Jest - Mock

About

Mocking is just replacing a function by another called a mocking function.

In Jest mocking function can replace the code executed but they also record all call made to them creating a command history.

Articles Related

Features

Function Call history

Mock function can register their call against their .mock property where assertion can be made.

Example from the Doc

The function to test

function forEach(items, callback) {
    for (let index = 0; index < items.length; index++) {
        callback(items[index]);
    }
}

The mock function definition

const mockCallback = jest.fn(x => 42 + x);

The injection

forEach([0, 1], mockCallback);

The Test based on .mock property that contains the history of the mock function call

test('Injection Test', () => {

    // The mock function was called twice
    expect(mockCallback.mock.calls.length).toBe(2);

    // The first argument of the first call to the function was 0
    expect(mockCallback.mock.calls[0][0]).toBe(0);

    // The first argument of the second call to the function was 1
    expect(mockCallback.mock.calls[1][0]).toBe(1);

    // The return value of the first call to the function was 42
    expect(mockCallback.mock.results[0].value).toBe(42);
}
)

Mock return values

Jest mock function can return mock value

Sync

From Doc

Define a mock function

const myMock = jest.fn();
console.log(myMock());
// > undefined

Add some value to a stack of values

myMock
  .mockReturnValueOnce(10) 
  .mockReturnValueOnce('x')
  .mockReturnValue(true);

Destack them

console.log(myMock(), myMock(), myMock(), myMock());
// > 10, 'x', true, true

Async

How to mock a function that return a promise.

Data

const users = [{name: 'Bob'}];
const resp = {data: users};

Mock function definition

const myMock = jest.fn();
myMock.mockResolvedValue(resp);

Mock module / function

Mocking module happens via variable or path injection

Example from the doc

Mocking:

// After the import, you can define that a variable will be mocked
import lib from '../lib';
jest.mock('lib');
//
// Or before the import
jest.mock('../lib'); 
const lib = require('../lib');

lib and all sub function will become a mocked function/library where all mock function can be called to set up a mock.

// Adding return value to a subfunction
lib.subfunction
  .mockReturnValueOnce(true)
  .mockReturnValueOnce(false);
// Mocking the function
lib.subfunction.mockImplementation(() => 42);
// Mocking the function differently for each call
lib.subfunction
  .mockImplementationOnce(cb => cb(null, true))
  .mockImplementationOnce(cb => cb(null, false));