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Version: 28.x

Funciones Mock

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 only testing the output. Puedes crear una falsa función con jest.fn(). Si no se da la implementación, la función mockeda devolverá undefined cuando se invoque.

info

The TypeScript examples from this page will only work as documented if you explicitly import Jest APIs:

import {expect, jest, test} from '@jest/globals';

Métodos​

Referencia​

mockFn.getMockName()​

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

mockFn.mock.calls​

An array containing the call arguments of all calls that have been made to this mock function. Each item in the array is 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.results​

An array containing the results of all calls that have been made to this mock function. Each entry in this array is an object containing a type property, and a value property. type will be one of the following:

  • 'return' - Indicates that the call completed by returning normally.
  • 'throw' - Indicates that the call completed by throwing a value.
  • 'incomplete' - Indicates that the call has not yet completed. This occurs if you test the result from within the mock function itself, or from within a function that was called by the mock.

The value property contains the value that was thrown or returned. value is undefined when type === 'incomplete'.

Una matriz que contiene todas las instancias de objeto que han se han creado instancias de esta función mockeada utilizando new.

[
  {
    type: 'return',
    value: 'result1',
  },
  {
    type: 'throw',
    value: {
      /* Error instance */
    },
  },
  {
    type: 'return',
    value: 'result2',
  },
];

mockFn.mock.instances​

Por ejemplo: una función mockeada que ha creado una instancia dos veces, tendría la siguiente matriz de mock.instances:

Restablece toda la información almacenada en las matrices mockFn.mock.calls y mockFn.mock.instances.

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.mock.contexts​

An array that contains the contexts for all calls of the mock function.

A context is the this value that a function receives when called. The context can be set using Function.prototype.bind, Function.prototype.call or Function.prototype.apply.

Por ejemplo:

const mockFn = jest.fn();

const boundMockFn = mockFn.bind(thisContext0);
boundMockFn('a', 'b');
mockFn.call(thisContext1, 'a', 'b');
mockFn.apply(thisContext2, ['a', 'b']);

mockFn.mock.contexts[0] === thisContext0; // true
mockFn.mock.contexts[1] === thisContext1; // true
mockFn.mock.contexts[2] === thisContext2; // true

mockFn.mock.lastCall​

An array containing the call arguments of the last call that was made to this mock function. If the function was not called, it will return undefined.

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.lastCall array that looks like this:

['arg3', 'arg4'];

mockFn.mockClear()​

Clears all information stored in the mockFn.mock.calls, mockFn.mock.instances, mockFn.mock.contexts and mockFn.mock.results arrays. Often this is useful when you want to clean up a mocks usage data between two assertions.

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

danger

Beware that mockFn.mockClear() will replace mockFn.mock, not just reset the values of its properties! You should, therefore, avoid assigning mockFn.mock to other variables, temporary or not, to make sure you don't access stale data.

mockFn.mockReset()​

Does everything that mockFn.mockClear() does, and also removes any mocked return values or implementations.

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

The resetMocks configuration option is available to reset mocks automatically before each test.

info

Resetting a mock created with jest.spyOn() will result in a function with no return value.

mockFn.mockRestore()​

Does everything that mockFn.mockReset() does, and also restores the original (non-mocked) implementation.

Esto es útil para casos donde se desee simular funciones en ciertas pruebas y restaurar la implementación original para otras.

The restoreMocks configuration option is available to restore mocks automatically before each test.

info

mockFn.mockRestore() only works when the mock was created with jest.spyOn(). De modo que se debe realizar la restauración de manera independiente a Jest cuando se crean mocks con jest.fn().

mockFn.mockImplementation(fn)​

Acepta una función que debe ser utilizada como la implementación de la funcion a mockear. El mockeo de sí mismo aún grabará todas las llamadas que entran y casos que provienen de sí mismo, la única diferencia es que la aplicación también se ejecutará cuando se llama a la funcion mock.

tip

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

const mockFn = jest.fn(scalar => 42 + scalar);

mockFn(0); // 42
mockFn(1); // 43

mockFn.mockImplementation(scalar => 36 + scalar);

mockFn(2); // 38
mockFn(3); // 39

.mockImplementation() can also be used to mock class constructors:

SomeClass.js
module.exports = class SomeClass {
  method(a, b) {}
};
SomeClass.test.js
const SomeClass = require('./SomeClass');

jest.mock('./SomeClass'); // this happens automatically with automocking

const mockMethod = jest.fn();
SomeClass.mockImplementation(() => {
  return {
    method: mockMethod,
  };
});

const some = new SomeClass();
some.method('a', 'b');

console.log('Calls to method: ', mockMethod.mock.calls);

mockFn.mockImplementationOnce(fn)​

Acepta una función que se utilizará como una implementación de la funcion de mock para una llamada a la función mockeada. Puede ser encadenado para que varias llamadas de función produzcan resultados distintos.

