var a = []; // Create a new empty array.
a[5] = 5; // Perfectly legal JavaScript that resizes the array.
for (var i = 0; i < a.length; i++) {
// Iterate over numeric indexes from 0 to 5, as everyone expects.
console.log(a[i]);
}
/**
* 输出:
* undefined (5次)
* 5
*/var a = [];
a[5] = 5;
for (var x in a) {
// Shows only the explicitly set index of "5", and ignores 0-4
console.log(x);
}
/**
* 输出:
* 5
*/// Somewhere deep in your JavaScript library...
Array.prototype.foo = 1;
// 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);
}
/**
* 输出:
* 0
* 1
* 2
* 3
* 4
* foo
*/加一个 hasOwnProperty (它可是JavaScript中唯一一个处理属性而不查找原型链的函数) 过滤一下还是可以:
if (a.hasOwnProperty(x)){
console.log(x);
}
