为什么lodash.each比原生forEach更快？ [英] Why is lodash.each faster than native forEach?

问题描述

我试图找到运行具有自己范围的for循环的最快方法。我比较的三种方法是：

I was trying to find the fastest way of running a for loop with its own scope. The three methods I compared were:

var a = "t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t".split();

// lodash .each -> 1,294,971 ops/sec
lodash.each(a, function(item) { cb(item); });

// native .forEach -> 398,167 ops/sec
a.forEach(function(item) { cb(item); });

// native for -> 1,140,382 ops/sec
var lambda = function(item) { cb(item); };
for (var ix = 0, len = a.length; ix < len; ix++) {
  lambda(a[ix]);
}

这是在OS X上的Chrome 29上。您可以在这里自行运行测试：

This is on Chrome 29 on OS X. You can run the tests yourself here:

http://jsben.ch/BQhED

lodash的 .each 几乎是本机 .forEach的两倍？而且，它如何比的普通更快？巫术？黑魔法？

How is lodash's .each almost twice as fast as native .forEach? And moreover, how is it faster than the plain for? Sorcery? Black magic?

推荐答案

_。each（）不完全兼容到 []。forEach（）。请参阅以下示例：

_.each() is not fully compatible to [].forEach(). See the following example:

var a = ['a0'];
a[3] = 'a3';
_.each(a, console.log); // runs 4 times
a.forEach(console.log); // runs twice -- that's just how [].forEach() is specified

http://jsfiddle.net/BhrT3/

所以lodash的实施缺少 if（... in ...） check，这可能解释了性能差异。

So lodash's implementation is missing an if (... in ...) check, which might explain the performance difference.

如上面的评论中所述，与原始的差异主要是由测试中的附加函数查找引起的。使用此版本可获得更准确的结果：

As noted in the comments above, the difference to native for is mainly caused by the additional function lookup in your test. Use this version to get more accurate results:

for (var ix = 0, len = a.length; ix < len; ix++) {
  cb(a[ix]);
}

http://jsperf.com/lo-dash-each-vs-native-foreach/15

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