jest-webpack

A helper tool and webpack plugin to integrate jest and webpack.

Add it to your project as a developer dependency. And run it like webpack or webpack-dev-server from the command line or as a package.json script.

Install

npm install --save-dev jest-webpack

or with yarn

yarn add -D jest-webpack

Update package.json

Add it as a package.json script

{
  "name": "my-package",
  "scripts": {
    "test": "jest-webpack"

Run it

npm test

Run it with jest options

npm test -- --testPathPattern test-just-this-file

Status

jest-webpack currently works with a lot of jest options out of the bag since there is no special handling needed and they can just be passed to jest by the tool. You can see what is so far specifically tested so far in mzgoddard#3.

How it works?

jest-webpack uses a webpack plugin to add related plugins that are responsible for 4 operations.

1. The TestEntriesPlugin finds test files that jest will operate on and creates entry chunks for them.
2. The EntryReferencePlugin creates additional entries for any other files that are depended on by those test files. These entries return objects pointing to the various transformations of that file. If a css file is depended on by both css-loader and css-loader/locals for example, both of the outputs of those loaders will be in the same file. Files then depending on those outputs reference the entry chunk and use the exported member for their needed version of the original file.
3. The EmitChangedAssetsPlugin removes entry chunks that already exist in the destination folder. It also includes package.json for the tested project.
4. The RunJestWhenDonePlugin runs jest when webpack is done. It runs jest from the destination folder so jest uses the webpack transformed files. This lets jest determine what files changed and which runs to test again instead of testing all the files again.

This way of integrating jest and webpack is fairly transparent, requiring little modification to a webpack project. Used along with source-map-support, you can also get source maps.

Special Thanks

Thanks to Colch for letting me take over development of a jest-webpack integration as the jest-webpack npm package.