javaScript中遍历数组的方式：
1.for循环
let arr=["A","B","C"];
for (let i=0;i<arr.length;i++){
console.log(arr[i]);}
2.for (let item of arr)
for (let item of arr){
console.log(arr[i]);} 输出所有值；
3.for (let item in arr)
for (let item in arr){
console.log(arr[i]);} 输出所有下标；
ES6新增
for (let item of arr.keys){
console.log(arr[i]);}遍历数组中每个值的键(下标)；
for (let item of arr.values){
console.log(arr[i]);}遍历数组中每个值；
for (let item of arr.entries){
console.log(arr[i]);}遍历出数组每个值和值对应的键(下标)；