forEach for...in for...of 遍历

v-for=”( item, key, index ) in Object”

Object.keys(obj) 获取键值

forEach(val, index) 不改变原数组

for…in 遍历Object的键值

let obj = {a: '1', b: '2', c: '3', d: '4'}
for (let o in obj) {
    console.log(o)    //遍历的实际上是对象的属性名称 a,b,c,d
    console.log(obj[o])  //这个才是属性对应的值1，2，3，4
}

for…of (ES6)

let arr = ['China', 'America', 'Korea']
for (let o of arr) {
    console.log(o) //China, America, Korea
}

直接遍历对象 (error)

let obj = {a: '1', b: '2', c: '3', d: '4'}
for (let o of obj) {
    console.log(o)   //Uncaught TypeError: obj[Symbol.iterator] is not a function
}

字符串

let str = 'love'
for (let o of str) {
    console.log(o) // l,o,v,e
}

Object.key()

let obj = {a: '1', b: '2', c: '3', d: '4'}
for (let o of Object.keys(obj)) {
    console.log(o) // a,b,c,d
}

Object.values()

let obj = {a: '1', b: '2', c: '3', d: '4'}
for (let o of Object.values(obj)) {
    console.log(o) // 1,2,3,4
}

new Map()

new Map([[key,value],[key,value]])

let iterable = new Map([["a", 1], ["b", 2], ["c", 3]]);

for (let [key, value] of iterable) {
  console.log(value);
}
// 1
// 2
// 3

for (let entry of iterable) {
  console.log(entry);
}
// [a, 1]
// [b, 2]
// [c, 3]

new Set()

类似于数组，但它所有元素都是唯一的

let iterable = new Set([1, 1, 2, 2, 3, 3]);

for (let value of iterable) {
  console.log(value);
}
// 1
// 2
// 3