Unit Testing in NodeJS with Mocha and Chai

Getting Started

• Install mocha and chai in your project folder

npm install mocha --save

npm install chai --save

• Create a folder named test
• Add a file named start.js
• Add "mocha" to "test" in package.json

"test": "mocha"

• Add the following example to start.js

const expect = require('chai').expect;
it('should add numbers correctly', function () {
    const num1 = 2;
    const num2 = 3;
    expect(num1 + num2).to.equal(5);
})

it('should nod give a result of 6', function () {
    const num1 = 3;
    const num2 = 3;
    expect(num1 + num2).not.to.equal(5);
})

• Run the test

npm test

Real Scenario Example
Say, we have a middleware named is-auth.js

module.exports = (req, res, next) => {
  const authHeader = req.get('Authorization');
  if(!authHeader) {
    throw new Error('Not authenticated.');
  }
}

Add the following to a new file in test folder — auth-test.js

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

const authMiddleware = require('../middleware/is-auth');

it('should throw error if authorization header is NOT present', function () {
    const req = {
        get: function (headerName) {
            return null;
        }
    };

    expect(authMiddleware.bind(this, req, {}, () => { })).to.throw('Not authenticated.');
})

it('should throw error if authorization header is only one word', function () {
    const req = {
        get: function (headerName) {
            return 'xyz';
        }
    };

    expect(authMiddleware.bind(this, req, {}, () => { })).to.throw();
})

Run the test

npm test

Output

project@1.0.0 test C:\project
> mocha

  √ should throw error if authorization header is NOT present
  √ should throw error if authorization header is only one word
  √ should add numbers correctly
  √ should nod give a result of 6

  4 passing (31ms)

Describe

Adding it blocks in describe function gives additional information of where the ojutput is coming from
Add the following to auth-test.js

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

const authMiddleware = require('../middleware/is-auth');

describe('Auth middleware', function () {
    it('should throw error if authorization header is NOT present', function () {
        const req = {
            get: function (headerName) {
                return null;
            }
        };

        expect(authMiddleware.bind(this, req, {}, () => { })).to.throw('Not authenticated.');
    })

    it('should throw error if authorization header is only one word', function () {
        const req = {
            get: function (headerName) {
                return 'xyz';
            }
        };

        expect(authMiddleware.bind(this, req, {}, () => { })).to.throw();
    })
})

Output

project@1.0.0 test C:\project
> mocha

  √ should add numbers correctly
  √ should nod give a result of 6
  Auth middleware
    √ should throw error if authorization header is NOT present
    √ should throw error if authorization header is only one word

  4 passing (52ms)

What should not be tested and what should be

Functions from third party should not be tested by you. We must only test if our code behaves correctly on including the function.

Sinon

For restoring state of functions back if made changes for testing

Example,

sinon.stub(jwt, 'verify); // verify is the function of jwt library
jwt.verify.returns({userId: 'abc'}); // makes verify return {userId: 'abc'}
authMiddleware(req, {}, () => {}); // calls authMiddleware
expect(req).to.have.property('userId'); // checks if userId exists
expect(req).to.have.property('userId', 'abc'); // checks if value of userId is 'abc'
expect(jwt.verify.called).to.be.true; // checks if jwt.verfify function was called
jwt.verify.restore(); // restores state  of jwt.verify back to what it was before testing