Mocha.js: Getting Started


$ npm install mocha
$ mkdir test
$ $EDITOR test/test.js # or open with your favorite editor

In your editor:

var assert = require('assert');
describe('Array', function () {
  describe('#indexOf()', function () {
    it('should return -1 when the value is not present', function () {
      assert.equal([1, 2, 3].indexOf(4), -1);

Back in the terminal:

$ ./node_modules/mocha/bin/mocha

      ✓ should return -1 when the value is not present

  1 passing (9ms)

Set up a test script in package.json:

"scripts": {
  "test": "mocha"

Then run tests with:

$ npm test

Mocha.js: Run Cycle Overview

A brief outline on the order Mocha’s components are executed. Worth noting that all hooks, describe and it callbacks are run in the order they are defined (i.e. found in the file).

run 'mocha spec.js'
spawn child process
|--------------> inside child process
  process and apply options
  run spec file/s
  |--------------> per spec file
    suite callbacks (e.g., 'describe')
    'before' root-level pre-hook
    'before' pre-hook
    |--------------> per test
      'beforeEach' root-level pre-hook
      'beforeEach' pre-hook
      test callbacks (e.g., 'it')
      'afterEach' post-hook
      'afterEach' root-level post-hook
    |<-------------- per test end
    'after' post-hook
    'after' root-level post-hooks
  |<-------------- per spec file end
|<-------------- inside child process end

Mocha.js: before, beforeEach, after, afterEach

You can use before these to setup and restore any test helpers or components. You can even call these agains in nested describe.

describe('My feature test', function() {
    let classToTest;
    beforeEach(function() {
        classToTest = new ClassToTest();
    afterEach(function() {
        classToTest = null;

Mocha.js: Only and Skip

You use it to perform a test, it allows it.only will ensure that only this test will executed, it.skip will skip the test.

it.only('should be able to get valueA', function() {

it.skip('this test is going to be skipped', function() {

Mocha.js: Root Hook Plugins Cheat Sheet


A Root Hook Plugin file is a script which exports (via module.exports) a mochaHooks property. It is loaded via --require <file>.

With CommonJS

exports.mochaHooks = {
  beforeEach(done) {
    // do something before every test

With ES Modules

export const mochaHooks = {
  beforeEach(done) {
    // do something before every test

Available Root Hooks


In serial mode (Mocha’s default), before all tests begin, once only

In parallel mode, run before all tests begin, for each file


In both modes, run before each test


In serial mode, run after all tests end, once only

In parallel mode, run after all tests end, for each file


In both modes, run after every test

Multiple Root Hooks In A Single Plugin

export const mochaHooks = {
  beforeEach: [
    function (done) {
      // do something before every test,
      // then run the next hook in this array
    async function () {
      // async or Promise-returning functions allowed

Multiple Root Hook Plugins

Multiple root hook plugins can be registered by using --require multiple times.

Migrating Tests To Use Root Hook Plugins

  1. Find your root hooks (hooks defined outside of a suite–usually describe() callback).
  2. Create a new file, e.g., test/hooks.js.
  3. Move your root hooks into test/hooks.js.
  4. In test/hooks.js, make your hooks a member of an exported mochaHooks property.
  5. Use --require test/hooks.js (even better: use a config file with {"require": "test/hooks.js"}) when running your tests.

Mocha.js: Reporters Cheat Sheet


Alias: Specspec

This is the default reporter. The Spec reporter outputs a hierarchical view nested just as the test cases are.

Dot Matrix

Alias: Dotdot

The Dot Matrix reporter is a series of characters which represent test cases. Failures highlight in red exclamation marks (!), pending tests with a blue comma (,), and slow tests as yellow. Good if you prefer minimal output.


Alias: Nyannyan


Alias: TAPtap

Landing Strip

Alias: Landinglanding


Alias: Listlist

The List reporter outputs a simple specifications list as test cases pass or fail, outputting the failure details at the bottom of the output.


Alias: Progressprogress

The Progress reporter implements a simple progress-bar


Alias: JSONjson

The JSON reporter outputs a single large JSON object when the tests have completed (failures or not).

