10个常见的前端手写功能，你全都会吗？

万丈高楼平地起，地基打的牢，才能永远立于不败之地。今天给大家带来的是10个常见的 JavaScript 手写功能，重要的地方已添加注释。有的是借鉴别人的，有的是自己写的，如有不正确的地方，欢迎多多指正。

1、防抖

function debounce(fn, delay) {  let timer  return function (...args) {    if (timer) {      clearTimeout(timer)
    }
    timer = setTimeout(() => {
      fn.apply(this, args)
    }, delay)
  }
}// 测试function task() {  console.log('run task')
}const debounceTask = debounce(task, 1000)window.addEventListener('scroll', debounceTask)

2、节流

function throttle(fn, delay) {  let last = 0 // 上次触发时间
  return function (...args) {    const now = Date.now()    if (now - last > delay) {
      last = now
      fn.apply(this, args)
    }
  }
}// 测试function task() {  console.log('run task')
}const throttleTask = throttle(task, 1000)window.addEventListener('scroll', throttleTask)

3、深拷贝

JSON 方法

// 不支持值为undefined、函数和循环引用的情况const cloneObj = JSON.parse(JSON.stringify(obj))

递归拷贝

function deepClone(obj, cache = new WeakMap()) {  if (obj === null || typeof obj !== 'object') return obj  if (obj instanceof Date) return new Date(obj)  if (obj instanceof RegExp) return new RegExp(obj)  
  if (cache.has(obj)) return cache.get(obj) // 如果出现循环引用，则返回缓存的对象，防止递归进入死循环
  let cloneObj = new obj.constructor() // 使用对象所属的构造函数创建一个新对象
  cache.set(obj, cloneObj) // 缓存对象，用于循环引用的情况

  for (let key in obj) {    if (obj.hasOwnProperty(key)) {
      cloneObj[key] = deepClone(obj[key], cache) // 递归拷贝
    }
  }  return cloneObj
}// 测试const obj = { name: 'Jack', address: { x: 100, y: 200 } }
obj.a = obj // 循环引用const newObj = deepClone(obj)console.log(newObj.address === obj.address) // false

4、手写 Promise

class MyPromise {  constructor(executor) {    this.status = 'pending' // 初始状态为等待
    this.value = null // 成功的值
    this.reason = null // 失败的原因
    this.onFulfilledCallbacks = [] // 成功的回调函数数组
    this.onRejectedCallbacks = [] // 失败的回调函数数组
    let resolve = value => {      if (this.status === 'pending') {        this.status = 'fulfilled'
        this.value = value;        this.onFulfilledCallbacks.forEach(fn => fn()) // 调用成功的回调函数
      }
    }    let reject = reason => {      if (this.status === 'pending') {        this.status = 'rejected'
        this.reason = reason        this.onRejectedCallbacks.forEach(fn => fn()) // 调用失败的回调函数
      }
    };    try {      executor(resolve, reject)
    } catch (err) {      reject(err)
    }
  }  then(onFulfilled, onRejected) {    return new MyPromise((resolve, reject) => {      if (this.status === 'fulfilled') {        setTimeout(() => {          const x = onFulfilled(this.value);
          x instanceof MyPromise ? x.then(resolve, reject) : resolve(x)
        })
      }      if (this.status === 'rejected') {        setTimeout(() => {          const x = onRejected(this.reason)
          x instanceof MyPromise ? x.then(resolve, reject) : resolve(x)
        })
      }      if (this.status === 'pending') {        this.onFulfilledCallbacks.push(() => { // 将成功的回调函数放入成功数组
          setTimeout(() => {            const x = onFulfilled(this.value)
            x instanceof MyPromise ? x.then(resolve, reject) : resolve(x)
          })
        })        this.onRejectedCallbacks.push(() => { // 将失败的回调函数放入失败数组
          setTimeout(() => {            const x = onRejected(this.reason)
            x instanceof MyPromise ? x.then(resolve, reject) : resolve(x)
          })
        })
      }
    })
  }
}// 测试function p1() {  return new MyPromise((resolve, reject) => {    setTimeout(resolve, 1000, 1)
  })
}function p2() {  return new MyPromise((resolve, reject) => {    setTimeout(resolve, 1000, 2)
  })
}p1().then(res => {  console.log(res) // 1
  return p2()
}).then(ret => {  console.log(ret) // 2})

5、异步控制并发数

function limitRequest(urls = [], limit = 3) {  return new Promise((resolve, reject) => {    const len = urls.length
    let count = 0

