Mock functions are also known as "spies", because they let you spy on the behavior of a function that is called indirectly by some other code, rather than just testing the output. You can create a mock function with jest.fn(). If no implementation is given, the mock function will return undefined when invoked.

Methods

• mockFn.getMockName()
• mockFn.mock.calls
• mockFn.mock.instances
• mockFn.mockClear()
• mockFn.mockReset()
• mockFn.mockRestore()
• mockFn.mockImplementation(fn)
• mockFn.mockImplementationOnce(fn)
• mockFn.mockName(value)
• mockFn.mockReturnThis()
• mockFn.mockReturnValue(value)
• mockFn.mockReturnValueOnce(value)

Reference

mockFn.getMockName()

available in Jest 22.0.0+

Returns the mock name string set by calling mockFn.mockName(value).

mockFn.mock.calls

An array that represents all calls that have been made into this mock function. Each call is represented by an array of arguments that were passed during the call.

For example: A mock function f that has been called twice, with the arguments f('arg1', 'arg2'), and then with the arguments f('arg3', 'arg4') would have a mock.calls array that looks like this:

[['arg1', 'arg2'], ['arg3', 'arg4']];

mockFn.mock.instances

An array that contains all the object instances that have been instantiated from this mock function using new.

For example: A mock function that has been instantiated twice would have the following mock.instances array:

const mockFn = jest.fn();

const a = new mockFn();
const b = new mockFn();

mockFn.mock.instances[0] === a; // true
mockFn.mock.instances[1] === b; // true

mockFn.mockClear()

Resets all information stored in the mockFn.mock.calls and mockFn.mock.instances arrays.

Often this is useful when you want to clean up a mock's usage data between two assertions.

Beware that mockClear will replace mockFn.mock, not just mockFn.mock.calls and mockFn.mock.instances. You should therefore avoid assigning mockFn.mock to other variables, temporary or not, to make sure you don't access stale data.

The clearMocks configuration option is available to clear mocks automatically between tests.

mockFn.mockReset()

Resets all information stored in the mock, including any initial implementation and mock name given.

This is useful when you want to completely restore a mock back to its initial state.

Beware that mockReset will replace mockFn.mock, not just mockFn.mock.calls and mockFn.mock.instances. You should therefore avoid assigning mockFn.mock to other variables, temporary or not, to make sure you don't access stale data.

mockFn.mockRestore()

Removes the mock and restores the initial implementation.

This is useful when you want to mock functions in certain test cases and restore the original implementation in others.

Beware that mockFn.mockRestore only works when mock was created with jest.spyOn. Thus you have to take care of restoration yourself when manually assigning jest.fn().

mockFn.mockImplementation(fn)

Accepts a function that should be used as the implementation of the mock. The mock itself will still record all calls that go into and instances that come from itself – the only difference is that the implementation will also be executed when the mock is called.

Note: jest.fn(implementation) is a shorthand for jest.fn().mockImplementation(implementation).

For example:

const mockFn = jest.fn().mockImplementation(scalar => 42 + scalar);
// or: jest.fn(scalar => 42 + scalar);

const a = mockFn(0);
const b = mockFn(1);

a === 42; // true
b === 43; // true

mockFn.mock.calls[0][0] === 0; // true
mockFn.mock.calls[1][0] === 1; // true

mockImplementation can also be used to mock class constructors:

// SomeClass.js
module.exports = class SomeClass {
  m(a, b) {}
}

// OtherModule.test.js
jest.mock('./SomeClass');  // this happens automatically with automocking
const SomeClass = require('./SomeClass')
const mMock = jest.fn()
SomeClass.mockImplementation(() => {
  return {
    m: mMock
  }
})

const some = new SomeClass()
some.m('a', 'b')
console.log('Calls to m: ', mMock.mock.calls)

mockFn.mockImplementationOnce(fn)

Accepts a function that will be used as an implementation of the mock for one call to the mocked function. Can be chained so that multiple function calls produce different results.

var 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(() => defaultValue) or .mockImplementation(() => defaultValue) if they were called:

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

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

mockFn.mockName(value)

available in Jest 22.0.0+

Accepts a string to use in test result output in place of "jest.fn()" to indicate which mock function is being referenced.

For example:

const mockFn = jest.fn().mockName('mockedFunction');
// mockFn();
expect(mockFn).toHaveBeenCalled();

Will result in this error:

    expect(mockedFunction).toHaveBeenCalled()

    Expected mock function to have been called.

mockFn.mockReturnThis()

Just a simple sugar function for:

jest.fn(function() {
  return this;
});

mockFn.mockReturnValue(value)

Accepts a value that will be returned whenever the mock function is called.

const mock = jest.fn();
mock.mockReturnValue(42);
mock(); // 42
mock.mockReturnValue(43);
mock(); // 43

mockFn.mockReturnValueOnce(value)

Accepts a value that will be returned for one call to the mock function. Can be chained so that successive calls to the mock function return different values. When there are no more mockReturnValueOnce values to use, calls will return a value specified by mockReturnValue.

const myMockFn = jest.fn()
  .mockReturnValue('default')
  .mockReturnValueOnce('first call')
  .mockReturnValueOnce('second call');

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