Json Stream

Alias: JSONStreamjson-stream

The JSON Stream reporter outputs newline-delimited JSON “events” as they occur, beginning with a “start” event, followed by test passes or failures, and then the final “end” event.


Alias: Minmin

The Min reporter displays the summary only, while still outputting errors on failure. This reporter works great with --watch as it clears the terminal in order to keep your test summary at the top.


Alias: Docdoc

The Doc reporter outputs a hierarchical HTML body representation of your tests. Wrap it with a header, footer, and some styling, then you have some fantastic documentation!


Alias: Markdownmarkdown

The Markdown reporter generates a markdown TOC and body for your test suite. This is great if you want to use the tests as documentation within a Github wiki page, or a markdown file in the repository that Github can render.


Alias: XUnitxunit

The XUnit reporter is also available. It outputs an XUnit-compatible XML document, often applicable in CI servers.

By default, it will output to the console. To write directly to a file, use --reporter-option output=filename.xml.

To specify a custom report title, use --reporter-option suiteName="Custom name".

Html Reporter

Alias: HTMLhtml

The HTML reporter is not intended for use on the command-line.

Mocha.js: Interfaces Cheat Sheet


The BDD interface provides:










The TDD interface provides: 








The keys beforeafterbeforeEach, and afterEach are special-cased, object values are suites, and function values.


he QUnit-inspired interface matches the “flat” look of QUnit, where the test suite title is simply defined before the test-cases. Like TDD, it uses suite() and test(), but resembling BDD, it also contains before()after()beforeEach(), and afterEach()


The require interface allows you to require the describe and friend words directly using require and call them whatever you want. This interface is also useful if you want to avoid global variables in your tests.

Mocha.js: Running in the Browser Cheat Sheet

Every release of Mocha will have new builds of ./mocha.js and ./mocha.css for use in the browser.

Browser-specific methods

The following method(s) only function in a browser context:

mocha.allowUncaught() : If called, uncaught errors will not be absorbed by the error handler.

A typical setup might look something like the following, where we call mocha.setup('bdd') to use the BDD interface before loading the test scripts, running them onload with

  <meta charset="utf-8">
  <title>Mocha Tests</title>
  <link href="[email protected]/mocha.css" rel="stylesheet" />
  <div id="mocha"></div>

  <script src=""></script>
  <script src="[email protected]/mocha.js"></script>

  <script src="test.array.js"></script>
  <script src="test.object.js"></script>
  <script src="test.xhr.js"></script>


The browser may use the --grep as functionality. Append a query-string to your URL: ?grep=api.

Browser Configuration

Mocha options can be set via mocha.setup()

Mocha.js: Desktop Notification Cheat Sheet

Node-based notifications

Enable Mocha’s desktop notifications:

$ mocha --growl

Browser-based notifications

Enable Mocha’s web notifications with a slight modification to your client-side mocha HTML. Add a call to mocha.growl() prior to running your tests:

<!DOCTYPE html>
<html lang="en">
    <meta charset="utf-8" />
    <title>Mocha Tests</title>
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <link rel="stylesheet" href="" />
    <div id="mocha"></div>

    <script src=""></script>
    <script src=""></script>

    <script class="mocha-init">
      mocha.growl(); // <-- Enables web notifications
    <script src="test.spec.js"></script>
    <script class="mocha-exec">;

Mocha.js: The test/ Directory

By default, mocha looks for the glob "./test/*.js", so you may want to put your tests in test/ folder. If you want to include subdirectories, pass the --recursive option.

To configure where mocha looks for tests, you may pass your own glob:

$ mocha --recursive "./spec/*.js"

Some shells support recursive matching by using the globstar (**) wildcard. Bash >= 4.3 supports this with the globstar option which must be enabled to get the same results as passing the --recursive option (ZSH and Fish support this by default). With recursive matching enabled, the following is the same as passing --recursive:

$ mocha "./spec/**/*.js"

Note: Double quotes around the glob are recommended for portability.

