20/JS原理题

最新推荐文章于 2025-06-01 09:08:41 发布

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最新推荐文章于 2025-06-01 09:08:41 发布

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本文链接：https://blog.youkuaiyun.com/weixin_44017013/article/details/108363957

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实现一个call函数
// 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
}
实现一个apply函数
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
}
实现一个bind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))
}
}
}
instanceof的原理
// 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 === right) {
return true
}
leftValue = rightValue.proto
}
}
Object.create的基本实现原理
function create(obj) {
function F() {}
F.prototype = obj
return new F()
new本质
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}

实现一个基本的Promise
// 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:
}
}
}
实现浅拷贝
// 1. …实现
let copy1 = {…{x:1}}

// 2. Object.assign实现

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

实现一个基本的深拷贝
// 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
}

使用setTimeout模拟setInterval
// 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)
js实现一个继承方法// 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()
})()

实现一个基本的Event Bus
// 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

实现一个双向数据绑定
let obj = {}
let input = document.getElementById(‘input’)
let span = document.getElementById(‘span’)
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
}
})
input.addEventListener(‘keyup’, function(e) {
obj.text = e.target.value
})
完整实现可前往之前写的：这应该是最详细的响应式系统讲解了
实现一个简单路由
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.routesthis.currentHash
}
}
实现懒加载

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 ++) { // 得到图片顶部距离可视区顶部的距离 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') } } } setInterval(lazyLoad, 1000)

rem实现原理
function setRem () {
let doc = document.documentElement
let width = doc.getBoundingClientRect().width
// 假设设计稿为宽750，则rem为10px
let rem = width / 75
doc.style.fontSize = rem + ‘px’
}
手写实现AJAX
// 1. 简单实现

// 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)
}
})
}

实现拖拽
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
}
}
}
实现一个节流函数
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))

实现一个防抖函数
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))
最后
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