总结的一些js手写题，多写写可以巩固js语法知识

深拷贝

function deepCopy(obj){
    let res
    if(typeof obj === 'object'){
    	res = obj.constructor === Array ? [] : {}
        for(let k in obj){
             res = typeof obj[k] === 'object' ? deepCopy(obj[k]) : obj
        }
    }else{
    	res = obj
    }
    return res
}

数组扁平化的多种写法

//递归
function flatten(arr){
	let res = []
    	arr.forEach(item => {
    	if(Array.isArray(item)){
        	res = res.concat(flatten(item))
        }else{
        	res.push(item)
        }
    })
    return res
}
//用reduce
function flatten(arr){
	return arr.reduce((pre,item) => {
    	return pre.concat(Array.isArray(item) ? flatten(item) : item)
    },[])
}

手写reduce

Array.prototype.myReduce = function(fn,init){
	if(typeof fn !== 'function'){
    	throw new TypeError('error')
    }
    
    let initialArr = this   //拿到当前数组
    let arr = initialArr.concat()  //保证原始数组不被改变
    let index,newValue
    
    if(arguments.length === 2){
    	arr.unshift(init)   //unshift方法要记住啊！
        index = 0
    }else{
    	index = 1
    }
    
    if(arr.length === 1){
    	return newValue
    }
    
    while(arr.length > 1){
    	newValue = fn.call(null,arr[0],arr[1],index,initialArr)
        index++
        arr.splice(0,2,newValue)  //splice方法要记住啊！还有slice！！
    
    }
    return newValue
}

手写map

Array.prototype.myMap = function(fn){
	let newArr = []
    for(let i = 0; i < this.length; i++){
    	newArr.push(fn(this[i],i,this))
    }
    return newArr
}

手写fill

Array.prototype.myFill = function(value,start = 0, end = this.length){
	for(let i = start; i < end; i++){
    	this[i] = value
    }
}

手写filter

Array.prototype.myFilter = function(fn,context){
	let temp = []
    for(let i = 0; i < this.length; i++){
    	let res = fn.call(context,this[i],i)
    	if(res) temp.push(this[i])
    }
	return temp
}

手写find

//很简单了，但还是练习一下吧,找不到返回undefined
Array.prototype.myFind(fn){
	for(let i = 0; i < this.length; i++){{
    	let res = fn.call(this,this[i],i,this)
        if(res) return this[i]
    }
}

手写findIndex

//很简单了，但还是练习一下吧,找不到返回-1
Array.prototype.myFindIndex = function(fn){
	for(let i = 0; i < this.length; i++){{
    	let res = fn.call(this,this[i],i,this)
        if(res) return i
    }
    return -1
}

手写ajax请求

var xhr = new XMLHttpRequest()
var url = 'https://sdf.com'

xhr.onreadystatechange = function(){    //readyState(0-4)每变化一次就调用一次onreadystatechange函数
	if(xhr.readyState === 4){
		if(xhr.status === 200){
			console.log(xhr.responseText)
		}else{
			console.error(xhr.statusText)
		}
	}
}

xhr.open('GET',url,true)
xhr.send(null)

模拟实现instanceof

function instanceofObj(a,b){
	//模拟 a instanceof b
	let prototypeB = b.prototype
	let protoA = a.__proto__
	let state = false
	while(true){
		if(protoA === null){ //可能是undefinded
			state = false
			break
		}
		if(prototypeB === protoA){
			state = true
			break
		}
		protoA = protoA.__proto__
	}
	return state
}

console.log(instanceofObj([],Array))   //true

用setTimeout实现setInterval

function mysetInterval(fn,time){
	function interval(){
		setTimeout(interval,time)   //递归
		fn()
	}
	setTimeout(interval,time)
}

mysetInterval(function(){
	console.log(0)
},1000)

字符串逆序

func