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 for Testing JavaScript Applications

Configure Jest to Report Code Coverage on Project Files

Instructor: [00:00] As you begin adding tests to a project, it can be really helpful to know how much of your project is tested and where you could use a little bit more help adding new test. For example, we know that our auto scaling text file is being tested because we're rendering it here in several other test, but we don't know how much of this file is being tested.

[00:17] We also know that the app file is not being tested at all, but that's because we're the ones who wrote the test in the first place. It would be really nice if we could just get some insight into how well our code base is being tested.

[00:28] What we're going to do here is add the coverage flag to our test script. 
```js
"scripts": {
    "test": "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": "eslint --ignore-path .gitignore .",
    "validate": "npm run lint && npm run test && npm run build",
    "setup": "npm install && npm run validate"
```


Now if I run `npm t` to run that test script, Jest will automatically generate this coverage report that tells me a lot about the coverage for my code base.

[00:41] Part of what this does is it actually creates a coverage directory with all the coverage information for our coverage report and we can open this up in a browser. If we open coverage/lcov-report/index.html, 

```bash
open coverage/lcov-report/index.html
```


then that will open up in our browser right here and we can see that code coverage report right here.

[00:59] We can see in shared, it has utils right here. We're missing one bit of coverage right there, but we're running the rest of these lines five times. Then, in our auto-scaling text, it looks like we are missing a little case right here.

[01:12] In our calculator display, we have a little case right here that we're missing. It's nice we get to see the test cases that we're probably missing, the use case that we're missing in our tests.

[01:21] Under src directory here, we're seeing that calculator is not very well tested. We could probably add a couple of tests to that. Themes is run three times, so that one's fine.

[01:31] One problem that I see with this coverage report is it's including the test directory, which has our test utilities, and these test utilities are absolutely going to get 100 percent code coverage. Otherwise, we'd just remove them. Why would we have a test utility if we're not actually using it?

[01:47] This is a problem because this is going to get 100 percent code coverage. It's going to blow our numbers for our code coverage in our project. We don't need to know how well our test utilities are covered. We want to know how well our application code is covered.

[02:01] In addition, you'll notice that the src directory has calculator in themes, but the src directory also has an app and index. We're not getting any insight into how well those files are being covered, or the fact that they're actually not being covered, which should reduce the overall coverage for our project.

[02:18] It'd be really nice to solve this problem and get a more accurate report on our code coverage. The way that we're going to solve this is in our jest.config.js. We'll add a collectCoverageFrom, and this is an array of globs. We're going to say anything that is under the source directory that ends in js. 

```js
  moduleNameMapper: {
    '\\.module\\.css$': 'identity-obj-proxy',
    '\\.css$': require.resolve('./test/style-mock.js'),
  },
  setupFilesAfterEnv: ['@testing-library/jest-dom/extend-expect'],
  snapshotSerializers: ['jest-emotion'],
  collectCoverageFrom: ['**/src/**/*.js'],
```


That's what we want to collect coverage on. This will exclude the test directory with our test utilities in here. This will also exclude the test by default. Jest will automatically exclude the test. We don't need to worry about those. We certainly wouldn't want those included in our coverage report.

[02:55] Now if we `npm run test` to get our coverage of report regenerated, and then we open up that coverage report again, we can compare the old coverage report with the new one. You'll notice that our coverage numbers are getting reduced to reflect our actual coverage way more accurately. We have a much better sense of how well we're doing in our application.

[03:17] In a full-size application, this difference will be stark. Don't be ashamed if you find yourself with like two percent code coverage. You can work your way back up.

[03:25] Here now we have our shared. That has all the same stuff. There's actually nothing different about that one. The source directory now has, in addition to the themes and calculator, our index and our app which both have zero percent code coverage because they're not being included in any of our tests. They probably should be.

[03:42] One other thing that I'm going to do here is because these files are all generated in the coverage directory, they're not useful to be committed to src control. I'm going to go to our gitignore, and we're going to add a coverage line right here. We don't include the coverage report in Git.

```js
dist
node_modules
coverage
```

[03:58] In review to make all of this magic happen, we needed to add to our package JSON the coverage flag to our Jest script. You could add the coverage flag to your watch script, but then the coverage report will get regenerated.

[04:12] If you're only running the test for a couple of your files, that coverage report will get all kinds of messed up. Typically, I only run coverage when I run all of my tests with the regular test script.

[04:23] Another thing that we did here to make sure we're reporting coverage on all of our application files and none of our test files, they specify the correct coverage from so that all of our source files are included in the coverage. Then we also added, to our gitignore, that new coverage directory so we don't commit that to src control.


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

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

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.

Configure Jest to Report Code Coverage on Project Files

Transcript

Configure Jest to Report Code Coverage on Project Files