循环

for...in循环，适用数组或者对象

// 对象
for (var i in object) {
  console.log("key:",i,"value:",object[i])
}

// 数组
var arr = ['张三','李四']
for (var index in arr) {
    console.log("key:",index,"value:",arr[index])
}

forEach循环，适用数组

var arr = [1,2,3]
arr.forEach(function(val,index){

})

for...of循环，适用数组和对象

var arr = ['张三','李四']
for (var value of arr) {
    console.log("value:",value)
}