是否将某种JavaScript性能技巧归结为零？ [英] Is Subtracting Zero some sort of JavaScript performance trick?

问题描述

查看jQuery核心，我发现了以下代码约定：

Looking in the jQuery core I found the following code convention:

nth: function(elem, i, match){
    return match[3] - 0 === i;
},

我对片段匹配感到非常好奇[3] - 0

在谷歌上搜索'-0'并不是太高效，搜索'减零'带回鲍勃迪伦歌曲的参考。

Hunting around for '-0' on google isn't too productive, and a search for 'minus zero' brings back a reference to a Bob Dylan song.

所以，任何人都可以告诉我。这是某种性能技巧，还是有理由这样做而不是 parseInt 或 parseFloat ？

So, can anyone tell me. Is this some sort of performance trick, or is there a reason for doing this rather than a parseInt or parseFloat?

推荐答案

基于一些快速和肮脏的基准测试运行，1234 - 0 比 parseInt（1234）快约50％，比 +1234在Firefox 3.6中。

Based on a few quick and dirty benchmark runs, "1234" - 0 was about 50% faster than parseInt("1234") and 10% faster than +"1234" in Firefox 3.6.

更新：

我的快速而肮脏的基准测试并不是很有用，因为它只是在循环中转换字符串1234。我再次尝试使用随机数字列表，结果遍布地图。这台计算机上的三种方法都在400-500毫秒内，除非它们跳到1300毫秒！我认为垃圾收集正在干扰。下面是一些在Firebug中使用的代码，以防我做了一些愚蠢的事情：

My "quick and dirty" benchmark was not very useful because it was just converting the string "1234" in a loop. I tried again using a random list of numbers, and the results are all over the map. The three methods are all within 400-500 ms on this computer except when they jump to 1300 ms! I think garbage collection is interfering. Here is some code to play with in Firebug, in case I did something stupid:

function randomList() {
    var list = [];
    for (var i = 0; i < 1000000; i++) {
        list.push("" + Math.floor(Math.random()*4000000000));
    }
    return list;
}

function testParseInt(list) {
    console.log("Sanity check: parseInt('" + list[0] + "') = " + parseInt(list[0]) );
    var start = new Date();
    for (var string in list)
        var tmp = parseInt(string);
    var time = new Date() - start;
    console.log("parseInt(string): " + time);
}

function testMinusZero(list) {
    console.log("Sanity check: '" + list[0] + "' - 0 = " + (list[0] - 0));
    var start = new Date();
    for (var string in list)
        var tmp = string - 0;
    var time = new Date() - start;
    console.log("string - 0: " + time);
}

function testUnaryPlus(list) {
    console.log("Sanity check: +'" + list[0] + "' = " + (+list[0]));
    var start = new Date();
    for (var string in list)
        var tmp = +string;
    var time = new Date() - start;
    console.log("+string: " + time);
}

function testPlusZero(list) {
    console.log("Sanity check: '" + list[0] + "' + 0 = " + (list[0] + 0) + " Oh no!");
    var start = new Date();
    for (var string in list)
        var tmp = string + 0;
    var time = new Date() - start;
    console.log("string + 0: " + time);
}


var numbers = randomList();

testParseInt(numbers);
testMinusZero(numbers);
testUnaryPlus(numbers);
testPlusZero(numbers);

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