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Mocha Testing Cheatsheet

A quick reference guide to Mocha, a JavaScript test framework, covering setup, assertions, hooks, and advanced features for effective testing.

Mocha Setup and Basics

Installation & Setup

Install Mocha via npm:

npm install --global mocha

Or as a dev dependency:

npm install --save-dev mocha

Create a test directory and a test file (e.g., test.js).

Add a test script to your package.json:

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

Basic Test Structure

A basic test case includes describe (test suite) and it (test case) blocks:

describe('My Function', function() {
  it('should return true', function() {
    // Assertion here
  });
});

describe(string, function): Defines a suite of tests.

it(string, function): Defines an individual test case.

Running Tests

Run tests from the command line:

mocha

Or using the npm script:

npm test

Mocha will look for test files in the test directory by default.

To run a specific test file:

mocha test/my_test.js

Assertions and Hooks

Common Assertions

Mocha is often used with an assertion library like Chai. Here are some common assertions:

assert.equal(actual, expected, message): Checks equality.

assert.strictEqual(actual, expected, message): Checks strict equality.

assert.notEqual(actual, expected, message): Checks inequality.

assert.isTrue(value, message): Checks if a value is true.

assert.isFalse(value, message): Checks if a value is false.

assert.isNull(value, message): Checks if a value is null.

assert.isNotNull(value, message): Checks if a value is not null.

Chai Assertions (BDD Style)

Chai provides expect and should styles:

expect(actual).to.equal(expected): Checks equality.

expect(actual).to.be.a('string'): Checks type.

expect(actual).to.be.true: Checks if true.

actual.should.equal(expected) (requires chai.should()):

const chai = require('chai');
chai.should(); // Initialize should

const myVar = 5;
myVar.should.equal(5);

Hooks

Hooks are used to set up preconditions and clean up after tests.

before(function): Runs once before all tests in a suite.

after(function): Runs once after all tests in a suite.

beforeEach(function): Runs before each test in a suite.

afterEach(function): Runs after each test in a suite.

Example:

describe('My Suite', function() {
  beforeEach(function() {
    // Setup code before each test
  });

  afterEach(function() {
    // Cleanup code after each test
  });

  it('should do something', function() {
    // Test
  });
});

Asynchronous Testing

Testing Asynchronous Code

Mocha supports testing asynchronous code using callbacks, Promises, and async/await.

Using callbacks: it(string, function(done)) - Pass done and call it when the async operation completes.

it('should call done()', function(done) {
  setTimeout(function() {
    assert.ok(true);
    done();
  }, 50);
});

Using Promises: Return a Promise from the it block.

it('should return a promise', function() {
  return new Promise(function(resolve, reject) {
    setTimeout(function() {
      resolve();
    }, 50);
  });
});

Using async/await:

it('should use async/await', async function() {
  await new Promise(resolve => setTimeout(resolve, 50));
  assert.ok(true);
});

For Promises, use .then and .catch to handle fulfillment and rejection.

Timeouts

Mocha has a default timeout of 2000ms. You can change it using this.timeout(ms) within a test or hook.

it('should take a long time', function(done) {
  this.timeout(5000); // Set timeout to 5 seconds
  setTimeout(done, 4000);
});

To disable timeouts, use this.timeout(0).

Advanced Mocha Features

Pending Tests

You can define pending tests (tests without a function body) using it or xit.

it('should be implemented later');

xit('should also be implemented later', function() {
  // This test will be skipped
});

Skipping Tests

You can skip tests using .skip or this.skip().

it.skip('should skip this test', function() {
  // This test will be skipped
});

it('should skip conditionally', function() {
  if (condition) {
    this.skip();
  }
});

Only Running Specific Tests

You can run only specific tests using .only.

it.only('should only run this test', function() {
  // This test will be the only one run
});

describe.only('My Suite', function() {
  // Only tests in this suite will be run
});

Reporters

Mocha supports different reporters to format test results. Specify the reporter using the -R flag.

mocha -R spec

Common reporters include spec, list, progress, json, nyan, etc.
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