20道JS原理题助你面试一臂之力(必看)

// 思路：将要改变this指向的方法挂到目标this上执行并返回
Function.prototype.mycall = function (context) {
 if (typeof this !== 'function') {
 throw new TypeError('not funciton')
 }
 context = context || window
 context.fn = this
 let arg = [...arguments].slice(1)
 let result = context.fn(...arg)
 delete context.fn
 return result
} 

// 思路：将要改变this指向的方法挂到目标this上执行并返回
Function.prototype.myapply = function (context) {
 if (typeof this !== 'function') {
 throw new TypeError('not funciton')
 }
 context = context || window
 context.fn = this
 let result
 if (arguments[1]) {
 result = context.fn(...arguments[1])
 } else {
 result = context.fn()
 }
 delete context.fn
 return result
}

// 思路：类似call，但返回的是函数
Function.prototype.mybind = function (context) {
 if (typeof this !== 'function') {
 throw new TypeError('Error')
 }
 let _this = this
 let arg = [...arguments].slice(1)
 return function F() {
 // 处理函数使用new的情况
 if (this instanceof F) {
  return new _this(...arg, ...arguments)
 } else {
  return _this.apply(context, arg.concat(...arguments))
 }
 }
}

// 思路：右边变量的原型存在于左边变量的原型链上
function instanceOf(left, right) {
 let leftValue = left.__proto__
 let rightValue = right.prototype
 while (true) {
 if (leftValue === null) {
  return false
 }
 if (leftValue === rightValue) {
  return true
 }
 leftValue = leftValue.__proto__
 }
}

// 思路：将传入的对象作为原型
function create(obj) {
 function F() {}
 F.prototype = obj
 return new F()

function myNew (fun) {
 return function () {
 // 创建一个新对象且将其隐式原型指向构造函数原型
 let obj = {
  __proto__ : fun.prototype
 }
 // 执行构造函数
 fun.call(obj, ...arguments)
 // 返回该对象
 return obj
 }
}

function person(name, age) {
 this.name = name
 this.age = age
}
let obj = myNew(person)('chen', 18) // {name: "chen", age: 18}

// 未添加异步处理等其他边界情况
// ①自动执行函数，②三个状态，③then
class Promise {
 constructor (fn) {
 // 三个状态
 this.state = 'pending'
 this.value = undefined
 this.reason = undefined
 let resolve = value => {
  if (this.state === 'pending') {
  this.state = 'fulfilled'
  this.value = value
  }
 }
 let reject = value => {
  if (this.state === 'pending') {
  this.state = 'rejected'
  this.reason = value
  }
 }
 // 自动执行函数
 try {
  fn(resolve, reject)
 } catch (e) {
  reject(e)
 }
 }
 // then
 then(onFulfilled, onRejected) {
 switch (this.state) {
  case 'fulfilled':
  onFulfilled()
  break
  case 'rejected':
  onRejected()
  break
  default:
 }
 }
}

// 1. ...实现
let copy1 = {...{x:1}}

// 2. Object.assign实现

let copy2 = Object.assign({}, {x:1})

// 1. JOSN.stringify()/JSON.parse()
let obj = {a: 1, b: {x: 3}}
JSON.parse(JSON.stringify(obj))

// 2. 递归拷贝
function deepClone(obj) {
 let copy = obj instanceof Array ? [] : {}
 for (let i in obj) {
 if (obj.hasOwnProperty(i)) {
  copy[i] = typeof obj[i] === 'object' ? deepClone(obj[i]) : obj[i]
 }
 }
 return copy
}

// 可避免setInterval因执行时间导致的间隔执行时间不一致
setTimeout (function () {
 // do something
 setTimeout (arguments.callee, 500)
}, 500)

// 借用构造函数继承实例属性
function Child () {
 Parent.call(this)
}
// 寄生继承原型属性
(function () {
 let Super = function () {}
 Super.prototype = Parent.prototype
 Child.prototype = new Super()
})()

