How to test JavaScript application with Mocha and Chai

npm init -y

Initialize a new Node.js project in current directory.

function getHelloMessage() {
  return 'Hello, world!';
}

module.exports = {
  getHelloMessage
}

Create getHelloMessage() function that will be tested.

npm iex -D mocha chai

Install Mocha and Chai as development dependencies.

const { expect } = require('chai');
const { getHelloMessage } = require('./index');

describe('getHelloMessage()', () => {

  it('should return "Hello, world!"', () => {
    expect(getHelloMessage()).to.equal('Hello, world!');
  });

});

Create a test file index.test.js to test getHelloMessage() function.

1. The expect function is used to create assertions in the test cases.
2. Define a test suite for the getHelloMessage() function. The describe() function is used to group related test cases together, making the test output more organized and readable.
3. Define a test case for the getHelloMessage() function. The it() function is used to define a single test case. The first argument is a description of the test case, and the second argument is a function that contains the test logic.

{
  "name": "how-to-test-js-app-with-mocha-and-chai",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "mocha index.test.js"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "chai": "^5.1.2",
    "mocha": "^11.0.1"
  }
}

Add test script to package.json.

npm test

Run the test.

getHelloMessage
✔ should return "Hello, world!"

1 passing (3ms)