Mocha.js: Error Codes Cheat Sheet

ERR_MOCHA_INVALID_ARG_TYPEwrong type was passed for a given argument
ERR_MOCHA_INVALID_ARG_VALUEinvalid or unsupported value was passed for a given argument
ERR_MOCHA_INVALID_EXCEPTIONa falsy or otherwise underspecified exception was thrown
ERR_MOCHA_INVALID_INTERFACEinterface specified in options not found
ERR_MOCHA_INVALID_REPORTERreporter specified in options not found
ERR_MOCHA_NO_FILES_MATCH_PATTERNtest file(s) could not be found
ERR_MOCHA_UNSUPPORTEDrequested behavior, option, or parameter is unsupported

Mocha.js: Timeouts Cheat Sheet


describe('a suite of tests', function() {

  it('should take less than 500ms', function(done){
    setTimeout(done, 300);

  it('should take less than 500ms as well', function(done){
    setTimeout(done, 250);


it('should take less than 500ms', function(done){
  setTimeout(done, 300);


describe('a suite of tests', function() {
  beforeEach(function(done) {
    this.timeout(3000); // A very long environment setup.
    setTimeout(done, 2500);

Mocha.js: Usage Cheat Sheet

Usage: mocha [debug] [options] [files]
-V, --versionoutput the version number
-A, --async-only force all tests to take a callback (async) or return a promise
-c, --colors force enabling of colors
-C, --no-colors force disabling of colors
-G, --growl enable growl notification support
-O, --reporter-options <k=v,k2=v2,...> reporter-specific options
-R, --reporter <name> specify the reporter to use (default: “spec”)
-S, --sort sort test files
-b, --bail bail after first test failure
-d, --debug enable node’s debugger, a synonym for node –debug
-g, --grep <pattern> only run tests matching <pattern>
-f, --fgrep <string> only run tests containing <string>
-gc, --expose-gc expose gc extension
-i, --invert inverts --grep and --fgrep matches
-r, --require <name> require the given module (default: [])
-s, --slow <ms> specify “slow” test threshold in milliseconds (default: 75)
-t, --timeout <ms> specify the test timeout threshold in milliseconds (default: 2000)
-u, --ui <name> specify user-interface (bdd|tdd|qunit|exports) (default: “bdd”)
-w, --watch watch files in the current working directory for changes
--check-leaks check for global variable leaks
--full-trace display the full stack trace
--compilers <ext>:<module>,... use the given module(s) to compile files (default: [])
--debug-brk enable node’s debugger breaking on the first line
--globals <names> allow the given comma-delimited global [names] (default: [])
--es_staging enable all staged features
--harmony<_classes,_generators,...> all node --harmony* flags are available
--preserve-symlinks Instructs the module loader to preserve symbolic links when resolving and caching modules
--icu-data-dirinclude ICU data
--inline-diffs display actual/expected differences inline within each string
--no-diff do not show a diff on failure
--inspect activate devtools in chrome
--inspect-brkactivate devtools in chrome and break on the first line
--interfaces output provided interfaces and exit
--no-deprecation silence deprecation warnings
--exit force shutdown of the event loop after test run: mocha will call process.exit
--no-timeoutsdisables timeouts, given implicitly with –debug/–inspect
--no-warnings silence all node process warnings
--opts <path> specify opts path (default: “test/mocha.opts”)
--perf-basic-prof enable perf linux profiler (basic support)
--napi-modules enable experimental NAPI modules
--prof log statistical profiling information
--log-timer-events Time events including external callbacks
--recursive include sub directories
--reporters output provided reporters and exit
--retries <times> specify number of times to retry a failed test case (default: 0)
--throw-deprecation throw an exception anytime a deprecated function is used
--trace trace function calls
--trace-deprecation show stack traces on deprecations
--trace-warnings show stack traces on node process warnings
--use_strictenforce strict mode
--watch-extensions <ext>,... specify extensions to monitor with –watch (default: [“js”])
--delay wait for async suite definition
--allow-uncaught enable uncaught errors to propagate
--forbid-only causes test marked with only to fail the suite
--forbid-pending causes pending tests and test marked with skip to fail the suite
--file <file> adds file be loaded prior to suite execution (default: [])
--exclude <file> adds file or glob pattern to ignore (default: [])
-h, --helpoutput usage information

Mocha.js: Testing Mocha

To run Mocha’s tests, you will need GNU Make or compatible; Cygwin should work.

$ cd /path/to/mocha
$ npm install
$ npm test

To use a different reporter:

$ REPORTER=nyan npm test