# 迭代器和生成器

当一个对象实现了Symbol.iterator属性时，我们认为它是可迭代的。

* for..of 语句
* for..of vs. for..in 语句

## 代码生成

编译器会生成一个简单的for循环做为for..of循环，比如：

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

生成的代码为

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


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