本文介绍了为什么使用“for...in"?数组迭代是个坏主意?的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着跟版网的小编来一起学习吧！
问题描述
有人告诉我不要在 JavaScript 中将 for...in 与数组一起使用.为什么不呢?
I've been told not to use for...in with arrays in JavaScript. Why not?
推荐答案
原因是一个构造:
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]);
}
/* Will display:
undefined
undefined
undefined
undefined
undefined
5
*/有时可能完全不同:
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);
}
/* Will display:
5
*/还要考虑 JavaScript 库可能会做这样的事情，这会影响您创建的任何数组:
Also consider that JavaScript libraries might do things like this, which will affect any array you create:
// 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);
}
/* Will display:
0
1
2
3
4
foo
*/这篇关于为什么使用“for...in"?数组迭代是个坏主意?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持跟版网！
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