Skip to content

Latest commit

 

History

History
71 lines (46 loc) · 2.2 KB

NOTES.md

File metadata and controls

71 lines (46 loc) · 2.2 KB
Raw

Notes

Jest unit test setup

A minimal reproducible bug is available at topheman/jest-esm-experiments.

In the last few years, you must have been using ES6+ features of the JavaScript language on your projects. Tools like Babel and Webpack have made this possible and are now mainstream thanks to toolkits like Create React App.

This project doesn't rely on transpilation or bundler (no Babel, no Webpack), it uses directly the latest APIs of the browsers.

Prerequisite

Due to the following error:

TypeError: Jest: a transform must export a `process` function.

Downgrading to [email protected] - source

BUT ...

When I setup Jest (which is one of those great zero config tools), it didn't understand the import statements (logic, it doesn't exist yet in Node).

So, I installed the great esm package from @jdalton. It enables the support for ECMAScript modules in Node - that way, import and export statements you wrote will be understood by Node.

My jest.config.js would look like this:

module.exports = {
  transform: {
    "\\.m?jsx?$": "esm"
  },
  transformIgnorePatterns: []
};

And I would get the following error:

  • Always on Travis
  • In local (only if I added a 4th test file)
TypeError: Cannot read property 'next' of undefined

After some debugging, I think the problem is with the esm loader. The src/js/common.js file being imported:

  • directly in one of the tests
  • indirectly by the components I'm testing in the 2 other tests

The Fix

I installed babel-jest

yarn add --dev babel-jest @babel/core @babel/preset-env

Added a babel.config.js (only to be used for unit tests):

module.exports = {
  presets: [["@babel/preset-env", { targets: { node: "current" } }]]
};

And finally, you can choose to test relying on:

  • babel (works for the moment): npm test - calls npm run test:babel
  • esm (for investigation): npm run test:esm