后增量与前增量 - Javascript优化 [英] post increment vs pre increment - Javascript Optimization

问题描述

当我偶然发现这个名为 JSpeed - Javascript 优化的项目时，我正在浏览 Google 代码.

I was browsing Google Code when I chanced upon this project called JSpeed - optimization for Javascript.

我注意到其中一项优化是将 for 循环语句中的 i++ 更改为 ++i.

I noticed one of the optimization was to change i++ to ++i in for loop statements.

优化前

for (i=0;i<1;i++) {}

for (var i = 0, j = 0; i < 1000000; i++, j++) {
    if (i == 4) {
        var tmp = i / 2;
    }

    if ((i % 2) == 0) {
        var tmp = i / 2;
        i++;
    }
}
var arr = new Array(1000000);
for (i = 0; i < arr.length; i++) {}

优化后

for(var i=0;i<1;++i){}
for(var i=0,j=0;i<1000000;++i,++j){if(i==4){var tmp=i>>1;}
if((i&1)==0){var tmp=i>>1;i++;}}
var arr=new Array(1000000);for(var i=0,arr_len=arr.length;i<arr_len;++i){}

我知道 pre 和 post 增量的作用，但知道这如何加快代码速度吗?

I know what pre and post increments do, but any idea how does this speeds the code up?

推荐答案

这是我读到的，可以回答你的问题:preincrement (++i) 给 i，然后返回i；相比之下，i++ 返回i，然后给它加一个，理论上 导致在应用增量操作之前创建一个临时变量来存储 i 的值".

This is what I read and could answer your question: "preincrement (++i) adds one to the value of i, then returns i; in contrast, i++ returns i then adds one to it, which in theory results in the creation of a temporary variable storing the value of i before the increment operation was applied".

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