## for-in 循环
只用于 object/map/hash 的遍历
对 `Array` 用 `for-in` 循环有时会出错. 因为它并不是从 `0` 到 `length - 1` 进行遍历, 而是所有出现在对象及其原型链的键值. 下面就是一些失败的使用案例:
```
function printArray(arr) {
for (var key in arr) {
print(arr[key]);
}
}
printArray([0,1,2,3]); // This works.
var a = new Array(10);
printArray(a); // This is wrong.
a = document.getElementsByTagName('*');
printArray(a); // This is wrong.
a = [0,1,2,3];
a.buhu = 'wine';
printArray(a); // This is wrong again.
a = new Array;
a[3] = 3;
printArray(a); // This is wrong again.
```
而遍历数组通常用最普通的 for 循环.
```
function printArray(arr) {
var l = arr.length;
for (var i = 0; i < l; i++) {
print(arr[i]);
}
}
```
