如上面的知识图谱所示，一个常见的测试框架通常需要实现这些功能:
● before/after 钩子函数: 如beforeEach，afterEach，
● Mock方法： 函数Mock，时间mock等。
● 断言: 判断一个描述是否正确，在Jest中常为 expect(xxx).toBe(xxx) 的形式
● 测试覆盖率：提供多种形式的测试报告，如HTML，文本等形式

这些基本的测试功能是每一个测试框架都需要考虑实现的，各个测试框架除了Api的设计方式可能有所不同之外，其核心功能是一致的，在使用新的测试框架的时候你仍然可以通过上面的脑图按图索骥，抛开Api的表象，快速上手使用。

在本文中我们会介绍这些功能在Jest中的使用方式，让大家熟悉Jest的Api。

before/after钩子函数

beforeAll(() => { // 在所有测试用例前执行一次
  return initializeCityDatabase();
});

afterAll(() => { // 在所有测试用例后执行一次
  return clearCityDatabase();
});

beforeEach(() => { // 在每个测试用例前执行一次
  initializeCityDatabase();
});

afterEach(() => { // 在每个测试用例后执行一次
  clearCityDatabase();
});

函数Mock

函数Mock可以解耦要调用的函数，测试对函数的调用是否正确，比如调用函数传入的参数是否正确，是否被调用等。详细看文档

mock函数实现

const mockCallback = jest.fn(x => 42 + x);
forEach([0, 1], mockCallback);
// The mock function is called twice
expect(mockCallback.mock.calls.length).toBe(2); 
// mock的函数有特殊的mock属性包含了这个函数被调用的相关信息

mock返回值

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

Mock函数断言

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

// 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);

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

时间Mock

模拟时间流逝useFakeTimers

通常一些本地的时间方法比如setTimeout等不太适合测试环境，因为这些方法会依赖真实的时间流逝。jest可以交换这些时间函数，控制时间的推移，比方说:

beforeAll(() => {
  jest.useFakeTimers();
});

// 或者有多个测试用例使用在每个测试用例执行之前执行
beforeEach(() => {
  jest.useFakeTimers();
});

控制时间运行 runAllTimers

还有一些测试用例用于测试某个回调一秒后会被调用:

test('calls the callback after 1 second', () => {
  const timerGame = require('../timerGame');
  const callback = jest.fn();

  timerGame(callback);
	
	// 这个时间点还没有被调用
  expect(callback).not.toBeCalled();

  // 快进，让所有时间回调都执行
  jest.runAllTimers();

  // 现在回调被调用
  expect(callback).toBeCalled();
  expect(callback).toHaveBeenCalledTimes(1);
});

使用真实的时间useRealTimers

在测试过程中可能想要恢复时间正常流逝，可以使用useRealTimers()方法恢复

断言方法

通用断言ToBe

Object.js 判断

test('two plus two is four', () => {
  expect(2 + 2).toBe(4);
});

通用断言ToEqual

深比对

test('object assignment', () => {
  const data = {one: 1};
  data['two'] = 2;
  expect(data).toEqual({one: 1, two: 2});
});

翻转判断Not

test('adding positive numbers is not zero', () => {
  for (let a = 1; a < 10; a++) {
    for (let b = 1; b < 10; b++) {
      expect(a + b).not.toBe(0);
    }
  }
});

真假断言

● toBeNull matches only null
● toBeUndefined matches only undefined
● toBeDefined is the opposite of toBeUndefined
● toBeTruthy matches anything that an if statement treats as true
● toBeFalsy matches anything that an if statement treats as false

test('zero', () => {
  const z = 0;
  expect(z).not.toBeNull();
  expect(z).toBeDefined();
  expect(z).not.toBeUndefined();
  expect(z).not.toBeTruthy();
  expect(z).toBeFalsy();
});

数字断言

test('two plus two', () => {
  const value = 2 + 2;
  expect(value).toBeGreaterThan(3);
  expect(value).toBeGreaterThanOrEqual(3.5);
  expect(value).toBeLessThan(5);
  expect(value).toBeLessThanOrEqual(4.5);

  // toBe and toEqual are equivalent for numbers
  expect(value).toBe(4);
  expect(value).toEqual(4);
});

浮点数断言toBeCloseTo

test('adding floating point numbers', () => {
  const value = 0.1 + 0.2;
  //expect(value).toBe(0.3);           This won't work because of rounding error
  expect(value).toBeCloseTo(0.3); // This works.
});

字符串断言toMatch

test('there is no I in team', () => {
  expect('team').not.toMatch(/I/);
});

test('but there is a "stop" in Christoph', () => {
  expect('Christoph').toMatch(/stop/);
});

数组 & 迭代器断言toContain

const shoppingList = [
  'diapers',
  'kleenex',
  'trash bags',
  'paper towels',
  'beer',
];

test('the shopping list has beer on it', () => {
  expect(shoppingList).toContain('beer');
  expect(new Set(shoppingList)).toContain('beer');
});

异常断言toThrow

function compileAndroidCode() {
  throw new ConfigError('you are using the wrong JDK');
}

test('compiling android goes as expected', () => {
  expect(compileAndroidCode).toThrow();
  expect(compileAndroidCode).toThrow(ConfigError);

  // You can also use the exact error message or a regexp
  expect(compileAndroidCode).toThrow('you are using the wrong JDK');
  expect(compileAndroidCode).toThrow(/JDK/);
});