为什么向后遍历数组的速度比向前快 [英] Why is iterating through an array backwards faster than forwards

问题描述

鉴于此代码：

var arr = [];

for (var i = 0; i < 10000; ++i)
    arr.push(1);

前锋

for (var i = 0; i < arr.length; ++i) {}

向后

for (var i = arr.length - 1; i >= 0; --i) {}

硬编码转发

for (var i = 0; i < 10000; ++i) {}

为什么向后退得这么快？

Why is backwards so much faster?

这是测试：
http://jsperf.com/array-iteration-direction

推荐答案

因为你的远期条件必须收到你的长度属性每次都是数组，而另一个条件只需要检查大于零，一个非常快的任务。

Because your forwards-condition has to receive the length property of your array each time, whilst the other condition only has to check for "greater then zero", a very fast task.

当你的数组长度在循环期间没有改变时，你真的看看ns-perfomance，你可以使用

When your array length doesn't change during the loop, and you really look at ns-perfomance, you can use

for (var i=0, l=arr.length; i<l; i++)

BTW：而不是 for（var i = arr.length; i> 0; --i）您可以使用代替（var i = arr.length; i - > 0;）实际上你的数组从n-1到0，而不是从n到1。

BTW: Instead of for (var i = arr.length; i > 0; --i) you might use for (var i = arr.length; i-- > 0; ) which really runs through your array from n-1 to 0, not from n to 1.

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