es 中 for in for of

arr=[11,22,33,44,55,66,77,88]
for (const i in arr){
console.log(i)
if (i===4){
console.log(arr[i])
}
}

arr=[11,22,33,44,55,66,77,88]
for (const v of arr){
console.log(v)
}

arr=[11,22,33,44,55,66,77,88]
for (const i in arr){
console.log(i)
console.log(i==4)
console.log(i===4)
console.log(parseInt(i)===4)
}

for in 

i 不是整数型

深入浅出ES6（二）：迭代器和for-of循环 http://www.infoq.com/cn/articles/es6-in-depth-iterators-and-the-for-of-loop