# Neutrino Jest Preset
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`neutrino-preset-jest` is a Neutrino preset that supports testing JavaScript projects with the Jest test runner.

## Features

- Zero upfront configuration necessary to start testing
- Babel compilation that compiles your tests using the same Babel options used by your source code
- Source watching for re-running of tests on change
- Collecting test coverage information and generating report
- Easily extensible to customize your testing as needed

## Requirements

- Node.js v6.9+
- Yarn or npm client
- Neutrino v5, Neutrino build preset

## Installation

`neutrino-preset-jest` can be installed via the Yarn or npm clients. Inside your project, make sure
`neutrino` and `neutrino-preset-jest` are development dependencies. You will also be using
another Neutrino preset for building your application source code.

#### Yarn

```bash
❯ yarn add --dev neutrino-preset-jest
```

#### npm

```bash
❯ npm install --save-dev neutrino-preset-jest
```

### Installation: React Testing

Jest is often used in the testing of React components. If you are using this preset in a React application, which most
likely means you are using `neutrino-preset-react`, you should also consider adding React's TestUtils and potentially a
higher-level abstraction such as Airbnb's Enzyme. These should be development dependencies of your project.

#### Yarn

```bash
❯ yarn add --dev react-addons-test-utils enzyme
```

#### npm

```bash
❯ npm install --save-dev react-addons-test-utils enzyme
```

See the [React's Test Utils documentation](https://facebook.github.io/react/docs/test-utils.html) for specifics on React
testing with this approach.

## Project Layout

`neutrino-preset-jest` follows the standard [project layout](https://neutrino.js.org/project-layout) specified by Neutrino. This
means that by default all project test code should live in a directory named `test` in the root of the
project. Test files end in either `_test.js`, `.test.js`, `_test.jsx`, or `.test.jsx`.

## Quickstart

After adding the Jest preset to your Neutrino-built project, add a new directory named `test` in the root of the
project, with a single JS file named `simple_test.js` in it.

```bash
❯ mkdir test && touch test/simple_test.js
```

Edit your `test/simple_test.js` file with the following:

```js
describe('simple', () => {
  it('should be sane', () => {
    expect(!false).toBe(true);
  });
});
```

Now edit your project's package.json to add commands for testing your application. In this example,
let's pretend this is a Node.js project:

```json
{
  "scripts": {
    "test": "neutrino test --use neutrino-preset-node neutrino-preset-jest"
  }
}
```

Or if you have set up Neutrino with `neutrino.use` in your package.json:

```json
{
  "neutrino": {
    "use": [
      "neutrino-preset-node",
      "neutrino-preset-jest"
    ]
  }
}
```

Run the tests, and view the results in your console:

#### Yarn

```bash
❯ yarn test
 PASS  test/simple_test.js
  simple
    ✓ should be sane (2ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        0.936s
Ran all test suites.
✨  Done in 2.12s.
```

#### npm

```bash
❯ npm test

 PASS  test/simple_test.js
  simple
    ✓ should be sane (2ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        0.972s
Ran all test suites.
```

To run tests against files from your source code, simply import them:

```js
import thingToTest from '../src/thing';
```

For more details on specific Jest usage, please refer to their [documentation](https://facebook.github.io/jest/).

## Executing single tests

By default this preset will execute every test file located in your test directory ending in the appropriate file
extension. Use the command line [`files` parameters](https://neutrino.js.org/cli#neutrino-test) to execute individual tests.

## Watching for changes

`neutrino-preset-jest` can watch for changes on your source directory and subsequently re-run tests. Simply use the
`--watch` flag with your `neutrino test` command.

## Coverage reporting

Jest has an integrated coverage reporter, which requires no configuration. To collect test coverage information and
generate a report:

```bash
❯ neutrino test --coverage
```

You can also edit your package.json file and create a separate command for generating a coverage report, which can be
very helpful during continuous integration of your project:

```json
{
  "scripts": {
    "coverage": "neutrino test --coverage"
  }
}
```

See the [Jests documentation](https://facebook.github.io/jest/docs/configuration.html#collectcoveragefrom-array) for
more configuration options for generating coverage reports.

## Customizing

To override the test configuration, start with the documentation on [customization](https://neutrino.js.org/customization).
`neutrino-preset-jest` creates some conventions to make overriding the configuration easier once you are ready to make
changes.

### Rules

The following is a list of rules and their identifiers which can be overridden:

- `compile`: Compiles JS files from the `test` directory using adopted Babel settings from other build presets.
Contains a single loader named `babel`.

### Simple customization

By following the [customization guide](https://neutrino.js.org/customization/simple) and knowing the rule, and loader IDs above,
you can override and augment the build directly from package.json.

Jest configuration settings can also be modified directly from package.json, but it is not required.
`neutrino-preset-jest` will import Jest configuration from your package.json's `jest` object or `neutrino.options.jest`;
the format is defined on the [Jest documentation site](https://facebook.github.io/jest/docs/configuration.html).

_Example: Turn off bailing on test failures._

```json
{
  "neutrino": {
    "options": {
      "jest": {
        "bail": false
      }
    }
  }
}
```

### Advanced configuration

By following the [customization guide](https://neutrino.js.org/customization/advanced) and knowing the rule, and loader IDs above,
you can override and augment testing by creating a JS module which overrides the config. 

You can also modify Jest settings by overriding with any options Jest accepts. In a standalone Jest project this is
typically done in the package.json file, but `neutrino-preset-jest` allows advanced configuration through this
mechanism as well. This is stored in the `neutrino.options.jest` object, and takes the same configuration options as
outlined in the [Jest documentation](https://facebook.github.io/jest/docs/configuration.html).

_Example: Create a global `__DEV__` variable set to `true` in all test environments._

```js
module.exports = neutrino => {
  neutrino.options.jest.globals = {
    __DEV__: true
  };
};
```

## Contributing

This preset is part of the [neutrino-dev](https://github.com/mozilla-neutrino/neutrino-dev) repository, a monorepo
containing all resources for developing Neutrino and its core presets. Follow the
[contributing guide](https://neutrino.js.org/contributing) for details.

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