1const users = [{id: 1, name: 'Hugo'}, {id: 2, name: 'Francesco'}];
2
3test('we should have ids 1 and 2', () => {
4  expect(users).toEqual(
5    expect.arrayContaining([
6      expect.objectContaining({id: 1}),
7      expect.objectContaining({id: 2})
8    ])
9  );
10});

1const fruits = ['apple', 'cat'];
2
3test('should have array of string', () => {
4  expect(fruits).toEqual(
5    expect.arrayContaining([expect.any(String)])
6  );
7});

1import pizzas from '../data';
2
3// very basic test to notify the user if our pizza data has changed
4test('the pizza data is correct', () => {
5  expect(pizzas).toMatchSnapshot();
6  expect(pizzas).toHaveLength(4);
7  expect(pizzas.map(pizza => pizza.name)).toEqual([
8    'Chicago Pizza',
9    'Neapolitan Pizza',
10    'New York Pizza',
11    'Sicilian Pizza',
12  ]);
13});
14
15// let's test that each item in the pizza data has the correct properties
16for (let i = 0; i < pizzas.length; i += 1) {
17  it(`pizza[${i}] should have properties (id, name, image, desc, price)`, () => {
18    expect(pizzas[i]).toHaveProperty('id');
19    expect(pizzas[i]).toHaveProperty('name');
20    expect(pizzas[i]).toHaveProperty('image');
21    expect(pizzas[i]).toHaveProperty('desc');
22    expect(pizzas[i]).toHaveProperty('price');
23  });
24}
25
26// default jest mock function
27test('mock implementation of a basic function', () => {
28  const mock = jest.fn(() => 'I am a mock function');
29
30  expect(mock('Calling my mock function!')).toBe('I am a mock function');
31  expect(mock).toHaveBeenCalledWith('Calling my mock function!');
32});
33
34// let's mock the return value and test calls
35test('mock return value of a function one time', () => {
36  const mock = jest.fn();
37
38  // we can chain these!
39  mock.mockReturnValueOnce('Hello').mockReturnValueOnce('there!');
40
41  mock(); // first call 'Hello'
42  mock(); // second call 'there!'
43
44  expect(mock).toHaveBeenCalledTimes(2); // we know it's been called two times
45
46  mock('Hello', 'there', 'Steve'); // call it with 3 different arguments
47  expect(mock).toHaveBeenCalledWith('Hello', 'there', 'Steve');
48
49  mock('Steve'); // called with 1 argument
50  expect(mock).toHaveBeenLastCalledWith('Steve');
51});
52
53// let's mock the return value
54// difference between mockReturnValue & mockImplementation
55test('mock implementation of a function', () => {
56  const mock = jest.fn().mockImplementation(() => 'United Kingdom');
57  expect(mock('Location')).toBe('United Kingdom');
58  expect(mock).toHaveBeenCalledWith('Location');
59});
60
61// spying on a single function of an imported module, we can spy on its usage
62// by default the original function gets called, we can change this
63test('spying using original implementation', () => {
64  const pizza = {
65    name: n => `Pizza name: ${n}`,
66  };
67  const spy = jest.spyOn(pizza, 'name');
68  expect(pizza.name('Cheese')).toBe('Pizza name: Cheese');
69  expect(spy).toHaveBeenCalledWith('Cheese');
70});
71
72// we can mock the implementation of a function from a module
73test('spying using mockImplementation', () => {
74  const pizza = {
75    name: n => `Pizza name: ${n}`,
76  };
77  const spy = jest.spyOn(pizza, 'name');
78  spy.mockImplementation(n => `Crazy pizza!`);
79
80  expect(pizza.name('Cheese')).toBe('Crazy pizza!');
81  spy.mockRestore(); // back to original implementation
82  expect(pizza.name('Cheese')).toBe('Pizza name: Cheese');
83});
84
85// let's test pizza return output
86test('pizza returns new york pizza last', () => {
87  const pizza1 = pizzas[0];
88  const pizza2 = pizzas[1];
89  const pizza3 = pizzas[2];
90  const pizza = jest.fn(currentPizza => currentPizza.name);
91
92  pizza(pizza1); // chicago pizza
93  pizza(pizza2); // neapolitan pizza
94  pizza(pizza3); // new york pizza
95
96  expect(pizza).toHaveLastReturnedWith('New York Pizza');
97});
98
99// let's match some data against our object
100test('pizza data has new york pizza and matches as an object', () => {
101  const newYorkPizza = {
102    id: 3,
103    name: 'New York Pizza',
104    image: '/images/ny-pizza.jpg',
105    desc:
106      'New York-style pizza has slices that are large and wide with a thin crust that is foldable yet crispy. It is traditionally topped with tomato sauce and mozzarella cheese.',
107    price: 8,
108  };
109  expect(pizzas[2]).toMatchObject(newYorkPizza);
110});
111
112// async example, always return a promise (can switch out resolves with reject)
113test('expect a promise to resolve', async () => {
114  const user = {
115    getFullName: jest.fn(() => Promise.resolve('Karl Hadwen')),
116  };
117  await expect(user.getFullName('Karl Hadwen')).resolves.toBe('Karl Hadwen');
118});
119
120test('expect a promise to reject', async () => {
121  const user = {
122    getFullName: jest.fn(() =>
123      Promise.reject(new Error('Something went wrong'))
124    ),
125  };
126  await expect(user.getFullName('Karl Hadwen')).rejects.toThrow(
127    'Something went wrong'
128  );
129});
130

1test('id should match', () => {
2  const obj = {
3    id: '111',
4    productName: 'Jest Handbook',
5    url: 'https://jesthandbook.com'
6  };
7  expect(obj.id).toEqual('111');
8});
9