Test ES6 modules with Jest

Question

How to test ES6 modules with Jest.

---

Example:

sum.js

const sum = function (a, b) {
  return a + b;
}

export default sum;

sum.test.js

import sum from './sum';

test('adds 1 + 2 to equal 3', () => {
  expect(sum(1, 2)).toBe(3);
});

Answer 1

[Dec 2023 UPDATE]

Now you can support ES6 and ESM (ECMAScript modules) natively:

Step 1: Prevent Jest from trying to transform ESM code to CommonJS, updating your Jest config (package.json example below) with:

"jest": {
    "transform": {}
}

Step 2: To be able to parse ES modules without an external transformer (e.g., babel), start Node with the --experimental-vm-modules flag. This can be done by changing how Jest is started by the npm "test" script (again inside package.json):

"scripts": {
    "test": "node --experimental-vm-modules node_modules/jest/bin/jest.js"
}

And that's it. :)

You can even uninstall your transformer packages if you were using them just for the tests.

---

[OUTDATED answer, just for historic purposes]

The only requirement is to config your test environment to Babel, and add the es2015 transform plugin:

---

Step 1:

Add your test environment to .babelrc in the root of your project:

{
  "env": {
    "test": {
      "plugins": ["@babel/plugin-transform-modules-commonjs"]
    }
  }
}

Step 2:

Install the es2015 transform plugin:

npm install --save-dev @babel/plugin-transform-modules-commonjs

---

And that's it. Jest will enable compilation from ES modules to CommonJS automatically, without having to inform additional options to your jest property inside package.json.