JAVASCRIPT

mock an API call in Jest

// index.test.js
const getFirstAlbumTitle = require('./index');
const axios = require('axios');

jest.mock('axios');

it('returns the title of the first album', async () => {
  axios.get.mockResolvedValue({
    data: [
      {
        userId: 1,
        id: 1,
        title: 'My First Album'
      },
      {
        userId: 1,
        id: 2,
        title: 'Album: The Sequel'
      }
    ]
  });

  const title = await getFirstAlbumTitle();
  expect(title).toEqual('My First Album');
});

mock callback function jest

const bodyToAssertAgainst = {};
globals.request.post = jest.fn().mockImplementation((obj, cb) => {
    cb(null, bodyToAssertAgainst);
});

mock function jest

/*
The Mock Function
The goal for mocking is to replace something we don’t control with something 
we do, so it’s important that what we replace it with has all the features 
we need.

The Mock Function provides features to:

1. Capture calls
2. Set return values
3. Change the implementation

The simplest way to create a Mock Function instance is with jest.fn()
*/

test("returns undefined by default", () => {
  const mock = jest.fn();

  let result = mock("foo");

  expect(result).toBeUndefined();
  expect(mock).toHaveBeenCalled();
  expect(mock).toHaveBeenCalledTimes(1);
  expect(mock).toHaveBeenCalledWith("foo");
});

jest mock call

test("mock.calls", () => {
  const mockFn = jest.fn();
  mockFn(1, 2);

  expect(mockFn.mock.calls).toEqual([[1, 2]]);
});