    // 同时启动limit个任务
    while (limit > 0) {      start()
      limit -= 1
    }    function start() {      const url = urls.shift() // 从数组中拿取第一个任务
      if (url) {
        axios.post(url).then(res => {          // todo
        }).catch(err => {          // todo
        }).finally(() => {          if (count == len - 1) {            // 最后一个任务完成
            resolve()
          } else {            // 完成之后，启动下一个任务
            count++            start()
          }
        })
      }
    }

  })
}// 测试limitRequest(['http://xxa', 'http://xxb', 'http://xxc', 'http://xxd', 'http://xxe'])

6、继承

ES5 继承（寄生组合继承）

function Parent(name) {  this.name = name
}Parent.prototype.eat = function () {  console.log(this.name + ' is eating')
}function Child(name, age) {  Parent.call(this, name)  this.age = age
}Child.prototype = Object.create(Parent.prototype)Child.prototype.constructor = Child// 测试let xm = new Child('xiaoming', 12) 
console.log(xm.name) // xiaomingconsole.log(xm.age) // 12xm.eat() // xiaoming is eating

ES6 继承

class Parent {  constructor(name) {    this.name = name
  }  eat() {    console.log(this.name + ' is eating')
  }
}class Child extends Parent {  constructor(name, age) {    super(name)    this.age = age
  }
}// 测试let xm = new Child('xiaoming', 12) 
console.log(xm.name) // xiaomingconsole.log(xm.age) // 12xm.eat() // xiaoming is eating

7、数组排序

sort 排序

// 对数字进行排序，简写const arr = [3, 2, 4, 1, 5]
arr.sort((a, b) => a - b)console.log(arr) // [1, 2, 3, 4, 5]// 对字母进行排序，简写const arr = ['b', 'c', 'a', 'e', 'd']
arr.sort()console.log(arr) // ['a', 'b', 'c', 'd', 'e']

冒泡排序

function bubbleSort(arr) {  let len = arr.length
  for (let i = 0; i < len - 1; i++) {    // 从第一个元素开始，比较相邻的两个元素，前者大就交换位置
    for (let j = 0; j < len - 1 - i; j++) {      if (arr[j] > arr[j + 1]) {        let num = arr[j]
        arr[j] = arr[j + 1]
        arr[j + 1] = num
      }
    }    // 每次遍历结束，都能找到一个最大值，放在数组最后
  }  return arr
}//测试console.log(bubbleSort([2, 3, 1, 5, 4])) // [1, 2, 3, 4, 5]

8、数组去重

Set 去重

const newArr = [...new Set(arr)]// 或const newArr = Array.from(new Set(arr))

indexOf 去重

const newArr = arr.filter((item, index) => arr.indexOf(item) === index)

9、获取 url 参数

URLSearchParams 方法

// 创建一个URLSearchParams实例const urlSearchParams = new URLSearchParams(window.location.search);// 把键值对列表转换为一个对象const params = Object.fromEntries(urlSearchParams.entries());

split 方法

function getParams(url) {  const res = {}  if (url.includes('?')) {    const str = url.split('?')[1]    const arr = str.split('&')
    arr.forEach(item => {      const key = item.split('=')[0]      const val = item.split('=')[1]
      res[key] = decodeURIComponent(val) // 解码
    })
  }  return res
}// 测试const user = getParams('http://www.baidu.com?user=%E9%98%BF%E9%A3%9E&age=16')console.log(user) // { user: '阿飞', age: '16' }

10、发布订阅模式

class EventEmitter {  constructor() {    this.cache = {}
  }  on(name, fn) {    if (this.cache[name]) {      this.cache[name].push(fn)
    } else {      this.cache[name] = [fn]
    }
  }  off(name, fn) {    const tasks = this.cache[name]    if (tasks) {      const index = tasks.findIndex((f) => f === fn || f.callback === fn)      if (index >= 0) {
        tasks.splice(index, 1)
      }
    }
  }  emit(name, once = false) {    if (this.cache[name]) {      // 创建副本，如果回调函数内继续注册相同事件，会造成死循环
      const tasks = this.cache[name].slice()      for (let fn of tasks) {        fn();
      }      if (once) {        delete this.cache[name]
      }
    }
  }
}// 测试const eventBus = new EventEmitter()const task1 = () => { console.log('task1'); }const task2 = () => { console.log('task2'); }

eventBus.on('task', task1)
eventBus.on('task', task2)
eventBus.off('task', task1)setTimeout(() => {
  eventBus.emit('task') // task2}, 1000)