Jest is more than a testing framework. It’s a highly optimized, blazing fast platform with incredible parallelization for running tasks across many files in our project. It has a capability to run more than just tests. We can bring these features to our linting as well. Let’s see how we can bring our favorite Jest features (like watch mode) to ESLint with jest-runner-eslint.

Configure Jest for Testing JavaScript Applications

Run ESLint with Jest using jest-runner-eslint

Instructor: [00:01] Jest is more than just a testing framework. It's an entire platform for running tasks in parallel, because it's running all of your tests in parallel. You can see the benefits of this as your project and your test base grows bigger and bigger over time.

[00:12] In addition to being really efficient, running of all of your tasks in parallel, Jest also has an amazing watch mode you can use, to help you as you develop your software.

[00:20] One of the really awesome features of Jest is the ability to specify a custom runner. By default, Jest has a runner that will run the Jest framework tasks, but you can actually have runners that run ESLint prettier, or even go on Python tasks.

[00:33] It's really remarkable what you can do. Let's go ahead and try this out with our Linting. I'm going to npm install, and save as a dev dependency Jest runner ESLint. 

```bash
npm install --save-dev jest-runner-eslint
```

With that installed, I'm going to go up here into my test directory. I'm going to make a new file called jest.lint.js.

[00:50] This is going to be a little bit different configuration from our others. I'm going to say module that exports equals this object. We want the root, der, to be a path, join, of the der name up one directory. We want the root directory to be this main directory here.

[01:05] Let's go ahead and get our path from require path, the path node module here. We'll just specify our display name to be lint so we can differentiate this configuration from the others. Then we'll specify a runner to be Jest runner is Lint.

[01:20] Our test match will be all JavaScript files. We want everything from the root der/ star, star,/ star.js, so every JavaScript file in there. 

```js
const path = require('path')

module.exports = {
  rootDir: path.join(__dirname, '..'),
  displayName: 'lint',
  runner: 'jest-runner-eslint',
  testMatch: ['<rootDir>/**/*.js'],
}
```

We actually don't want every single JavaScript file in there because we also have the dis directory that has some JavaScript files. Coverage directory has a couple JavaScript files. We don't want to cover those.

[01:40] The way that we're doing that currently with our package.JSON is under Lint, we have ESLintignorepath.gitignore. We want to use the ignore path flag for the CLI. I'm going to configure Jest runner ESLint inside my package JSON here so that the CLI options, that Jest runner ESLint [inaudible] CLI, includes ignore path and .gitignore.

```js
 .
 .
 .
  "setup": "npm install && npm run validate"
},
"jest-runner-eslint": {
  "clipOptions": {
    "ignorePath": "./.gitignore
  }
}
```

[02:04] With that, I can run npm Jest. We'll specify the config is testjest.lint.js. 

```bash
npx jewst --config --config ./test/jest.lint.js
```

That's going to run lining across all the files in my project. Of course, if I make a lining error in index.js, for example, saying foo() and call that, that foo is not defined. That's a lining error. If I ran that script again, then we're going to get that lining error printed out in the terminal, just like we do if we're using the regular ESLint CLI.

[02:31] Let's go ahead and get rid of foo(). A cool thing about having this all configured in Jest is now we can add that to our jest.config.js here to have testjest.lint.js.

```js
    './src/shared/utils.js': {
      statements: 100,
      branches: 80,
      functions: 100,
      lines: 100,
    },
  },
  projects: [
    './test/jest.lint.js',
    './test/jest.client.js',
    './test/jest.server.js',
  ],
```

[02:44] Now if we run `npm t` to run our tests, that runs all of our tests in parallel, and that includes our linting. If I hit A to run all the tests, we're going to get our lining, our client, and our server, and it's all running in this beautiful watch mode that we have.

[02:59] We can hit the O key again, to only run the things that are impacted by the changes that we've made. We get that lining feedback, which is cool. On top of this, we can go to our package JSON and update our lint script. Now, it is jestconfigtestjest.lint.js.

```js
"scripts": {
    "test": "is-ci \"test:coverage\" \"test:watch\"",
    "test:coverage": "jest --coverage",
    "test:watch": "jest --watch",
    "test:debug": "node --inspect-brk ./node_modules/jest/bin/jest.js --runInBand --watch",
    "dev": "webpack-dev-server --mode=development",
    "build": "webpack --mode=production",
    "postbuild": "cp ./public/index.html ./dist/index.html",
    "start": "serve --no-clipboard --single --listen 8080 dist",
    "format": "prettier --ignore-path .gitignore --write \"**/*.+(js|json|css|html|md)\"",
    "lint": "jest --config test/jest.lint.js",
    "validate": "npm run lint && npm run test && npm run build",
    "setup": "npm install && npm run validate"
```

