unit testing - 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

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.