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

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

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

mockFn(); // 'first call'
mockFn(); // 'second call'
mockFn(); // 'default'
mockFn(); // 'default'

mockFn.mockName(name)​

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

Por ejemplo:

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

// mockFn();
expect(mockFn).toHaveBeenCalled();

Will result in this error:

expect(mockedFunction).toHaveBeenCalled()

Expected number of calls: >= 1
Received number of calls:    0

mockFn.mockReturnThis()​

Shorthand for:

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

mockFn.mockReturnValue(value)​

Shorthand for:

jest.fn().mockImplementation(() => 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)​

Shorthand for:

jest.fn().mockImplementationOnce(() => value);

Acepta un valor que se devolverá para una llamada a la función de prueba. 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 mockFn = jest
  .fn()
  .mockReturnValue('default')
  .mockReturnValueOnce('first call')
  .mockReturnValueOnce('second call');

mockFn(); // 'first call'
mockFn(); // 'second call'
mockFn(); // 'default'
mockFn(); // 'default'

mockFn.mockResolvedValue(value)​

Shorthand for:

jest.fn().mockImplementation(() => Promise.resolve(value));

Useful to mock async functions in async tests:

test('async test', async () => {
  const asyncMock = jest.fn().mockResolvedValue(43);

  await asyncMock(); // 43
});

mockFn.mockResolvedValueOnce(value)​

Shorthand for:

jest.fn().mockImplementationOnce(() => Promise.resolve(value));

Useful to resolve different values over multiple async calls:

test('async test', async () => {
  const asyncMock = jest
    .fn()
    .mockResolvedValue('default')
    .mockResolvedValueOnce('first call')
    .mockResolvedValueOnce('second call');

  await asyncMock(); // 'first call'
  await asyncMock(); // 'second call'
  await asyncMock(); // 'default'
  await asyncMock(); // 'default'
});

mockFn.mockRejectedValue(value)​

Shorthand for:

jest.fn().mockImplementation(() => Promise.reject(value));

Useful to create async mock functions that will always reject:

test('async test', async () => {
  const asyncMock = jest
    .fn()
    .mockRejectedValue(new Error('Async error message'));

  await asyncMock(); // throws 'Async error message'
});

mockFn.mockRejectedValueOnce(value)​

Shorthand for:

jest.fn().mockImplementationOnce(() => Promise.reject(value));

Useful together with .mockResolvedValueOnce() or to reject with different exceptions over multiple async calls:

test('async test', async () => {
  const asyncMock = jest
    .fn()
    .mockResolvedValueOnce('first call')
    .mockRejectedValueOnce(new Error('Async error message'));

  await asyncMock(); // 'first call'
  await asyncMock(); // throws 'Async error message'
});

TypeScript Usage​

tip

Please consult the Getting Started guide for details on how to setup Jest with TypeScript.

jest.fn(implementation?)​

Correct mock typings will be inferred if implementation is passed to jest.fn(). There are many use cases where the implementation is omitted. To ensure type safety you may pass a generic type argument (also see the examples above for more reference):

import {expect, jest, test} from '@jest/globals';
import type add from './add';
import calculate from './calc';

test('calculate calls add', () => {
  // Create a new mock that can be used in place of `add`.
  const mockAdd = jest.fn<typeof add>();

  // `.mockImplementation()` now can infer that `a` and `b` are `number`
  // and that the returned value is a `number`.
  mockAdd.mockImplementation((a, b) => {
    // Yes, this mock is still adding two numbers but imagine this
    // was a complex function we are mocking.
    return a + b;
  });

  // `mockAdd` is properly typed and therefore accepted by anything
  // requiring `add`.
  calculate(mockAdd, 1, 2);

  expect(mockAdd).toHaveBeenCalledTimes(1);
  expect(mockAdd).toHaveBeenCalledWith(1, 2);
});