// 组件通信，一个触发与监听的过程
class EventEmitter {
 constructor () {
 // 存储事件
 this.events = this.events || new Map()
 }
 // 监听事件
 addListener (type, fn) {
 if (!this.events.get(type)) {
  this.events.set(type, fn)
 }
 }
 // 触发事件
 emit (type) {
 let handle = this.events.get(type)
 handle.apply(this, [...arguments].slice(1))
 }
}

// 测试
let emitter = new EventEmitter()
// 监听事件
emitter.addListener('ages', age => {
 console.log(age)
})
// 触发事件
emitter.emit('ages', 18) // 18

let obj = {}
let input = document.getElementById('input')
let span = document.getElementById('span')
// 数据劫持
Object.defineProperty(obj, 'text', {
 configurable: true,
 enumerable: true,
 get() {
 console.log('获取数据了')
 return obj['text']
 },
 set(newVal) {
 console.log('数据更新了')
 input.value = newVal
 span.innerHTML = newVal
 }
})
// 输入监听
input.addEventListener('keyup', function(e) {
 obj.text = e.target.value
})

// hash路由
class Route{
 constructor(){
 // 路由存储对象
 this.routes = {}
 // 当前hash
 this.currentHash = ''
 // 绑定this，避免监听时this指向改变
 this.freshRoute = this.freshRoute.bind(this)
 // 监听
 window.addEventListener('load', this.freshRoute, false)
 window.addEventListener('hashchange', this.freshRoute, false)
 }
 // 存储
 storeRoute (path, cb) {
 this.routes[path] = cb || function () {}
 }
 // 更新
 freshRoute () {
 this.currentHash = location.hash.slice(1) || '/'
 this.routes[this.currentHash]()
 } 
}

<ul>
 <li><img src="./imgshttps://img.qb5200.com/download-x/default.png" data="./imgs/1.png" alt=""></li>
 <li><img src="./imgshttps://img.qb5200.com/download-x/default.png" data="./imgs/2.png" alt=""></li>
 <li><img src="./imgshttps://img.qb5200.com/download-x/default.png" data="./imgs/3.png" alt=""></li>
 <li><img src="./imgshttps://img.qb5200.com/download-x/default.png" data="./imgs/4.png" alt=""></li>
 <li><img src="./imgshttps://img.qb5200.com/download-x/default.png" data="./imgs/5.png" alt=""></li>
 <li><img src="./imgshttps://img.qb5200.com/download-x/default.png" data="./imgs/6.png" alt=""></li>
 <li><img src="./imgshttps://img.qb5200.com/download-x/default.png" data="./imgs/7.png" alt=""></li>
 <li><img src="./imgshttps://img.qb5200.com/download-x/default.png" data="./imgs/8.png" alt=""></li>
 <li><img src="./imgshttps://img.qb5200.com/download-x/default.png" data="./imgs/9.png" alt=""></li>
 <li><img src="./imgshttps://img.qb5200.com/download-x/default.png" data="./imgs/10.png" alt=""></li>
</ul>

let imgs = document.querySelectorAll('img')
// 可视区高度
let clientHeight = window.innerHeight || document.documentElement.clientHeight || document.body.clientHeight
function lazyLoad () {
 // 滚动卷去的高度
 let scrollTop = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop
 for (let i = 0; i < imgs.length; i ++) {
 // 图片在可视区冒出的高度
 let x = clientHeight + scrollTop - imgs[i].offsetTop
 // 图片在可视区内
 if (x > 0 && x < clientHeight+imgs[i].height) {
  imgs[i].src = imgs[i].getAttribute('data')
 } 
 }  
}
// addEventListener('scroll', lazyLoad) or setInterval(lazyLoad, 1000)

// 原始配置
function setRem () {
 let doc = document.documentElement
 let width = doc.getBoundingClientRect().width
 let rem = width / 75
 doc.style.fontSize = rem + 'px'
}
// 监听窗口变化
addEventListener("resize", setRem)

// 1. 简单流程

// 实例化
let xhr = new XMLHttpRequest()
// 初始化
xhr.open(method, url, async)
// 发送请求
xhr.send(data)
// 设置状态变化回调处理请求结果
xhr.onreadystatechange = () => {
 if (xhr.readyStatus === 4 && xhr.status === 200) {
 console.log(xhr.responseText)
 }
}

