Testing Express Applications

Supertest

Supertest was made by the developers of Express. We use this in conjunction with expect.

npm i supertest@2.0.0

In server.js

app.get('/users', (req, res) => {
        res.status(200).send([
            {
                name: 'Andrew Mead',
                age: 26
            }, {
                name: 'Nuwan Goonewardena',
                age: 25
            },{
                name: 'Alex Gordon',
                age: 23
            }
        ]);
    });

In server.test.js, we use (done) as it is an asynchronous function.

it('should return correct response', (done) => {
    request(app)
        .get('/users')
        .expect(200)
        .expect((res) => {
            expect(res.body).toInclude({
                name: 'Nuwan Goonewardena',
                age: 25
            })
        })
        .end(done);     
});

Using describe()

We use describe and pass in an arrow function with all our tests. This let's us categorize our tests to keep our test suite clean & organized.

describe('Utils', () => {
    it('should add two numbers', () => {
        var res = utils.add(33,11);
        expect(res).toBe(44).toBeA('number');
    });

We can nest describe functions to make it more scannable.

describe('Utils', () => {
    describe('#add', () => {
        it('should add two numbers', () => {
            var res = utils.add(33,11);
            expect(res).toBe(44).toBeA('number');
        });
        
        it('should async add two numbers', (done) => {
            utils.asyncAdd(4, 3, (sum) => {
                expect(sum).toBe(7).toBeA('number')
                done();
            });
        });
    });

Test Spies

Spies let you swap out a real function for a testing utility. When that test function gets called, we can make assertions about it, like being called with certain arguments.

const expect = require('expect');

describe('App', () => {
    it('should call the spy correctly', () => {
        var spy = expect.createSpy();
        spy();
        expect(spy).toHaveBeenCalled();
    });
});

You can also assert whether the spy() function was called with certain arguments:

    spy('Nuwan', 25);
    expect(spy).toHaveBeenCalledWith('Nuwan', 25);

Simulating functions

Replace function with spies, so that when a function gets called, it is called with the spy. This lets us assert that the spy function was called with certain arguments.

npm i rewire@2.5.2 --save-dev

const rewire = require('rewire');

rewire is used instead of require when you're loading in the file you want to mock out.

var app = rewire('./app');

This loads your file through require but it also adds two functions to to app.js. These methods are:

app.__set__
app.__get__

We use these to mock out various data inside of app.js. Inside of the describe function;

    var db = {
        saveUser: expect.createSpy()
    };
    app.__set__('db', db);
  
    it('should call saveUser with user object', () => {
        var email = 'nuwan@example.com';
        var password = '123abc'

        app.handleSignup(email, password);
        expect(db.saveUser).toHaveBeenCalledWith({email, password})
    });
});