Mocha - for running the tests
Chai - for asserting the results
Sinon - for managing side effects and external dependencies 

npm install --save-dev mocha chai



package.json

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



Then you can run tests with npm test. Mocha is then looking for tests under folder test.



Dummy test

test/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 not give a result of 6', function(){
    const num1 = 2;  
    const num2 = 3;
    expect(num1 + num2).not.to.equal(6);
})





Real test

middleware/is-auth.js

module.exports = (req, res, next) => {
    const authHeader = req.get('Authorization');
    is (!authHeader) {
        const error = new Error('Not authenticated.');
        error.statusCode = 401;
        throw error;
    }
    ....
}



test/auth-middleware.js

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

it('should throw an error if no authorization header is present', function() {
    const req = {
        get: function(headerName) {
            return null;
        }
    };
    expect(authMiddleware.bind(req, {}, () => {})).to.throw('Not authenticated.');
});