JS中循环遍历的常用方法

1.forEach的使用

循环遍历 修改数组

let a=["a","b","c","d","e","f"]
a.forEach((item,i)=>{
	//item = item.toUpperCase();//a= ["a","b","c","d","e","f"]
	a[i] = item.toUpperCase();//改变原数组中基本类型的值 a= ["A","B","C","D","E","F"]
})
console.log(a)
            let b=[
				{name:"zhangsan",id:1},
				{name:"lisi",id:2},
			]
			b.forEach((item,i)=>{
				//item = {  name:"wangwu",  id:3 } //无法改变原数组
				item.name="wangwu" //可以改变原数组
			}) 
			console.log(b)

2.遍历对象键值

Object.keys()、Object.values()、Object.entries() 使用方法

const obj = {  a: 1,  b: 2,  c: 3}
console.log(Object.keys(obj)) // 返回一个包含对象键的数组 ["a","b","c"]
console.log(Object.values(obj)) //函数来获取对象的所有属性值并将其转换成数组。[1,2,3]
console.log(Object.entries(obj)) //返回一个数组，成员是参数对象自身的属性的键值对数组 [["a",1],["b",2],["c",3]] 

3.展平嵌套数组 flat()

let arrs= [["a",1],["b",2],["c",3],[{d:'a',e:2},{a:1,b:2}]]
			console.log(arrs.flat())//默认展开一级，flat(n)要继续下钻n写下钻层级 ["a",1,"b",2,"c",3,{"d":"a","e":2},{"a":1,"b":2}]

4. 数组去重 new Set()、解析类数组为数组Array.from()

let arr=['123','234','345',234,"234"]
let a=new Set(arr)  //类似于数组但是实际上原型是Object { "123", "234", "345",  234 }  
 
console.log(Array.from(a)) //解析类数组为数组 ["123","234","345",234]