The Nx Plugin for Jest contains executors and generators allowing your workspace to use the powerful Jest testing capabilities.

![Jest logo](/shared/jest-logo.png)

[Jest](https://jestjs.io/) is an open source test runner created by Facebook. It has a lot of great features:

- Immersive watch mode for providing near instant feedback when developing tests.
- Snapshot testing for validating features.
- Great built-in reporter for printing out test results.

## Setting up Jest

By default, Nx will use Jest when creating applications and libraries.

```shell
nx g @nrwl/web:app frontend
```

### Adding Jest to an Existing Project

Add Jest to a project using the `jest-project` generator from `@nrwl/jest`.

First, install `@nrwl/jest`, if not already installed using your preferred package manager.

```shell
npm install --save-dev @nrwl/jest
```

```shell
yarn add --dev @nrwl/jest
```

Once installed, run the `jest-project` generator

```shell
nx g @nrwl/jest:jest-project --project=<project-name>
```

> Hint: You can use the `--dry-run` flag to see what will be generated.

Replacing `<project-name>` with the name of the project you're wanting to add Jest too.

## Using Jest

### Testing Applications

The recommended way to run/debug Jest tests via an editor

- [VSCode](https://marketplace.visualstudio.com/items?itemName=firsttris.vscode-jest-runner)
- [Webstorm](https://blog.jetbrains.com/webstorm/2018/10/testing-with-jest-in-webstorm/)

To run Jest tests via nx use

```shell
nx test frontend
```

### Watching for Changes

Using the `--watch` flag will run the tests whenever a file changes.

```shell
nx test frontend --watch
```

### Snapshot Testing

Jest has support for **Snapshot Testing**, a tool which simplifies validating data. Check out the [official Jest Documentation on Snapshot Testing](https://jestjs.io/docs/en/snapshot-testing).

Example of using snapshots:

```typescript
describe('SuperAwesomFunction', () => {
  it('should return the correct data shape', () => {
    const actual = superAwesomFunction();
    expect(actual).toMatchSnapshot();
  });
});
```

When using snapshots, you can update them with the `--updateSnapshot` flag, `-u` for short.

> By default, snapshots will be generated when there are not existing snapshots for the associated test.

```shell
nx test frontend -u
```

Snapshot files should be checked in with your code.

### Performance in CI

Typically, in CI it's recommended to use `nx affected --target=test --parallel=[# CPUs] -- --runInBand` for the best performance.

This is because each [jest process creates a workers based on system resources](https://jestjs.io/docs/cli#--maxworkersnumstring), running multiple projects via nx and using jest workers will create too many process overall causing the system to run slower than desired. Using the `--runInBand` flag tells jest to run in a single process.

## Configurations

### Jest

Primary configurations for Jest will be via the `jest.config.js` file that generated for your project. This file will extend the root `jest.config.js` file. Learn more about [Jest configurations](https://jestjs.io/docs/configuration#options).

### Nx

Nx Jest Plugin options can be configured via the [project config file](/reference/project-configuration) or via the [command line flags](/packages/jest).

> Hint: Use `--help` to see all available options
>
> ```shell
> nx test <project-name> --help
> ```

### Code Coverage

Enable code coverage with the `--coverage` flag or by adding it to the executor options in the [project configuration file](/reference/project-configuration).

By default, coverage reports will be generated in the `coverage/` directory under projects name. i.e. `coverage/apps/frontend`. Modify this directory with the `--coverageDirectory` flag. Coverage reporters can also be customized with the `--coverageReporters` flag.

> `coverageDirectory` and `coverageReporters` are configurable via the project configuration file as well.

### Global setup/teardown with nx libraries

In order to use Jest's global setup/teardown functions that reference nx libraries, you'll need to register the TS path for jest to resolve the libraries.
Nx provides a helper function that you can import within your setup/teardown file.

```typescript
import { registerTsProject } from 'nx/src/utils/register';
const cleanupRegisteredPaths = registerTsProject('.', 'tsconfig.base.json');

import { yourFancyFunction } from '@some-org/my-util-library';
export default async function () {
  yourFancyFunction();
}
// make sure to run the clean up!
cleanupRegisteredPaths();
```

## Debugging Failing Tests

If your code editor doesn't provide a way to debug your tests, you can leverage the Chrome DevTools to debug your tests with the `--inspect-brk` flag for node.

```shell
node --inspect-brk ./node_modules/@nrwl/cli/bin/nx test <project-name>
```

Enter [chrome://inspect](chrome://inspect) in Chrome address bar and inspect the target to attach to the node process. Visit the official [Jest documentation](https://jestjs.io/docs/en/troubleshooting#tests-are-failing-and-you-don-t-know-why) to find out more.

