20道JS原理題助你面試一臂之力(必看)

// 思路：將要改變this指向的方法掛到目標(biāo)this上執(zhí)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：將要改變this指向的方法掛到目標(biāo)this上執(zhí)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：類似call，但返回的是函數(shù)
Function.prototype.mybind = function (context) {
 if (typeof this !== 'function') {
 throw new TypeError('Error')
 }
 let _this = this
 let arg = [...arguments].slice(1)
 return function F() {
 // 處理函數(shù)使用new的情況
 if (this instanceof F) {
 return new _this(...arg, ...arguments)
 } else {
 return _this.apply(context, arg.concat(...arguments))
 }
 }
}

// 思路：右邊變量的原型存在于左邊變量的原型鏈上
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：將傳入的對象作為原型
function create(obj) {
 function F() {}
 F.prototype = obj
 return new F()

function myNew (fun) {
 return function () {
 // 創(chuàng)建一個新對象且將其隱式原型指向構(gòu)造函數(shù)原型
 let obj = {
 __proto__ : fun.prototype
 }
 // 執(zhí)行構(gòu)造函數(shù)
 fun.call(obj, ...arguments)
 // 返回該對象
 return obj
 }
}

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

// 未添加異步處理等其他邊界情況
// ①自動執(zhí)行函數(shù)，②三個狀態(tài)，③then
class Promise {
 constructor (fn) {
 // 三個狀態(tài)
 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
 }
 }
 // 自動執(zhí)行函數(shù)
 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. ...實現(xiàn)
let copy1 = {...{x:1}}

// 2. Object.assign實現(xiàn)

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

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

// 2. 遞歸拷貝
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因執(zhí)行時間導(dǎo)致的間隔執(zhí)行時間不一致
setTimeout (function () {
 // do something
 setTimeout (arguments.callee, 500)
}, 500)

// 借用構(gòu)造函數(shù)繼承實例屬性
function Child () {
 Parent.call(this)
}
// 寄生繼承原型屬性
(function () {
 let Super = function () {}
 Super.prototype = Parent.prototype
 Child.prototype = new Super()
})()

// 組件通信，一個觸發(fā)與監(jiān)聽的過程
class EventEmitter {
 constructor () {
 // 存儲事件
 this.events = this.events || new Map()
 }
 // 監(jiān)聽事件
 addListener (type, fn) {
 if (!this.events.get(type)) {
 this.events.set(type, fn)
 }
 }
 // 觸發(fā)事件
 emit (type) {
 let handle = this.events.get(type)
 handle.apply(this, [...arguments].slice(1))
 }
}

// 測試
let emitter = new EventEmitter()
// 監(jiān)聽事件
emitter.addListener('ages', age => {
 console.log(age)
})
// 觸發(fā)事件
emitter.emit('ages', 18) // 18

let obj = {}
let input = document.getElementById('input')
let span = document.getElementById('span')
// 數(shù)據(jù)劫持
Object.defineProperty(obj, 'text', {
 configurable: true,
 enumerable: true,
 get() {
 console.log('獲取數(shù)據(jù)了')
 return obj['text']
 },
 set(newVal) {
 console.log('數(shù)據(jù)更新了')
 input.value = newVal
 span.innerHTML = newVal
 }
})
// 輸入監(jiān)聽
input.addEventListener('keyup', function(e) {
 obj.text = e.target.value
})

// hash路由
class Route{
 constructor(){
 // 路由存儲對象
 this.routes = {}
 // 當(dāng)前hash
 this.currentHash = ''
 // 綁定this，避免監(jiān)聽時this指向改變
 this.freshRoute = this.freshRoute.bind(this)
 // 監(jiān)聽
 window.addEventListener('load', this.freshRoute, false)
 window.addEventListener('hashchange', this.freshRoute, false)
 }
 // 存儲
 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" alt=""></li>
 <li><img src="./imgs/default.png" data="./imgs/2.png" alt=""></li>
 <li><img src="./imgs/default.png" data="./imgs/3.png" alt=""></li>
 <li><img src="./imgs/default.png" data="./imgs/4.png" alt=""></li>
 <li><img src="./imgs/default.png" data="./imgs/5.png" alt=""></li>
 <li><img src="./imgs/default.png" data="./imgs/6.png" alt=""></li>
 <li><img src="./imgs/default.png" data="./imgs/7.png" alt=""></li>
 <li><img src="./imgs/default.png" data="./imgs/8.png" alt=""></li>
 <li><img src="./imgs/default.png" data="./imgs/9.png" alt=""></li>
 <li><img src="./imgs/default.png" data="./imgs/10.png" alt=""></li>
</ul>

