反向循环真的更快吗? [英] Are loops really faster in reverse?

问题描述

我已经听过很多次了.向后计数时 JavaScript 循环真的更快吗?如果是这样，为什么?我已经看到一些测试套件示例表明反向循环更快，但我找不到任何解释为什么！

I've heard this quite a few times. Are JavaScript loops really faster when counting backward? If so, why? I've seen a few test suite examples showing that reversed loops are quicker, but I can't find any explanation as to why!

我假设这是因为循环不再需要在每次检查属性是否完成时都评估它，它只检查最终的数值.

I'm assuming it's because the loop no longer has to evaluate a property each time it checks to see if it's finished and it just checks against the final numeric value.

即

for (var i = count - 1; i >= 0; i--)
{
  // count is only evaluated once and then the comparison is always on 0.
}

推荐答案

并不是 i-- 比 i++ 快.实际上，它们的速度都一样快.

It's not that i-- is faster than i++. Actually, they're both equally fast.

在升序循环中需要时间的是为每个 i 评估数组的大小.在这个循环中:

What takes time in ascending loops is evaluating, for each i, the size of your array. In this loop:

for(var i = array.length; i--;)

当你声明 i 时，你只计算一次 .length，而对于这个循环

You evaluate .length only once, when you declare i, whereas for this loop

for(var i = 1; i <= array.length; i++)

每次增加 i 时都会评估 .length，当您检查 i <= array.length 时.

you evaluate .length each time you increment i, when you check if i <= array.length.

在大多数情况下，您甚至不必担心这种优化.

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