前端常见功能方法实现1、防抖 2、节流 3、深拷贝 4、实现 Promise 5、异步控制并发数 6、ES5继承（寄生组

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)
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2、节流

function throttle(fn, delay) {
  let last = 0 // 上次触发时间
  return (...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)
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3、深拷贝

function deepClone(obj, cache = new WeakMap()) {
  if (typeof obj !== 'object') return obj
  if (obj === null) return obj
  if (cache.get(obj)) return cache.get(obj) // 防止循环引用，程序进入死循环
  if (obj instanceof Date) return new Date(obj)
  if (obj instanceof RegExp) return new RegExp(obj)
  
  // 找到所属原型上的constructor，所属原型上的constructor指向当前对象的构造函数
  let cloneObj = new obj.constructor()
  cache.set(obj, cloneObj) // 缓存拷贝的对象，用于处理循环引用的情况
  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
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4、实现 Promise

class MyPromise {
  constructor(executor) { // executor执行器
    this.status = 'pending' // 等待状态
    this.value = null // 成功或失败的参数
    this.fulfilledCallbacks = [] // 成功的函数队列
    this.rejectedCallbacks = [] // 失败的函数队列
    const that = this
    function resolve(value) { // 成功的方法
      if (that.status === 'pending') {
        that.status = 'resolved'
        that.value = value
        that.fulfilledCallbacks.forEach(myFn => myFn(that.value)) //执行回调方法
      }
    }
    function reject(value) { //失败的方法
      if (that.status === 'pending') {
        that.status = 'rejected'
        that.value = value
        that.rejectedCallbacks.forEach(myFn => myFn(that.value)) //执行回调方法
      }
    }
    try {
      executor(resolve, reject)
    } catch (err) {
      reject(err)
    }
  }
  then(onFulfilled, onRejected) {
    if (this.status === 'pending') {
      // 等待状态，添加回调函数到成功的函数队列
      this.fulfilledCallbacks.push(() => {
        onFulfilled(this.value)
      })
      // 等待状态，添加回调函数到失败的函数队列
      this.rejectedCallbacks.push(() => {
        onRejected(this.value)
      })
    }
    if (this.status === 'resolved') { // 支持同步调用
      console.log('this', this)
      onFulfilled(this.value)
    }
    if (this.status === 'rejected') { // 支持同步调用
      onRejected(this.value)
    }
  }
}

// 测试
function fn() {
  return new MyPromise((resolve, reject) => {
    setTimeout(() => {
      if(Math.random() > 0.6) {
        resolve(1)
      } else {
        reject(2)
      }
    }, 1000)
  })
}
fn().then(
  res => {
    console.log('res', res) // res 1
  },
  err => {
    console.log('err', err) // err 2
  })
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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

    // 同步启动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).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'])
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6、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.contructor = Child
Child.prototype.study = function () {
  console.log(this.name + ' is studying')
}

// 测试
let child = new Child('xiaoming', 16)
console.log(child.name) // xiaoming
child.eat() // xiaoming is eating
child.study() // xiaoming is studying
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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']
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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]
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8、数组去重

Set 去重

cosnt newArr = [...new Set(arr)] 
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Array.from 去重

const newArr = Array.from(new Set(arr))
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indexOf 去重

function resetArr(arr) {
  let res = []
  arr.forEach(item => {
    if (res.indexOf(item) === -1) {
      res.push(item)
    }
  })
  return res
}

// 测试
const arr = [1, 1, 2, 3, 3]
console.log(resetArr(arr)) // [1, 2, 3]
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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());
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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' }
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10、事件总线 | 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)
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11、HTML便签解码

htmlFormat(html) {
  if (!html) {
    return false;
  }
  var temp = "";
  if(html.length == 0) return "";
  temp = html.replace(/&amp;/g,"&");
  temp = temp.replace(/&lt;/g,"<");
  temp = temp.replace(/&gt;/g,">");
  temp = temp.replace(/&nbsp;/g," ");
  temp = temp.replace(/&#39;/g,"\'");
  temp = temp.replace(/&quot;/g,"\"");
  return temp;
}
//测试
let p = '&lt;p&gt;hello&lt;/p&gt;'
let q = htmlFormat(p)
console.log(q) //<p>hello</p>

12、小程序判断滑动方向及滑动距离

// 触摸开始事件
  touchStart = (e) => {
    let sx = e.touches[0].pageX
    let sy = e.touches[0].pageY
    const touchStart = [sx, sy]
    this.setState({ touchStart })
  }

// 触摸滑动事件
  touchMove = (e) => {
    const { touchStart } = this.state
    let sx = e.touches[0].pageX
    let sy = e.touches[0].pageY
    const touchMove = [sx, sy]
    console.error(touchStart[1] - touchMove[1])//滑动距离
  }

// 触摸结束事件
  touchEnd = () => {
    const { touchStart, touchMove } = this.state
    console.error(touchStart)
    console.error(touchMove)
    if (touchMove[0] === 0) return
    if (touchStart[0] - touchMove[0] > 0) {
      console.log('向左滑，这里可以调用方法，及页面跳转事件')
    } else if (touchStart[0] - touchMove[0] < 0) {
      console.log('向右滑，这里可以调用方法，及页面跳转事件')
    } else {
      console.log('向上或向下滑动')
    }
    console.error(touchStart[0] - touchMove[0]) //判断左右
    console.error(touchStart[1] - touchMove[1]) //判断上下
  }