let imgs = document.querySelectorAll('img')
// 可視區(qū)高度
let clientHeight = window.innerHeight || document.documentElement.clientHeight || document.body.clientHeight
function lazyLoad () {
 // 滾動卷去的高度
 let scrollTop = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop
 for (let i = 0; i < imgs.length; i ++) {
 // 圖片在可視區(qū)冒出的高度
 let x = clientHeight + scrollTop - imgs[i].offsetTop
 // 圖片在可視區(qū)內(nèi)
 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'
}
// 監(jiān)聽窗口變化
addEventListener("resize", setRem)

// 1. 簡單流程

// 實例化
let xhr = new XMLHttpRequest()
// 初始化
xhr.open(method, url, async)
// 發(fā)送請求
xhr.send(data)
// 設(shè)置狀態(tài)變化回調(diào)處理請求結(jié)果
xhr.onreadystatechange = () => {
 if (xhr.readyStatus === 4 && xhr.status === 200) {
 console.log(xhr.responseText)
 }
}

// 2. 基于promise實現(xiàn) 

function ajax (options) {
 // 請求地址
 const url = options.url
 // 請求方法
 const method = options.method.toLocaleLowerCase() || 'get'
 // 默認(rèn)為異步true
 const async = options.async
 // 請求參數(shù)
 const data = options.data
 // 實例化
 const xhr = new XMLHttpRequest()
 // 請求超時
 if (options.timeout && options.timeout > 0) {
 xhr.timeout = options.timeout
 }
 // 返回一個Promise實例
 return new Promise ((resolve, reject) => {
 xhr.ontimeout = () => reject && reject('請求超時')
 // 監(jiān)聽狀態(tài)變化回調(diào)
 xhr.onreadystatechange = () => {
 if (xhr.readyState == 4) {
 // 200-300 之間表示請求成功，304資源未變，取緩存
 if (xhr.status >= 200 && xhr.status < 300 || xhr.status == 304) {
 resolve && resolve(xhr.responseText)
 } else {
 reject && reject()
 }
 }
 }
 // 錯誤回調(diào)
 xhr.onerror = err => reject && reject(err)
 let paramArr = []
 let encodeData
 // 處理請求參數(shù)
 if (data instanceof Object) {
 for (let key in data) {
 // 參數(shù)拼接需要通過 encodeURIComponent 進行編碼
 paramArr.push(encodeURIComponent(key) + '=' + encodeURIComponent(data[key]))
 }
 encodeData = paramArr.join('&')
 }
 // get請求拼接參數(shù)
 if (method === 'get') {
 // 檢測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)
 // 發(fā)送請求
 if (method === 'get') xhr.send(null)
 else {
 // post 方式需要設(shè)置請求頭
 xhr.setRequestHeader('Content-Type','application/x-www-form-urlencoded;charset=UTF-8')
 xhr.send(encodeData)
 }
 })
}

window.onload = function () {
 // drag處于絕對定位狀態(tài)
 let drag = document.getElementById('box')
 drag.onmousedown = function(e) {
 var e = e || window.event
 // 鼠標(biāo)與拖拽元素邊界的距離 = 鼠標(biāo)與可視區(qū)邊界的距離 - 拖拽元素與邊界的距離
 let diffX = e.clientX - drag.offsetLeft
 let diffY = e.clientY - drag.offsetTop
 drag.onmousemove = function (e) {
 // 拖拽元素移動的距離 = 鼠標(biāo)與可視區(qū)邊界的距離 - 鼠標(biāo)與拖拽元素邊界的距離
 let left = e.clientX - diffX
 let top = e.clientY - diffY
 // 避免拖拽出可視區(qū)
 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：在規(guī)定時間內(nèi)只觸發(fā)一次
function throttle (fn, delay) {
 // 利用閉包保存時間
 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('節(jié)流')
}
addEventListener('scroll', throttle(fn, 1000)) 

// 思路:在規(guī)定時間內(nèi)未觸發(fā)第二次，則執(zhí)行
function debounce (fn, delay) {
 // 利用閉包保存定時器
 let timer = null
 return function () {
 let context = this
 let arg = arguments
 // 在規(guī)定時間內(nèi)再次觸發(fā)會先清除定時器后再重設(shè)定時器
 clearTimeout(timer)
 timer = setTimeout(function () {
 fn.apply(context, arg)
 }, delay)
 }
}

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