es6 for for can to get index？

Question

因为是标准化的es6，但如何使用索引？

let list = [4, 5, 6];

for (let i in list) {
   console.log(i); // "0", "1", "2",
}

for (let i of list) {
   console.log(i); // "4", "5", "6"
}