How to test JavaScript application with Mocha and Chai

Technologies used

• Node - 23.5.0
• Chai - 5.2.0
• Mocha - 11.2.2

1. npm init -y

   Initialize a new Node.js project in current directory.

2. function getHelloMessage() {
     return 'Hello, world!';
   }
   
   module.exports = {
     getHelloMessage
   }

   Create getHelloMessage() function that will be tested.

3. npm i -D mocha@11.2.2 chai@5.2.0

   Install Mocha and Chai as development dependencies.

4. const { getHelloMessage } = require('./index');
   
   const assert = require('assert');
   
   describe('getHelloMessage', () => {
   
     it('should return "Hello, world!"', () => {
       const actualResult = getHelloMessage();
       assert.equal('Hello, world!', actualResult);
     });
   });

   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.

5. {
     "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": "^5.2.0",
       "mocha": "^11.2.2"
     }
   }

   Add test script to package.json.

6. npm test

   Run the test.

   getHelloMessage
   ✔ should return "Hello, world!"
   
   1 passing (3ms)