Mock Functions

Mock functions make it easy to test the links between code by erasing the actual implementation of a function, capturing calls to the function (and the parameters passed in those calls), capturing instances of constructor functions when instantiated with new, and allowing test-time configuration of return values.

There are two ways to get your hands on mock functions: Either by require()ing a mocked component (via jest.mock('moduleName')) or by explicitly requesting one from jest.fn() in your test:

const myMock = jest.fn();
myMock('a', 'b');
// > [ [1], ['a', 'b'] ]

.mock property

All mock functions have this special .mock property, which is where data about how the function has been called is kept. The .mock property also tracks the value of this for each call, so it is possible to inspect this as well:

const myMock = jest.fn();

const a = new myMock();
const b = {};
const bound = myMock.bind(b);

// > [ <a>, <b> ]

These mock members are very useful in tests to assert how these functions get called, or instantiated:

// The function was called exactly once

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

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

// This function was instantiated exactly twice

// The object returned by the first instantiation of this function
// had a `name` property whose value was set to 'test'

Mock Return Values

Mock functions can also be used to inject test values into your code during a test:

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


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

Mock functions are also very effective in code that uses a functional continuation-passing style. Code written in this style helps avoid the need for complicated stubs that recreate behavior of the real component they're standing in for, in favor of injecting values directly into the test right before they're used.

const filterTestFn = jest.fn();

// Make the mock return `true` for the first call,
// and `false` for the second call

const result = [11, 12].filter(filterTestFn);

// > [11]
// > [ [11], [12] ]

Most real-world examples actually involve getting ahold of a mock function on a dependent component and configuring that, but the technique is the same. In these cases, try to avoid the temptation to implement logic inside of any function that's not directly being tested.

Mock Implementations

Still, there are cases where it's useful to go beyond the ability to specify return values and full-on replace the implementation of a mock function. This can be done with jest.fn or the mockImplementationOnce method on mock functions.

const myMockFn = jest.fn(cb => cb(null, true));

myMockFn((err, val) => console.log(val));
// > true

myMockFn((err, val) => console.log(val));
// > true

The mockImplementation method is useful when you need to define the default implementation of a mock function that is created from another module:

// foo.js
module.exports = function() {
  // some implementation;

// test.js
jest.mock('../foo'); // this happens automatically with automocking
const foo = require('../foo');

// foo is a mock function
foo.mockImplementation(() => 42);
// > 42

When you need to recreate a complex behavior of a mock function such that multiple function calls produce different results, use the mockImplementationOnce method:

const myMockFn = jest.fn()
  .mockImplementationOnce(cb => cb(null, true))
  .mockImplementationOnce(cb => cb(null, false));

myMockFn((err, val) => console.log(val));
// > true

myMockFn((err, val) => console.log(val));
// > false

When the mocked function runs out of implementations defined with mockImplementationOnce, it will execute the default implementation set with jest.fn (if it is defined):

const myMockFn = jest.fn(() => 'default')
  .mockImplementationOnce(() => 'first call')
  .mockImplementationOnce(() => 'second call');

console.log(myMockFn(), myMockFn(), myMockFn(), myMockFn());
// > 'first call', 'second call', 'default', 'default'

For cases where we have methods that are typically chained (and thus always need to return this), we have a sugary API to simplify this in the form of a .mockReturnThis() function that also sits on all mocks:

const myObj = {
  myMethod: jest.fn().mockReturnThis(),

// is the same as

const otherObj = {
  myMethod: jest.fn(function() {
    return this;

Custom Matchers

Finally, in order to make it simpler to assert how mock functions have been called, we've added some custom matcher functions for you:

// The mock function was called at least once

// The mock function was called at least once with the specified args
expect(mockFunc).toBeCalledWith(arg1, arg2);

// The last call to the mock function was called with the specified args
expect(mockFunc).lastCalledWith(arg1, arg2);

These matchers are really just sugar for common forms of inspecting the .mock property. You can always do this manually yourself if that's more to your taste or if you need to do something more specific:

// The mock function was called at least once

// The mock function was called at least once with the specified args
expect(mockFunc.mock.calls).toContain([arg1, arg2]);

// The last call to the mock function was called with the specified args
expect(mockFunc.mock.calls[mockFunc.mock.calls.length - 1]).toEqual(
  [arg1, arg2]

// The first arg of the last call to the mock function was `42`
// (note that there is no sugar helper for this specific of an assertion)
expect(mockFunc.mock.calls[mockFunc.mock.calls.length - 1][0]).toBe(42);

For a complete list of matchers, check out the reference docs.

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