JavaScript: module/require is not defined

Question

JavaScript: module/require is not defined

4.6k Views Asked by mattu At 18 April 2025 at 14:25

I'd like to have Jest testing for some JavaScript code that I'm using in the frontend of my web app. From what I understand, I need to export and import functions from my module so I can test them in Jest, and Jest (only) supports CommonJS module syntax out of the box. As a result, my code looks as follows:

<!-- index.html -->
<!DOCTYPE html>
<html><body><div>Hello World</div>
<script src="./utils.js">
    console.log('foo: ' + foo());
</script>
</body></html>

// utils.js
function foo() {return 42;}
module.exports = {foo};

// __test__/utils.test.js
const utils = require('../utils');
describe('utils', () => {
    test('foo', () => {expect(utils.foo()).toBe(42);});
});

The testing part works, but when I open index.html in the browser I get

Uncaught ReferenceError: module is not defined

My understanding is that the frontend does not support CommonJS module syntax (according to Client on node: Uncaught ReferenceError: require is not defined ). How should I write my utils.js file and exports such that I can:

have Jest testing for components of utils.js
use functions defined in utils.js in the browser

Just to be clear, I do not want to run my tests from the browser.

For context: in my real use-case I run the app using Express and Node.js but this minimal example should capture the underlying problem.

Also, see this related question: Uncaught ReferenceError: module/require is not defined

Original Q&A

There are 2 best solutions below

OtherGuru On 05 August 2021 at 17:53

I was getting similar error with jest on exporting from my main js file

Following worked well for me

if (typeof module !== 'undefined') module.exports = { foo };

PS: I have given up on modularization of JS code as js modules throw cors errors as my (html, js) code runs in a browser process that is remoted to windows application, and more importantly html, js code loads from disk instead of a server.

**ISD** · Accepted Answer

Try checking if module exists:

// utils.js

function foo() { return 42; }

if(module) module.exports = {foo}; // On node.js, use exports
else if(window) window.foo = foo; // In browser, use window
else console.error('Unknown environment');

And in browser:

<!-- index.html -->
<!doctype html>
<html>
  <body>
    <script src="./utils.js"></script>
    <script>
      // Use your foo() here!
      console.log('foo: ', window.foo());
    </script>
  </body>
</html>

JavaScript: module/require is not defined

There are 2 best solutions below

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