Mocha JavaScript Test Framework: Guide and Examples

Mocha is a JavaScript test framework for Node.js. It runs tests in sequence and shows results. It works with any assertion library and supports asynchronous tests.

Benefits of Mocha Framework for JavaScript Testing

Mocha helps you run JavaScript tests with control and order. It works with any assertion library, so you can pick the one you want.

Let’s look at an example to understand it:

const assert = require('assert');

describe('Math tests', function() {
  it('adds numbers', function() {
    assert.strictEqual(2 + 3, 5);
  });
});

This test checks if 2 + 3 equals 5.

Here is the output in the terminal:

Install and Set Up the Mocha Framework to Test in JavaScript

Install Mocha with npm. Run this command in your project folder.

npm install --save-dev mocha

Add a test script package.json after installation in the same root of your code.

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

Mocha with Assertion Libraries (Chai Example)

Create a file named test.js. Write a test inside it.

const assert = require('assert');

describe('Array', function() {
  it('returns -1 if value not found', function() {
    assert.strictEqual([1, 2, 3].indexOf(4), -1);
  });
});

This test checks if indexOf(4) gives -1 for an array with numbers 1, 2, 3. Here is an image that shows you the result:

Run Test with the CLI of the Mocha Framework in JavaScript

Mocha works well with Chai. Chai gives different styles for writing assertions.

Install Chai with npm.

npm install chai --save-dev

Here’s how it works in practice.

const chai = require('chai');
const expect = chai.expect;

describe('Chai test', function() {
  it('checks type of value', function() {
    expect('hello').to.be.a('string');
  });
});

This test checks if 'hello' is a string.

Run tests with the Mocha CLI. Use the command below.

npx mocha

The command finds test files and runs them. It shows the results in your terminal.

Here is the result:

Organize Tests in Projects

Keep tests in a folder named test in your project. Mocha looks inside this folder by default. Place each test file inside it.

Use names that describe the test purpose.

Wrapping Up

You learned the role of Mocha and how to install it. Also, you understood how to write a test and how to run it.

Here is a quick recap:

• Mocha runs JavaScript tests with sequence and control.
• It works with assertion libraries like Chai.
• It runs tests from the CLI with npx mocha.
• It supports the test organization inside a test folder.

FAQs

npm install --save-dev mocha

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

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