模拟函数

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模拟函数让您能够通过以下方式测试代码间的关联：抹去函数的实际实现、捕获对该函数的调用（及调用时传递的参数）、捕获通过new实例化的构造函数实例，并允许在测试时配置返回值。

有两种方式创建模拟函数：在测试代码中直接创建模拟函数，或编写manual mock来覆写模块依赖。

使用模拟函数

假设我们正在测试一个forEach函数的实现，该函数会对给定数组中的每个元素调用回调函数。

forEach.js

export function forEach(items, callback) {
  for (const item of items) {
    callback(item);
  }
}

要测试该函数，我们可以使用模拟函数并检查模拟状态，以验证回调函数是否按预期被调用。

forEach.test.js

const forEach = require('./forEach');

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

test('forEach mock function', () => {
  forEach([0, 1], mockCallback);

  // The mock function was called twice
  expect(mockCallback.mock.calls).toHaveLength(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属性

所有模拟函数都具有特殊的.mock属性，其中保存着函数如何被调用以及返回了哪些值的信息。.mock属性还会追踪每次调用时的this值，因此也能对此进行检查：

const myMock1 = jest.fn();
const a = new myMock1();
console.log(myMock1.mock.instances);
// > [ <a> ]

const myMock2 = jest.fn();
const b = {};
const bound = myMock2.bind(b);
bound();
console.log(myMock2.mock.contexts);
// > [ <b> ]

这些模拟成员在测试中非常有用，可用于断言函数的调用方式、实例化过程或返回值：

// The function was called exactly once
expect(someMockFunction.mock.calls).toHaveLength(1);

// 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');

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

// The function was called with a certain `this` context: the `element` object.
expect(someMockFunction.mock.contexts[0]).toBe(element);

// This function was instantiated exactly twice
expect(someMockFunction.mock.instances.length).toBe(2);

// The object returned by the first instantiation of this function
// had a `name` property whose value was set to 'test'
expect(someMockFunction.mock.instances[0].name).toBe('test');

// The first argument of the last call to the function was 'test'
expect(someMockFunction.mock.lastCall[0]).toBe('test');

模拟返回值

模拟函数还能在测试期间向代码注入测试值：

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

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

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

对于采用函数式延续传递风格（continuation-passing style）的代码，模拟函数同样非常高效。这种编码风格有助于避免创建复杂的存根（stubs）来模拟真实组件的行为，而是支持在即将使用时直接将值注入测试。

const filterTestFn = jest.fn();

// Make the mock return `true` for the first call,
// and `false` for the second call
filterTestFn.mockReturnValueOnce(true).mockReturnValueOnce(false);

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

console.log(result);
// > [11]
console.log(filterTestFn.mock.calls[0][0]); // 11
console.log(filterTestFn.mock.calls[1][0]); // 12

多数实际案例涉及获取依赖组件上的模拟函数并进行配置，但技术原理相同。在这些情况下，请避免在非直接测试的函数内部实现逻辑的诱惑。

模块模拟

假设我们有一个从API获取用户的类。该类使用axios调用API，然后返回包含所有用户的data属性：

users.js

import axios from 'axios';

class Users {
  static all() {
    return axios.get('/users.json').then(resp => resp.data);
  }
}

export default Users;

为了避免实际调用API（导致测试缓慢且脆弱），我们可以使用jest.mock(...)函数自动模拟axios模块。

模拟模块后，可为.get提供返回测试断言数据的mockResolvedValue。实际上我们是在声明：希望axios.get('/users.json')返回伪造的响应。

users.test.js

import axios from 'axios';
import Users from './users';

jest.mock('axios');

test('should fetch users', () => {
  const users = [{name: 'Bob'}];
  const resp = {data: users};
  axios.get.mockResolvedValue(resp);

  // or you could use the following depending on your use case:
  // axios.get.mockImplementation(() => Promise.resolve(resp))

  return Users.all().then(data => expect(data).toEqual(users));
});

局部模拟

可以对模块的子集进行模拟，同时保留其余部分的实际实现：

foo-bar-baz.js

export const foo = 'foo';
export const bar = () => 'bar';
export default () => 'baz';

//test.js
import defaultExport, {bar, foo} from '../foo-bar-baz';

jest.mock('../foo-bar-baz', () => {
  const originalModule = jest.requireActual('../foo-bar-baz');

  //Mock the default export and named export 'foo'
  return {
    __esModule: true,
    ...originalModule,
    default: jest.fn(() => 'mocked baz'),
    foo: 'mocked foo',
  };
});

test('should do a partial mock', () => {
  const defaultExportResult = defaultExport();
  expect(defaultExportResult).toBe('mocked baz');
  expect(defaultExport).toHaveBeenCalled();

  expect(foo).toBe('mocked foo');
  expect(bar()).toBe('bar');
});

模拟实现

某些场景下，仅指定返回值还不够，需要完全替换模拟函数的实现。这可通过jest.fn或模拟函数的mockImplementationOnce方法实现。

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

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

当需要为从其他模块创建的模拟函数定义默认实现时，mockImplementation方法非常有用：

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);
foo();
// > 42

需要重现模拟函数的复杂行为（例如多次调用产生不同结果）时，请使用mockImplementationOnce方法：

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

当模拟函数耗尽所有通过mockImplementationOnce定义的实现后，将执行通过jest.fn设置的默认实现（若已定义）：

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

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

对于通常需要链式调用（因此必须始终返回this）的方法，我们提供了语法糖API.mockReturnThis()来简化操作，该函数同样存在于所有模拟对象上：

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

// is the same as

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

模拟名称

您可以选择为模拟函数命名，这样在测试错误输出中就会显示该名称而非 'jest.fn()'。若需快速识别测试输出中报错的模拟函数，请使用 .mockName()。

const myMockFn = jest
  .fn()
  .mockReturnValue('default')
  .mockImplementation(scalar => 42 + scalar)
  .mockName('add42');

自定义匹配器

最后，为了让断言模拟函数的调用情况变得更加轻松，我们为您提供了一些定制匹配器函数：

// The mock function was called at least once
expect(mockFunc).toHaveBeenCalled();

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

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

// All calls and the name of the mock is written as a snapshot
expect(mockFunc).toMatchSnapshot();

这些匹配器是对检查 .mock 属性的常见操作进行的语法糖封装。如果您更倾向手动操作或需要执行更具体的检查，完全可以自行实现：

// The mock function was called at least once
expect(mockFunc.mock.calls.length).toBeGreaterThan(0);

// The mock function was called at least once with the specified args
expect(mockFunc.mock.calls).toContainEqual([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 assertion)
expect(mockFunc.mock.calls[mockFunc.mock.calls.length - 1][0]).toBe(42);

// A snapshot will check that a mock was invoked the same number of times,
// in the same order, with the same arguments. It will also assert on the name.
expect(mockFunc.mock.calls).toEqual([[arg1, arg2]]);
expect(mockFunc.getMockName()).toBe('a mock name');

完整匹配器列表请查阅参考文档。