前言
本文引自于javascript公眾號(hào),致力于讓更多的人了解大廠常見面試題绩脆,其中加入了解析思路萤厅,提供給大家。
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
}
解析:
- 不了解call用法的請(qǐng)看:call 方法和 apply 方法的作用及意義靴迫;
- 如果僅僅想改變
this
的指向:
Function.prototype.mycall = function (context) {
context = context || window
context.fn = this // 此時(shí)的this代表的是foo函數(shù)
context.fn() // 僅僅是調(diào)用一下foo函數(shù)
console.log(context, this); // 此時(shí)我們打印一下惕味,會(huì)發(fā)現(xiàn)fn是context下的一個(gè)方法,很自然的我們知道this指向了context對(duì)象玉锌,也就是傳遞過來的obj名挥。(不明白的,得惡補(bǔ)一下this指向主守。)
}
// 舉例:
const obj = {
name: '小米'
};
function foo() {
console.log(this.name);
}
foo.myCall(obj);
此時(shí)我們打印一下
console.log(context, this)
禀倔,會(huì)發(fā)現(xiàn)fn
是context
下的一個(gè)方法,很自然的我們知道this
指向了context
對(duì)象丸逸,也就是傳遞過來的obj
蹋艺。(不明白的,得惡補(bǔ)一下this指向黄刚。)
image.png
3.其他的方法就是為了傳參存在捎谨,就是傳單個(gè)怎么獲取的問題,這個(gè)稍微研究一下就理解了。
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
}
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))
}
}
}
// 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 === right) {
return true
}
leftValue = rightValue.__proto__
}
}
簡(jiǎn)單來說:如
A instanceof B
涛救,就是A是不是B的實(shí)例畏邢;實(shí)現(xiàn)原理就是:變量查找A的原型鏈,找到與B. prototype上相等的即A是B的實(shí)例检吆,如果找不到則不是舒萎。 深入原理:自行了解原型鏈。
function create(obj) {
function F() {}
F.prototype = obj
return new F()
}
用空函數(shù)F創(chuàng)建實(shí)例蹭沛,避免了多次調(diào)用父類的情況臂寝。
function myNew (fun) {
return function () {
// 創(chuàng)建一個(gè)新對(duì)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(duì)象
return obj
}
}
// ①自動(dòng)執(zhí)行函數(shù),②三個(gè)狀態(tài)摊灭,③then
class Promise {
constructor (fn) {
// 三個(gè)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(dòng)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(shí)現(xiàn)
let copy1 = {...{x:1}}
// 2. Object.assign實(shí)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(shí)間導(dǎo)致的間隔執(zhí)行時(shí)間不一致
setTimeout (function () {
// do something
setTimeout (arguments.callee, 500)
}, 500)
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()
})()
// 組件通信咆贬,一個(gè)觸發(fā)與監(jiān)聽的過程
class EventEmitter {
constructor () {
// 存儲(chǔ)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(cè)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')
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
}
})
input.addEventListener('keyup', function(e) {
obj.text = e.target.value
})
class Route{
constructor(){
// 路由存儲(chǔ)對(duì)象
this.routes = {}
// 當(dāng)前hash
this.currentHash = ''
// 綁定this,避免監(jiān)聽時(shí)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(chǔ)
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(dòng)卷去的高度
let scrollTop = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop
for (let i = 0; i < imgs.length; i ++) {
// 得到圖片頂部距離可視區(qū)頂部的距離
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')
}
}
}
setInterval(lazyLoad, 1000)
function setRem () {
let doc = document.documentElement
let width = doc.getBoundingClientRect().width
// 假設(shè)設(shè)計(jì)稿為寬750帚呼,則rem為10px
let rem = width / 75
doc.style.fontSize = rem + 'px'
}
// 1. 簡(jiǎn)單實(shí)現(xiàn)
// 實(shí)例化
let xhr = new XMLHttpRequest()
// 初始化
xhr.open(method, url, async)
// 發(fā)送請(qǐng)求
xhr.send(data)
// 設(shè)置狀態(tài)變化回調(diào)處理請(qǐng)求結(jié)果
xhr.onreadystatechange = () => {
if (xhr.readyStatus === 4 && xhr.status === 200) {
console.log(xhr.responseText)
}
}
// 2. 基于promise實(shí)現(xiàn)
function ajax (options) {
// 請(qǐng)求地址
const url = options.url
// 請(qǐng)求方法
const method = options.method.toLocaleLowerCase() || 'get'
// 默認(rèn)為異步true
const async = options.async
// 請(qǐng)求參數(shù)
const data = options.data
// 實(shí)例化
const xhr = new XMLHttpRequest()
// 請(qǐng)求超時(shí)
if (options.timeout && options.timeout > 0) {
xhr.timeout = options.timeout
}
// 返回一個(gè)Promise實(shí)例
return new Promise ((resolve, reject) => {
xhr.ontimeout = () => reject && reject('請(qǐng)求超時(shí)')
// 監(jiān)聽狀態(tài)變化回調(diào)
xhr.onreadystatechange = () => {
if (xhr.readyState == 4) {
// 200-300 之間表示請(qǐng)求成功掏缎,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(cuò)誤回調(diào)
xhr.onerror = err => reject && reject(err)
let paramArr = []
let encodeData
// 處理請(qǐng)求參數(shù)
if (data instanceof Object) {
for (let key in data) {
// 參數(shù)拼接需要通過 encodeURIComponent 進(jìn)行編碼
paramArr.push(encodeURIComponent(key) + '=' + encodeURIComponent(data[key]))
}
encodeData = paramArr.join('&')
}
// get請(qǐng)求拼接參數(shù)
if (method === 'get') {
// 檢測(cè)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(qǐng)求
if (method === 'get') xhr.send(null)
else {
// post 方式需要設(shè)置請(qǐng)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(duì)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(dòng)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
}
}
}
function throttle (fn, delay) {
// 利用閉包保存時(shí)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))
function debounce (fn, delay) {
// 利用閉包保存定時(shí)器
let timer = null
return function () {
let context = this
let arg = arguments
// 在規(guī)定時(shí)間內(nèi)再次觸發(fā)會(huì)先清除定時(shí)器后再重設(shè)定時(shí)器
clearTimeout(timer)
timer = setTimeout(function () {
fn.apply(context, arg)
}, delay)
}
}
function fn () {
console.log('防抖')
}
addEventListener('scroll', debounce(fn, 1000))