124 lines
3.6 KiB
Markdown
124 lines
3.6 KiB
Markdown
# Jest Plugin
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[Jest](https://jestjs.io/) is an open source test runner created by Facebook. It has a lot of great features:
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- Immersive watch mode for providing near instant feedback when developing tests.
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- Snapshot testing for validating features.
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- Great built-in reporter for printing out test results.
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## Setting up Jest
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By default, Nx will use Jest when creating applications and libraries.
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```shell
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nx g @nrwl/web:app frontend
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```
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### Adding Jest to an Existing Project
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Add Jest to a project using the `jest-project` generator from `@nrwl/jest`.
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First, install `@nrwl/jest`, if not already installed using your preferred package manager.
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```shell
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npm install --save-dev @nrwl/jest
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```
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```shell
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yarn add --dev @nrwl/jest
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```
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Once installed, run the `jest-project` generator
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```shell
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nx g @nrwl/jest:jest-project --project=<project-name>
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```
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> Hint: You can use the `--dry-run` flag to see what will be generated.
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Replacing `<project-name>` with the name of the project you're wanting to add Jest too.
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## Using Jest
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### Testing Applications
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Run the Jest test with
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```shell
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nx test frontend
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```
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### Watching for Changes
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Using the `--watch` flag will run the tests whenever a file changes.
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```shell
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nx test frontend --watch
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```
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### Snapshot Testing
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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).
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Example of using snapshots:
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```typescript
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describe('SuperAwesomFunction', () => {
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it('should return the correct data shape', () => {
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const actual = superAwesomFunction();
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expect(actual).toMatchSnapshot();
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});
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});
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```
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When using snapshots, you can update them with the `--updateSnapshot` flag, `-u` for short.
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> By default, snapshots will be generated when there are not existing snapshots for the associated test.
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```shell
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nx test frontend -u
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```
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Snapshot files should be checked in with your code.
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## Configurations
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### Jest
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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).
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### Nx
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Nx Jest Plugin options can be configured via the [project config file](/configuration/projectjson) or via the [command line flags](/jest/jest).
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> Hint: Use `--help` to see all available options
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>
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> ```shell
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> nx test <project-name> --help
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> ```
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### Code Coverage
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Enable code coverage with the `--coverage` flag or by adding it to the executor options in the [project configuration file](/configuration/projectjson).
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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.
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> `coverageDirectory` and `coverageReporters` are configurable via the project configuration file as well.
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## Debugging Failing Tests
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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.
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```shell
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node --inspect-brk ./node_modules/@nrwl/cli/bin/nx test <project-name>
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```
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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.
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## More Documentation
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- [Jest Docs](https://jestjs.io/)
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- [@nrwl/jest options](/jest/jest)
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