## More Documentation

- [Jest Docs](https://jestjs.io/)
- [@nrwl/jest options](/packages/jest)


Overview

jest

Pass Without Tests

Code Coverage

Stop After 'n' Failed

Exit the test suite immediately after `n` number of failing tests. (https://jestjs.io/docs/cli#--bail)

Runs tests related to the changes since the provided branch or commit hash. If the current branch has diverged from the given branch, then only changes made locally will be tested. (https://jestjs.io/docs/cli#--changedsince)

Whether to run Jest in continuous integration (CI) mode. This option is on by default in most popular CI environments. It will prevent snapshots from being written unless explicitly requested. (https://jestjs.io/docs/cli#--ci)

Deletes the Jest cache directory and then exits without running tests. Will delete Jest's default cache directory. _Note: clearing the cache will reduce performance_.

Indicates that test coverage information should be collected and reported in the output. (https://jestjs.io/docs/cli#--coverageboolean)

Forces test results output color highlighting (even if `stdout` is not a TTY). Set to false if you would like to have no colors. (https://jestjs.io/docs/cli#--colors)

Forces test results output highlighting even if stdout is not a TTY. (https://jestjs.io/docs/cli#--colors)

The directory where Jest should output its coverage files.

A list of reporter names that Jest uses when writing coverage reports. Any istanbul reporter.

**EXPERIMENTAL**: Detect memory leaks in tests. After executing a test, it will try to garbage collect the global object used, and fail if it was leaked

Attempt to collect and print open handles preventing Jest from exiting cleanly (https://jestjs.io/docs/cli#--detectopenhandles)

The path of the Jest configuration. (https://jestjs.io/docs/en/configuration)

Prints the test results in `JSON`. This mode will send all other test output and user messages to `stderr`. (https://jestjs.io/docs/cli#--json)

Logs the heap usage after every test. Useful to debug memory leaks. Use together with --runInBand and --expose-gc in node.

Specifies the maximum number of workers the worker-pool will spawn for running tests. This defaults to the number of the cores available on your machine. Useful for CI. (its usually best not to override this default) (https://jestjs.io/docs/cli#--maxworkersnumstring)

Attempts to identify which tests to run based on which files have changed in the current repository. Only works if you're running tests in a `git` or `hg` repository at the moment. (https://jestjs.io/docs/cli#--onlychanged)

Write test results to a file when the `--json` option is also specified. (https://jestjs.io/docs/cli#--outputfilefilename)

Will not fail if no tests are found (for example while using `--testPathPattern`.) (https://jestjs.io/docs/cli#--passwithnotests)

Run tests with specified reporters. Reporter options are not available via CLI. Example with multiple reporters: `jest --reporters="default" --reporters="jest-junit"`. (https://jestjs.io/docs/cli#--reporters)

Run all tests serially in the current process (rather than creating a worker pool of child processes that run tests). This is sometimes useful for debugging, but such use cases are pretty rare. Useful for CI. (https://jestjs.io/docs/cli#--runinband)

The name of a setup file used by Jest. (use Jest config file https://jestjs.io/docs/en/configuration#setupfilesafterenv-array)

Print your Jest config and then exits. (https://jestjs.io/docs/en/cli#--showconfig)

Prevent tests from printing messages through the console. (https://jestjs.io/docs/cli#--silent)

Adds a location field to test results.  Used to report location of a test in a reporter. { "column": 4, "line": 5 } (https://jestjs.io/docs/cli#--testlocationinresults)

Run only tests with a name that matches the regex pattern. (https://jestjs.io/docs/cli#--testnamepatternregex)

An array of regexp pattern strings that is matched against all tests paths before executing the test. Only run those tests with a path that does not match with the provided regexp expressions. (https://jestjs.io/docs/cli#--testpathignorepatternsregexarray)

An array of regexp pattern strings that is matched against all tests paths before executing the test. (https://jestjs.io/docs/cli#--testpathpatternregex)

Node module that implements a custom results processor. (https://jestjs.io/docs/en/configuration#testresultsprocessor-string)

Default timeout of a test in milliseconds. Default value: `5000`. (https://jestjs.io/docs/cli#--testtimeoutnumber)

The name of the Typescript configuration file. Set the tsconfig option in the jest config file. 

Use this flag to re-record snapshots. Can be used together with a test suite pattern or with `--testNamePattern` to re-record snapshot for test matching the pattern. (https://jestjs.io/docs/cli#--updatesnapshot)

Display individual test results with the test suite hierarchy. (https://jestjs.io/docs/cli#--verbose)

Watch files for changes and rerun tests related to changed files. If you want to re-run all tests when a file has changed, use the `--watchAll` option. (https://jestjs.io/docs/cli#--watch)

Watch files for changes and rerun all tests when something changes. If you want to re-run only the tests that depend on the changed files, use the `--watch` option. (https://jestjs.io/docs/cli#--watchall)

Jest Builder

init

Use JavaScript instead of TypeScript for config files

Do not add dependencies to `package.json`.

Jest init

jest-project

Use `babel-jest` instead of `ts-jest` (Deprecated: Use `--compiler=babel` instead).

The compiler to use for source and tests.

Skips the serializers required to snapshot angular templates.

[Deprecated]: Skips the setup file required for angular. (Use `--setup-file`)

@nrwl/jest:jest-project

Usage

Options

babelJest

compiler

js

project

skipSetupFile

setupFile

skipSerializers

supportTsx

skipFormat

skipPackageJson

testEnvironment