[03:16] Even better than that is now we no longer need to run our lining as part of the validate script because that's going to happen naturally through the test script. We get all the beautiful benefits of Jest watch mode and even the capability of only running glinting on the files that are relevant to the changes that we've made.

```js
"scripts": {
    "test": "is-ci \"test:coverage\" \"test:watch\"",
    "test:coverage": "jest --coverage",
    "test:watch": "jest --watch",
    "test:debug": "node --inspect-brk ./node_modules/jest/bin/jest.js --runInBand --watch",
    "dev": "webpack-dev-server --mode=development",
    "build": "webpack --mode=production",
    "postbuild": "cp ./public/index.html ./dist/index.html",
    "start": "serve --no-clipboard --single --listen 8080 dist",
    "format": "prettier --ignore-path .gitignore --write \"**/*.+(js|json|css|html|md)\"",
    "lint": "jest --config test/jest.lint.js",
    "validate": "npm run test && npm run build",
    "setup": "npm install && npm run validate"
```

[03:34] In review to make this happen, we installed Jest runner ESLint. We configured a Jest configuration that uses Jest runner ESLint with a runner configuration option.

[03:45] We specified our test match to be all JavaScript files in our root directory, which is the root of our project. We specified lint as the display name so we can differentiate this from our other test configurations.

[03:57] We added our test configuration to the projects under our root configuration here. We also made sure that Jest runner ESLint was configured so it uses git ignore for the ignore path. Then we could update our lint script to run with Jest and we were able to remove the lint script because our linting is run as part of our tests now.

[04:18] This might seem like just an extra dependency that's not totally necessary. Depending on your project, it might not be totally necessary. It can be nice for bigger projects where you have tons of files that you're linting and you want to scope down the files that you're linting to just the ones that you're working with at the time you're committing your code.


Jest is a fully featured testing framework with a developer experience that is second to none. It’s remarkably simple and flexible at the same time. For simple use cases, you often don’t need to configure anything, install and enjoy the built-in coverage and watch mode support.

In a real-world application though, you’ll often have needs specific to your application, especially when testing browser-based applications. You'll need to handle Webpack loaders, dynamic imports, and custom module resolution which Node.js does not support.

In this course we’ll go over ways you can optimize your Jest configuration to make testing real-world JavaScript applications a delight. We’ll cover what’s already been mentioned in addition to Babel support, code coverage, how to make watch mode even more helpful, and how to run test suites with entirely different configurations.

Intro to Configure Jest for Testing JavaScript Applications

We start in a small, but real-world application that’s built with webpack and React. We’ll go over installing the Jest testing framework locally to our project, adding an npm script to make it easy to run, and adding an example test file.

Install and Run Jest

Jest automatically loads and applies our babel configuration. However, because our project takes advantage of tree shaking with webpack, our babel configuration disables compiling modules. For Jest to work on our code, we need to make sure that our babel configuration compiles modules during our tests. We’ll use the NODE_ENV environment variable to generate the proper configuration.

Compile Modules with Babel in Jest Tests

In our application we’re testing code that should run in the browser, but Jest is intended to test JavaScript that runs in the browser or in node. Let’s create a custom jest config file so we can customize the test environment.

Configure Jest’s Test Environment for Testing Node or Browser Code

In most real-world applications using webpack, you’re likely using a few loaders. In this lesson we’ll see how to make Jest load our modules properly with a moduleNameMapper so we can test our code that makes use of these loaders.

Support Importing CSS files with Jest’s moduleNameMapper

If you’re using CSS modules with webpack, then we can improve our moduleNameMapper to improve our experience and capabilities testing our components by including the css module property name in our tests using identity-obj-proxy.

Support using Webpack CSS Modules with Jest

Snapshot testing is a way to simplify writing and maintaining assertions. As noted in [the Jest documentation](http://jestjs.io/docs/en/snapshot-testing.html#snapshot-testing-with-jest): “The snapshot artifact should be committed alongside code changes, and reviewed as part of your code review process. Jest uses pretty-format to make snapshots human-readable during code review. On subsequent test runs Jest will simply compare the rendered output with the previous snapshot. If they match, the test will pass. If they don't match, either the test runner found a bug in your code that should be fixed, or the implementation has changed and the snapshot needs to be updated.” Let’s see how these work and in what situations we can and should use them.

This lesson uses a simple demo before hopping into the application code testing DOM nodes. This example code isn't available.

Generate a Serializable Value with Jest Snapshots

Part of the power of snapshots is the ability to provide custom serializers. Let’s check out how to use jest-emotion to include our emotion CSS styles in our React component snapshots so we can be made aware of the impact of our CSS changes on our components.

