模拟函数
- info" level="5">info
方法
参考
mockFn.mockImplementationOnce(fn)
mockFn.mockName(name)
mockFn.mockReturnThis()
mockFn.mockReturnValue(value)
mockFn.mockReturnValueOnce(value)
mockFn.mockResolvedValue(value)
mockFn.mockResolvedValueOnce(value)
mockFn.mockRejectedValue(value)
mockFn.mockRejectedValueOnce(value)
TypeScript Usage
- tip
jest.fn(implementation?)

模拟函数

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. You can create a mock function with jest.fn(). If no implementation is given, the mock function will return undefined when invoked. 你可以通过 jest.fn() 创建 mock 函数。当调用没有执行，mock 函数会返回undefined

info" class="reference-link">info

The TypeScript examples from this page will only work as document if you import jest from '@jest/globals':

import {jest} from '@jest/globals';

方法

参考
TypeScript Usage
- jest.fn(implementation?)

参考

`mockFn.getMockName()`

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

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

For example: A mock function f that has been called three times, returning 'result1', throwing an error, and then returning 'result2', would have a mock.results array that looks like this:

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

`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.mock.contexts`

一个包含mock函数所有调用上下文的数组。

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.

例如：

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.

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.

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

`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. (Note that resetting a spy will result in a function with no return value). (注意重置一个 spy 将导致一个没有返回值的函数)。

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

`mockFn.mockRestore()`

Does everything that mockFn.mockReset() does, and also restores the original (non-mocked) 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 the mock was created with jest.spyOn(). Thus you have to take care of restoration yourself when manually assigning jest.fn().

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

`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.

tip

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

JavaScript
TypeScript

const mockFn = jest.fn(scalar => 42 + scalar);
mockFn(0); // 42
mockFn(1); // 43
mockFn.mockImplementation(scalar => 36 + scalar);
mockFn(2); // 38
mockFn(3); // 39

const mockFn = jest.fn((scalar: number) => 42 + scalar);
mockFn(0); // 42
mockFn(1); // 43
mockFn.mockImplementation(scalar => 36 + scalar);
mockFn(2); // 38
mockFn(3); // 39

.mockImplementation() 也可以用于模拟类构造器：

JavaScript
TypeScript

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);

SomeClass.ts

export class SomeClass {
  method(a: string, b: string): void {}
}

SomeClass.test.ts

import {SomeClass} from './SomeClass';
jest.mock('./SomeClass'); // this happens automatically with automocking
const mockMethod = jest.fn<(a: string, b: string) => void>();
SomeClass.mockImplementation(() => {
  return {
    method: mockMethod,
  };
});
const some = new SomeClass();
some.method('a', 'b');
console.log('Calls to method: ', mockMethod.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. Can be chained so that multiple function calls produce different results.

JavaScript
TypeScript

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

const mockFn = jest
  .fn<(cb: (a: null, b: boolean) => void) => void>()
  .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)`

用一个字符串来代替 'jest.fn()' 在测试结果的输出，用于表示正在引用哪个mock函数。

例如：

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

Will result in this error:

expect(mockedFunction).toHaveBeenCalled()
Expected mock function "mockedFunction" to have been called, but it was not called.

`mockFn.mockReturnThis()`

Syntactic sugar function for:

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

`mockFn.mockReturnValue(value)`

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

JavaScript
TypeScript

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

const mock = jest.fn<() => number>();
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. 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. When there are no more mockReturnValueOnce values to use, calls will return a value specified by mockReturnValue.

JavaScript
TypeScript

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

const mockFn = jest
  .fn<() => string>()
  .mockReturnValue('default')
  .mockReturnValueOnce('first call')
  .mockReturnValueOnce('second call');
mockFn(); // 'first call'
mockFn(); // 'second call'
mockFn(); // 'default'
mockFn(); // 'default'

`mockFn.mockResolvedValue(value)`

Syntactic sugar function for:

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

Useful to mock async functions in async tests:

JavaScript
TypeScript

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

test('async test', async () => {
  const asyncMock = jest.fn<() => Promise<number>>().mockResolvedValue(43);
  await asyncMock(); // 43
});

`mockFn.mockResolvedValueOnce(value)`

Syntactic sugar function for:

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

Useful to resolve different values over multiple async calls:

JavaScript
TypeScript

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'
});

test('async test', async () => {
  const asyncMock = jest
    .fn<() => Promise<string>>()
    .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)`

Syntactic sugar function for:

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

Useful to create async mock functions that will always reject:

JavaScript
TypeScript

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

test('async test', async () => {
  const asyncMock = jest
    .fn<() => Promise<never>>()
    .mockRejectedValue(new Error('Async error message'));
  await asyncMock(); // throws 'Async error message'
});

`mockFn.mockRejectedValueOnce(value)`

Syntactic sugar function for:

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

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

JavaScript
TypeScript

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'
});

test('async test', async () => {
  const asyncMock = jest
    .fn<() => Promise<string>>()
    .mockResolvedValueOnce('first call')
    .mockRejectedValueOnce(new Error('Async error message'));
  await asyncMock(); // 'first call'
  await asyncMock(); // throws 'Async error message'
});

TypeScript Usage

tip

请参阅 [入门](GettingStarted. md#using-typescript) 指南，了解如何使用 TypeScript 设置Jest。

`jest.fn(implementation?)`

Correct mock typings will be inferred, if implementation is passed to jest.fn(). There are many use cases there 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).toBeCalledTimes(1);
  expect(mockAdd).toBeCalledWith(1, 2);
});