diff --git a/client/src/client/workers/test-evaluator.js b/client/src/client/workers/test-evaluator.js
index 172b42e533..2a8a1eaec6 100644
--- a/client/src/client/workers/test-evaluator.js
+++ b/client/src/client/workers/test-evaluator.js
@@ -43,6 +43,7 @@ const __utils = (() => {
   };
 })();
 
+/* Run the test if there is one.  If not just evaluate the user code */
 self.onmessage = async e => {
   /* eslint-disable no-unused-vars */
   const { code = '' } = e.data;
@@ -64,10 +65,13 @@ self.onmessage = async e => {
       if (__userCodeWasExecuted) {
         // rethrow error, since test failed.
         throw err;
-      } else {
-        // report errors to dev console (not the editor console, since the test
-        // may still pass)
+      } else if (e.data.testString) {
+        // report errors to dev console if tests are running (since some
+        // challenges should pass with code that throws errors)
         __utils.oldLog(err);
+      } else {
+        // user is editing code, so both consoles should report errors
+        console.log(err.toString());
       }
       testResult = eval(e.data.testString);
     }
diff --git a/client/src/templates/Challenges/redux/execute-challenge-saga.js b/client/src/templates/Challenges/redux/execute-challenge-saga.js
index d68a77ca3b..e4dbf9a771 100644
--- a/client/src/templates/Challenges/redux/execute-challenge-saga.js
+++ b/client/src/templates/Challenges/redux/execute-challenge-saga.js
@@ -29,7 +29,8 @@ import {
   buildChallenge,
   getTestRunner,
   challengeHasPreview,
-  updatePreview
+  updatePreview,
+  isJavaScriptChallenge
 } from '../utils/build';
 
 export function* executeChallengeSaga() {
@@ -118,6 +119,7 @@ function* executeTests(testRunner, tests, testTimeout = 5000) {
   return testResults;
 }
 
+// updates preview frame and the fcc console.
 function* previewChallengeSaga() {
   yield delay(700);
 
@@ -134,16 +136,17 @@ function* previewChallengeSaga() {
     const proxyLogger = args => consoleProxy.put(args);
     const challengeData = yield select(challengeDataSelector);
 
-    // try to build even if there's no preview so build errors will be reported.
     const buildData = yield buildChallengeData(challengeData);
-    // then only continue if there is a preview.
-    if (!challengeHasPreview(challengeData)) {
-      return;
+    // evaluate the user code in the preview frame or in the worker
+    if (challengeHasPreview(challengeData)) {
+      const document = yield getContext('document');
+      yield call(updatePreview, buildData, document, proxyLogger);
+    } else if (isJavaScriptChallenge(challengeData)) {
+      const runUserCode = getTestRunner(buildData, proxyLogger);
+      // without a testString the testRunner just evaluates the user's code
+      yield call(runUserCode, null, 5000);
     }
-    const document = yield getContext('document');
-    yield call(updatePreview, buildData, document, proxyLogger);
-    // We don't want to see the default console, so we initialise and output in
-    // one call.
+    // To avoid seeing the default console, initialise and output in one call.
     yield all([put(initConsole('')), put(logsToConsole('// console output'))]);
   } catch (err) {
     console.error(err);
diff --git a/client/src/templates/Challenges/utils/build.js b/client/src/templates/Challenges/utils/build.js
index 07d5689df4..2f6240d356 100644
--- a/client/src/templates/Challenges/utils/build.js
+++ b/client/src/templates/Challenges/utils/build.js
@@ -177,3 +177,7 @@ export function challengeHasPreview({ challengeType }) {
     challengeType === challengeTypes.modern
   );
 }
+
+export function isJavaScriptChallenge({ challengeType }) {
+  return challengeType === challengeTypes.js;
+}