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Neutrino Mocha Preset

@neutrinojs/mocha is a Neutrino preset that supports testing JavaScript projects with the Mocha test runner.

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  • Zero upfront configuration necessary to start testing
  • Babel compilation that compiles your tests using the same Babel options used by your source code
  • Easily extensible to customize your testing as needed


  • Node.js ^8.10 or 10+
  • Yarn v1.2.1+, or npm v5.4+
  • Neutrino 9 and one of the Neutrino build presets
  • webpack 4
  • Mocha 6


The fastest way to get started is by using the create-project scaffolding tool. See the Create new project docs for more details.

Don’t want to use the CLI helper? No worries, we have you covered with the manual installation.

Manual Installation

First follow the manual installation instructions for your chosen build preset.

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


❯ yarn add --dev @neutrinojs/mocha mocha


❯ npm install --save-dev @neutrinojs/mocha mocha

After that, add a new directory named test in the root of the project, with a single JS file named simple_test.js in it.

❯ mkdir test && touch test/simple_test.js

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

import assert from 'assert';

describe('simple', () => {
  it('should be sane', () => {
    assert.equal(true, !false);

Now update your project's .neutrinorc.js to add the @neutrinojs/mocha preset. In this example, let's pretend this is a Node.js project:

const node = require('@neutrinojs/node');
const mocha = require('@neutrinojs/mocha');

module.exports = {
  use: [node(), mocha()],

Create a .mocharc.js file in the root of the project, that will be used by the Mocha CLI:

// .mocharc.js
const neutrino = require('neutrino');

process.env.NODE_ENV = process.env.NODE_ENV || 'test';

module.exports = neutrino().mocha();

Then add these scripts entries to your package.json to simplify running Mocha:

  "scripts": {
    "test": "mocha",
    "test:watch": "mocha --watch"

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


❯ yarn test

    ✓ should be sane

  1 passing (426ms)


❯ npm test

    ✓ should be sane

  1 passing (409ms)

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

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

For more details on specific Mocha usage, please refer to their documentation.

Project Layout

@neutrinojs/mocha follows the standard 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 _test.js by default.

Executing single tests

By default this preset will execute every test file located in your test directory ending in _test.js. Pass specific test filenames to the Mocha CLI to override this.

Watching for changes

@neutrinojs/mocha can watch for changes on your source directory and subsequently re-run tests. Simply pass --watch to the Mocha CLI (for example by using the test:watch scripts entry above).

Preset options

You can provide custom options and have them merged with this preset's default options, which are subsequently passed to Mocha. You can modify Mocha settings from .neutrinorc.js by overriding with any options Mocha accepts. In a standalone Mocha project this is done in the .mocharc.js file, but @neutrinojs/mocha allows configuration through this mechanism as well. This accepts the same configuration options as outlined in the Mocha documentation.

Example: Enable bailing on test failures.

module.exports = {
  use: [
      bail: true,


To override the test configuration, start with the documentation on customization. @neutrinojs/mocha creates some conventions to make overriding the configuration easier once you are ready to make changes.


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

Name Description NODE_ENV
compile Compiles JS files from the test directory using adopted Babel settings from other build presets. Contains a single loader named babel. all

Override configuration

By following the customization guide and knowing the rule, and loader IDs above, you can override and augment testing by providing a function to your .neutrinorc.js use array. You can also make this change from the Neutrino API when using the use method.

Example: Add a custom Babel plugin when testing:

const mocha = require('@neutrinojs/mocha');

module.exports = {
  use: [
    neutrino => {
      if (process.env.NODE_ENV === 'test') {
          .tap(options =>
            merge(options, {
              env: {
                test: {
                  plugins: ['custom-babel-plugin'],

Webstorm Mocha Runner

Webstorm has a mocha runner that enables running mocha tests within the IDE. The Mocha runner also allows for debugging of Mocha tests in Webstorm. Basic setup can be found at Webstorm's Mocha Run/Debug documentation.

  • Create a mocha runner using Webstorm's Mocha Run/Debug documentation. The defaults should be fine with the following exceptions:
  • choose File patterns radio button
  • Test file patterns: ./test/*_test.js (or whichever pattern you need for your tests)

Now you should be able to run your tests or add a breakpoint to your tests and debug them.


This preset is part of the neutrino repository, a monorepo containing all resources for developing Neutrino and its core presets and middleware. Follow the contributing guide for details.