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.

Configure Jest for Testing JavaScript Applications

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

Instructor: [00:01] Imagine I'm writing this test, and I want to do a little bit of debug on this getScale function. I'm thinking, "OK, why is it returning 1 here? Why is that Node not defined?" I'm going to console.log(node). Maybe I'll console.log('in here'). I'll console.log('not in there'). 

```js
function getScale(node) {
console.log(node)
  if (!node) {
  console.log('in here')
    return 1
  }
  console.log('not in there')
  const parentNode = node.parentNode

  const availableWidth = parentNode.offsetWidth
  const actualWidth = node.offsetWidth
  const actualScale = availableWidth / actualWidth

  if (actualScale < 1) {
    return actualScale * 0.9
  }
```


I want to just `npm run test:watch`. We want to see that output right there.

[00:26] We've got a bunch of these outputs. We've got a Node up here. Let's go ahead, and we'll just run autoscaling tech. We'll say `autoscaling`. Then, I'll just run the one. We're getting undefined, and then we're getting in here.

[00:38] Node is undefined. It's a little bit harder to debug. As cool as it is that Jest will give us this output to tell us exactly where that console.log happened, it would be nice if I could just step through the code in the same familiar development environment that I have when I'm running in the browser.

[00:54] What I really want to be able to do is just add a debugger statement in here and have my browser dev tools stop right there. I can do that in the browser, but I can't do it when I'm running my test. Or can I?

[01:04] That's what we're going to add here. I'm going to add a new script called test:debug. Node has built into it the special flag called --inspect-brk, which is inspect break, which means Node will start up a process, and before running any of the code, it will add a break point. Then you can hook up the Chrome debugger to debug that Node process.


[01:25] What is the process we want to have run? We want to have the Jest process run, but we can't pass Jest because that would just be passed as an argument to Node. Node wouldn't know what to do with Jest. We need to pass it a path to the Jest binary.

[01:40] Where is that Jest binary? Let's go up here into our Node modules. We'll go down to Jest. We can find that in here. We've got Jest right here. Here, we have the bin directory. There's the jest.js. That's the file we want to have Node run.

[01:57] We'll do node_modules/jest/bin/jest.js. Jest runs all of our files in parallel. We're going to run runInBand because running in parallel won't work very well with what we're trying to do here because it spawns new Node processes, so that won't work very well.

[02:15] By running in band, it ensures that all of our test run in the same Node process so we can debug that process with our Chrome developer tools. We're also going to add the watch flag here because that makes the development experience a lot easier. We'll go ahead and save that.

```js
"scripts": {
    "test": "jest",
    "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"
```

[02:29] I'm going to quit this. We'll `npm run test:debug` instead. That's going to give me this output saying that a debugger is listening, and we can go look up some help there. I can show you how we can get this up and running. If you open up Chrome and go to chrome://inspect, that will take you to this special view where I'll show you the different debugger sessions available.

[02:53] One of those is Node. I can click on Inspect right here and that will pop open the Chrome Developer Tools. Then I can play through here and it will stop on that debugger statement that I left in my code and I can see, "Oh OK, here's the Node," and if I back up, "Oh, that's where it's getting called."

[03:10] The nodeRef, "Oh, we're not initializing that to anything. This is the first time it's getting called." That explains why our nodeRef.current is undefined.

[03:18] We can look all throughout the call stack and if we go far enough, we can actually find our test utility right here and ultimately our calculator test, which is running the code that ultimately runs into this auto scanning text test. Then we can just close this.

[03:35] Another way that we can start up the DevTools is by opening Chrome and then opening the DevTools with the Inspect button here. There should be a Node button that we can click right in our DevTools. That will pop open the DevTools as well.

[03:48] In review, to make all of this magic happen, we simply added a script called test:debug and then we used Node with the --inspect-brk flag to point to the Jest binary. Now when you said, "Hey Jest, don't run in parallel, just run in band," and it will run with a --watch flag. As we're making changes, our DevTools can stay open and we can interact with our code through the DevTools with Chrome.


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

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

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.

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

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