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，③thenclass 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="./imgs/default.png" data="./imgs/1.png" <li><img src="./imgs/default.png" data="./imgs/2.png" <li><img src="./imgs/default.png" data="./imgs/3.png" <li><img src="./imgs/default.png" data="./imgs/4.png" <li><img src="./imgs/default.png" data="./imgs/5.png" <li><img src="./imgs/default.png" data="./imgs/6.png" <li><img src="./imgs/default.png" data="./imgs/7.png" <li><img src="./imgs/default.png" data="./imgs/8.png" <li><img src="./imgs/default.png" data="./imgs/9.png" <li><img src="./imgs/default.png" data="./imgs/10.png" style="margin: 0px; padding: 0px; outline: none; line-height: 25.2px; font-size: 14px; width: 660px; overflow: hidden; clear: both; font-family: tahoma, arial, "Microsoft YaHei";">	
let imgs = document.querySelectorAll('img')// 可视区高度let clientHeight = window.innerHeight || document.documentElement.clientHeight || document.body.clientHeightfunction 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))