a) === JSON.stringify(b);
   /* eslint-enable no-unused-vars */
   try {
-    // eslint-disable-next-line no-eval
-    const testResult = eval(e.data.script);
+    let testResult;
+    let __userCodeWasExecuted = false;
+    /* eslint-disable no-eval */
+    try {
+      testResult = eval(`
+        ${e.data.build}
+        __userCodeWasExecuted = true;
+        ${e.data.testString}
+      `);
+    } catch (err) {
+      if (__userCodeWasExecuted) {
+        throw err;
+      }
+      testResult = eval(e.data.testString);
+    }
+    /* eslint-enable no-eval */
     if (typeof testResult === 'function') {
       await testResult(fileName => __toString(e.data.sources[fileName]));
     }
diff --git a/client/src/templates/Challenges/redux/execute-challenge-saga.js b/client/src/templates/Challenges/redux/execute-challenge-saga.js
index 719715ac42..585a4e3462 100644
--- a/client/src/templates/Challenges/redux/execute-challenge-saga.js
+++ b/client/src/templates/Challenges/redux/execute-challenge-saga.js
@@ -89,7 +89,7 @@ function* executeJSChallengeSaga(proxyLogger) {
     return yield call(executeTests, async(testString, testTimeout) => {
       try {
         return await testWorker.execute(
-          { script: build + '\n' + testString, code, sources },
+          { build, testString, code, sources },
           testTimeout
         );
       } finally {
diff --git a/curriculum/test/test-challenges.js b/curriculum/test/test-challenges.js
index 290b7f6e30..17bdafb91d 100644
--- a/curriculum/test/test-challenges.js
+++ b/curriculum/test/test-challenges.js
@@ -320,13 +320,12 @@ async function evaluateJsTest({ solution, files, test }) {
 
   const { build, sources } = await buildJSChallenge(files);
   const code = sources && 'index' in sources ? sources['index'] : '';
-  const script = build + '\n' + test.testString;
 
   const testWorker = createWorker('test-evaluator');
 
   try {
     const { pass, err } = await testWorker.execute(
-      { script, code, sources },
+      { testString: test.testString, build, code, sources },
       5000
     );
     if (!pass) {