原理解释
因为for...in...会遍历原型链，而且对象属性中"enumerable=true"的属性会被遍历出来，而实际一般遍历数组我们只希望取到数组的直接属性——索引值就可以了。
与此相关
Object.prototype.hasOwnProperty
This method can be used to determine whether an object has the specified property as a direct property of that object; unlike the in operator, this method does not check down the object's prototype chain.
代码如下
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<script>
// Somewhere deep in your JavaScript library...
Array.prototype.foo = 1;
var ret=Object.getOwnPropertyDescriptor(Array.prototype,"foo");
console.log(ret);//是因为foo的属性描述符中enumerable属性为true,即该属性可遍历。
// Now you have no idea what the below code will do.
var a = [1, 2, 3, 4, 5];
for (var x in a){
// Now foo is a part of EVERY array and
// will show up here as a value of 'x'.
console.log(x);
}
</script>
</body>
</html>
新发现
为什么length属性没有被for...in...遍历出来呢？
这也与属性描述符有关系。
Object.getOwnPropertyDescriptor(a,"length")
Object {value: 5, writable: true, enumerable: false, configurable: false}
