javascript - Javascript 'for-in' 函数是否只返回一个索引？

Question

这是我尝试运行的方法:

function SayHello() {
    cars = new Array();
    cars[0] = "Toyota";
    cars[1] = "Mitsubishi";
    cars[2] = "Honda";

    for (car in cars) {
        alert(car);
    }
}

返回:

0
1
2

当我将代码更改为:

function SayHello() {
    cars = new Array();
    cars[0] = "Toyota";
    cars[1] = "Mitsubishi";
    cars[2] = "Honda";

    for (car in cars) {
        alert(cars[car]);
    }
}

它正确地返回了名称。

我的问题是，for-in 循环是否只是以有序的方式返回一个索引？谢谢。

Answer 1

是的，迭代器的值是属性的名称。然而，使用它来遍历数组是非常不受欢迎的。例如，考虑一下:

x = ['a', 'b', 'c'];

x.foo = 'bar';

for (i in x) alert(i);  // 0, 1, 2, foo

它用于遍历对象的成员:

x = { a : 'apple', b : 'banana', c : 'carrot' };

for (i in x) {
    // and it's best to check that the property actually exists
    // on this object, not just on one of its prototypal ancestors:
    if (x.hasOwnProperty(i)) {
        alert(i);  // 'a', 'b', 'c'
    }
}

有关原因的更多信息 YUI Blog