for in, for of

for in 遍历 key(index), for of 遍历 value.

e.g: 

let itemList = [1,2,3,4,5];

for (let a of itemList) {

  console.log(a)

}

res: 1,2,3,4,5

let itemList = [1,2,3,4,5];

for (let a in itemList) {

  console.log(a)

}

res: 0,1,2,3,4