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.

Configure Jest for Testing JavaScript Applications

Support Running Multiple Configurations with Jest’s Projects Feature

Instructor: [00:00] I don't know about you, but to me, this is utter madness. It is a nightmare. I much preferred having just a couple of scripts here. Where we have one test script that will determine whether to run coverage or watch based off ci.

[00:12] Then we have our coverage in our watch groups, and a debug script. That was so much nicer, and we added all this complexity just because we wanted to run a test in a different environment. Jest actually supports running multiple configurations in a single test run.

[00:27] That makes all of this possible. We've restored our beautiful scripts that we've had before. Now, we're going to make jest support this. To make this happen, I'm going to add a jestconfig.js at the root of our app again.

[00:40] That means I can go back into my eslintrc.js and update this to our jest.common.js to js.config.js, that we had before. Inside this jestconfig, we're going to do a moduleexports. We're going to keep that jestcommon where it is. We're going to mix in, require, test jestcommon. We get lots of those same things that we had before.


[01:00] Then we're going to actually go to jest-common.js. We're going to grab the `collectCoverageFrom` and move that to jest-config.js and then we'll go into our jest-client.js file. We're going to move our coverage threshold because combining these two configurations also has the effect of combining the coverage report, which is also a wonderful thing.

[01:19] Let's go ahead and put that in jest.config.js. Finally, to make all this work together, we're going to say projects as a new configuration object. We'll say test jest.client and test jest.server. 

```js
module.exports = {
...require('./test/jest-common'),
collectCoverageFrom: ['**/src/**/*.js'],
coverageThreshold: {
  global: {
    statements: 15,
    branches: 10,
    functions: 15,
    lines: 15,
  },
  './src/shared/utils.js': {
    statements: 100,
    branches: 80,
    functions: 100,
    lines: 100,
  },
 },
 projects: ['./test/jest.clients.js', './test/jest.server.js'],
}
```

We save that. Now, we run `npm t` to run our test in watch mode. Like magic, we have our test running in watch mode.

[01:41] We are, in fact, running the auto-scaling-text from our severed test as well as all of our tests from our regular test. These are actually running in totally different environments, which is a fantastic feature from the jest testing platform. If we run CI as one and `npm t` to run our test in coverage mode.

``` bash
CI=1 npm t
```

It will collect coverage from all of our test runs.

[02:05] That coverage report will be unified across all of the test for both of these configurations. I am way happier with this. I want to review to make all these magic happen, we added a new jest configuration here. The real key here is to use projects pointing to individual jest configurations.

[02:23] Each one of these server and client are both using the jest common, which are going to give us our root directory, the module directories, and the moduleNameMapper which make sense or all of our Jest configurations.

[02:36] Then the jest.client.js, you can specify the environment, a couple of other specific things just for the client-side stuff, and the jest.server.js can specify a couple of specific things for the server-side stuff. Then the jest.configuration.js here is responsible for global style configuration.
I 
[02:50] We can determine what those global configurations are like our coverage by running npx.jest show config. 

```bash
npx jest --showConfig
```

This is going to load up our Jest configuration and show us the results configuration for Jest.

[03:04] Lots of these are some defaults. Here, in particular, the thing you need to pay attention to is the `globalConfig`. Anything inside of the global config is what needs to be configured inside of the jest.config.js configuration which is your main entry for your configuration.

[03:19] Things like our collect coverage from, our coverage reporters, the coverage directory, the coverage threshold, all of these things are considered global configuration and should be specified within the configuration that's responsible for our projects.

[03:33] One last thing I want to show you about this, is if we run `npm t` with watch turned on, we run all of the tests, it's cool that we get to run all the tests across these configurations, but it's hard to determine quickly which one of these tests lines is coming from which configuration, especially if you have a lot of these.

[03:51] We're going to add a label to this. If we go to our Jest server, we add a display name of server and then we go to our client-side, and add a display name of client.

[04:05] Because we've changed our configuration, we need to restart this again, so run  `npm t` to get that started up again. Now, we're going to get a server and a client label on each one of these lines, which will make it a little easier to differentiate these test lines.


**jest.server.js**
```js
module.exports = {
  ...require('./jest-common'),
  displayName: 'server',
  testEnvironment: 'jest-environment-node',
  testMatch: ['**/__server_tests__/**/*.js'],
}
```

**jest.client.js**
```js
Learn more or give us feedback
module.exports = {
  ...require('./jest-common'),
  displayName: 'client',
  testEnvironment: 'jest-environment-jsdom',
  setupFilesAfterEnv: ['@testing-library/jest-dom/extend-expect'],
  snapshotSerializers: ['jest-emotion'],
}
```


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

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.

Run ESLint with Jest using jest-runner-eslint

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.

Support Running Multiple Configurations with Jest’s Projects Feature

Transcript

Support Running Multiple Configurations with Jest’s Projects Feature