allow moduleFileExtensions without 'js' for custom runners

CHANGELOG.md

@@ -13,6 +13,7 @@
 - `[jest-cli]` Print log entries when logging happens after test environment is torn down ([#7731](https://github.com/facebook/jest/pull/7731))
 - `[jest-config]` Do not use a uuid as `name` since that breaks caching ([#7746](https://github.com/facebook/jest/pull/7746))
 - `[jest-config]` Make sure `normalize` can consume `Defaults` without warnings ([#7742](https://github.com/facebook/jest/pull/7742))
+- `[jest-config]` Allow `moduleFileExtensions` without 'js' for custom runners ([#7751](https://github.com/facebook/jest/pull/7751))
 
 ### Chore & Maintenance
 

packages/jest-config/src/__tests__/normalize.test.js

@@ -1448,16 +1448,32 @@ describe('moduleFileExtensions', () => {
     ]);
   });
 
-  it('throws if missing `js`', () => {
+  it('throws if missing `js` but using jest-runner', () => {
+    [undefined, 'jest-runner'].forEach(runner =>
+      expect(() =>
+        normalize(
+          {
+            rootDir: '/root/',
+            moduleFileExtensions: ['json', 'jsx'],
+            runner,
+          },
+          {},
+        ),
+      ).toThrowError("moduleFileExtensions must include 'js'"),
+    );
+  });
+
+  it('does not throw if missing `js` with a custom runner', () => {
     expect(() =>
       normalize(
         {
           rootDir: '/root/',
           moduleFileExtensions: ['json', 'jsx'],
+          runner: './', // does not need to be a valid runner for this validation
         },
         {},
       ),
-    ).toThrowError("moduleFileExtensions must include 'js'");
+    ).not.toThrow();
   });
 });
 

packages/jest-config/src/normalize.js

@@ -650,8 +650,12 @@ export default function normalize(
       case 'moduleFileExtensions': {
         value = options[key];
 
-        // If it's the wrong type, it can throw at a later time
-        if (Array.isArray(value) && !value.includes('js')) {
+        if (
+          Array.isArray(value) && // If it's the wrong type, it can throw at a later time
+          (options.runner === undefined ||
+            options.runner === DEFAULT_CONFIG.runner) && // Only require 'js' for the default jest-runner
+          !value.includes('js')
+        ) {
           const errorMessage =
             `  moduleFileExtensions must include 'js':\n` +
             `  but instead received:\n` +