UPDATE: `jest-emotion` has been renamed to `@emotion/jest` and you add the serializer with `'@emotion/jest/serializer'` instead of simply `'jest-emotion'`.

Test an Emotion Styled UI with Custom Jest Snapshot Serializers

Webpack’s resolve.modules configuration is a great way to make common application utilities easily accessible throughout your application. We can emulate this same behavior in Jest using the moduleDirectories configuration option.

Support Custom Module Resolution with Jest moduleDirectories

Often you’ll have some setup that you want to run before your tests start. Let’s see how we can use the setupFilesAfterEnv configuration in Jest to take care of test boilerplate and establish a good testing environment for our tests.

Configure Jest to Run Setup for All Tests with Jest setupFilesAfterEnv

Every large testbase has common utilities that make testing easier to accomplish. Whether that be a custom render function or a way to generate random data. Let’s see how we can make accessing these custom utilities throughout the tests easier using Jest’s moduleDirectories feature.

Support a Test Utilities File with Jest moduleDirectories

As the codebase grows it makes our test runs take longer and longer. Naturally people will run the tests less frequently to avoid waiting. With Jest’s powerful and interactive watch mode we can easily run the tests that we want to focus on without having to run all the tests in the codebase.

Use Jest Watch Mode to Speed Up Development

Sometimes it can be a real challenge to determine what’s going on when testing your code. It can be really helpful to step through your code in a debugger. In this lesson we’ll see how to use Jest’s --runInBand flag with node’s --inspect-brk to debug our tests in Chrome’s debugger.

Step through Code in Jest using the Node.js Debugger and Chrome DevTools

Jest comes with code coverage reporting built-into the framework, let’s see how quick and easy it is to add code coverage reporting to our project and take a look at the generated report. 

Configure Jest to Report Code Coverage on Project Files

You may have an idea of what Code Coverage is, but let’s take a quick look at how it’s recorded so we can understand how to analyze code coverage reports.

Analyze Jest Code Coverage Reports

Wherever you are at with code coverage you generally don’t want that level to go down. Let’s add coverage thresholds globally as well as in specific files to ensure we never drop below a certain level of coverage.

Set a Code Coverage Threshold in Jest to Maintain Code Coverage Levels

The coverage report generated by Jest is fantastic, but it’d be great to track that coverage over time and be able to review that coverage at a glance, maybe even put it up on a display in the office! Codecov.io is a fantastic service that can consume our code coverage report and it integrates great with GitHub. Let’s see how we can extend our existing Travis build configuration to send the coverage report to Codecov.io.

Report Jest Test Coverage to Codecov through TravisCI

In CI, we don’t want to start the tests in watch mode, but locally we normally want to run the tests in watch mode. We can have separate scripts, but it’d be great to not have to remember which script to run locally. Let’s use is-ci-cli to run the right script in the right environment when running the test script.

Run Jest Watch Mode by Default Locally with is-ci-cli

Sometimes you may have situations where configuration needs to be different for certain tests. In this lesson we’ll take a look at how we could create a custom configuration for tests that are intended to run in a node environment.

Run Tests with a Different Configuration using Jest’s --config Flag and testMatch Option

Sometimes you may find it useful to have more than one configuration in a project (for example, running some tests in a node environment and others in the jsdom environment). In this lesson we’ll learn about Jest’s projects feature to have jest run both of these configurations at once.

Support Running Multiple Configurations with Jest’s Projects Feature

It’s great that we can run multiple projects in our watch mode and that we can scope the tests down to specific tests, but sometimes it’s nice to be able to quickly switch between projects in watch mode. Let’s see how this works with jest-watch-select-projects.

Test Specific Projects in Jest Watch Mode with jest-watch-select-projects

Jest’s watch mode is [pluggable](http://jestjs.io/docs/en/watch-plugins.html) and jest-watch-typeahead is one plugin that you definitely don’t want to live without. It enhances the watch mode experience to help you know which tests will be run based on your filter. Making it even easier to run only the tests you’re concerned with running as you develop your codebase.

Filter which Tests are Run with Typeahead Support in Jest Watch Mode

Running the project tests on commit is a great way to avoid breaking the application accidentally and leverage the mechanism for confidence you have from your testbase. However, as the testbase grows this can take a very long time and slow productivity down. Let’s see how Jest is capable of running only the tests and linting only the files that are affected by the files we’re committing with husky and lint-staged to speed up our local test runs as well as help us avoid accidentally committing code that breaks our application.

Run ESLint with Jest using jest-runner-eslint

Transcript

Run ESLint with Jest using jest-runner-eslint