// 2. 基于promise实现 

function ajax (options) {
 // 请求地址
 const url = options.url
 // 请求方法
 const method = options.method.toLocaleLowerCase() || 'get'
 // 默认为异步true
 const async = options.async
 // 请求参数
 const data = options.data
 // 实例化
 const xhr = new XMLHttpRequest()
 // 请求超时
 if (options.timeout && options.timeout > 0) {
 xhr.timeout = options.timeout
 }
 // 返回一个Promise实例
 return new Promise ((resolve, reject) => {
 xhr.ontimeout = () => reject && reject('请求超时')
 // 监听状态变化回调
 xhr.onreadystatechange = () => {
  if (xhr.readyState == 4) {
  // 200-300 之间表示请求成功，304资源未变，取缓存
  if (xhr.status >= 200 && xhr.status < 300 || xhr.status == 304) {
   resolve && resolve(xhr.responseText)
  } else {
   reject && reject()
  }
  }
 }
 // 错误回调
 xhr.onerror = err => reject && reject(err)
 let paramArr = []
 let encodeData
 // 处理请求参数
 if (data instanceof Object) {
  for (let key in data) {
  // 参数拼接需要通过 encodeURIComponent 进行编码
  paramArr.push(encodeURIComponent(key) + '=' + encodeURIComponent(data[key]))
  }
  encodeData = paramArr.join('&')
 }
 // get请求拼接参数
 if (method === 'get') {
  // 检测url中是否已存在 ? 及其位置
  const index = url.indexOf('?')
  if (index === -1) url += '?'
  else if (index !== url.length -1) url += '&'
  // 拼接url
  url += encodeData
 }
 // 初始化
 xhr.open(method, url, async)
 // 发送请求
 if (method === 'get') xhr.send(null)
 else {
  // post 方式需要设置请求头
  xhr.setRequestHeader('Content-Type','application/x-www-form-urlencoded;charset=UTF-8')
  xhr.send(encodeData)
 }
 })
}

window.onload = function () {
 // drag处于绝对定位状态
 let drag = document.getElementById('box')
 drag.onmousedown = function(e) {
 var e = e || window.event
 // 鼠标与拖拽元素边界的距离 = 鼠标与可视区边界的距离 - 拖拽元素与边界的距离
 let diffX = e.clientX - drag.offsetLeft
 let diffY = e.clientY - drag.offsetTop
 drag.onmousemove = function (e) {
  // 拖拽元素移动的距离 = 鼠标与可视区边界的距离 - 鼠标与拖拽元素边界的距离
  let left = e.clientX - diffX
  let top = e.clientY - diffY
  // 避免拖拽出可视区
  if (left < 0) {
  left = 0
  } else if (left > window.innerWidth - drag.offsetWidth) {
  left = window.innerWidth - drag.offsetWidth
  }
  if (top < 0) {
  top = 0
  } else if (top > window.innerHeight - drag.offsetHeight) {
  top = window.innerHeight - drag.offsetHeight
  }
  drag.style.left = left + 'px'
  drag.style.top = top + 'px'
 }
 drag.onmouseup = function (e) {
  this.onmousemove = null
  this.onmouseup = null
 }
 }
}

// 思路：在规定时间内只触发一次
function throttle (fn, delay) {
 // 利用闭包保存时间
 let prev = Date.now()
 return function () {
 let context = this
 let arg = arguments
 let now = Date.now()
 if (now - prev >= delay) {
  fn.apply(context, arg)
  prev = Date.now()
 }
 }
}

function fn () {
 console.log('节流')
}
addEventListener('scroll', throttle(fn, 1000)) 

// 思路:在规定时间内未触发第二次，则执行
function debounce (fn, delay) {
 // 利用闭包保存定时器
 let timer = null
 return function () {
 let context = this
 let arg = arguments
 // 在规定时间内再次触发会先清除定时器后再重设定时器
 clearTimeout(timer)
 timer = setTimeout(function () {
  fn.apply(context, arg)
 }, delay)
 }
}

function fn () {
 console.log('防抖')
}
addEventListener('scroll', debounce(fn, 1000))