Array 遍历

https://www.typescriptlang.org/docs/handbook/iterators-and-generators.html

可以迭代的类型： Array ， Map ， Set ， String，Int32Array， Uint32Array，etc.

1. for ... of vs for ... in

for (let i in list) {} // i 是 index / key
for (let i of list) {} // i 是 value

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"
}

• Code Generation

1. Targeting ES5 and ES3

当编译目标语言为 ES5 and ES3
for ... of 转换成标准的 C
only allowed on values of Array type

let numbers = [1,2,3];
for (let num of numbers) {
console.log(num);
}

// generated as

var numbers = [1,2,3];
for(var _i = 0; _i < numbers.length; _i++){
console.log(num);
}

2. Targeting ECMAScript 2015 and higher
   the compiler will generate for ... of