为什么UnderscoreJS使用toString.call（）而不是typeof？ [英] Why does UnderscoreJS use toString.call() instead of typeof?

问题描述

在UnderscoreJS的引擎盖下，我看到：

Looking under the hood in UnderscoreJS, I see:

  _.isFunction = function(obj) {
    return toString.call(obj) == '[object Function]';
  };

  _.isString = function(obj) {
    return toString.call(obj) == '[object String]';
  };

  _.isNumber = function(obj) {
    return toString.call(obj) == '[object Number]';
  };

这似乎是一个奇怪的选择。为什么不使用typeof来确定值是字符串，函数还是数字？使用toString是否有性能提升？旧浏览器不支持typeof吗？

This seems like an odd choice. Why not just use typeof to determine whether a value is a string, function, or number? Is there a performance gain by using toString? Is typeof not supported by older browsers?

推荐答案

实际上这是因为检查更快[[Class]] 通过 toString 进行检查。也可以减少错误，因为toString为您提供了精确的类......

Well actually this is because it is faster to check the [[Class]] by checking with toString. Also there could be less mistakes, since toString gives you the exact Class ...

检查：

var fn = function() { 
    console.log(typeof(arguments)) // returns object
    console.log(arguments.toString()) // returns object Arguments
}

你可以在这里看到下划线typeof vs toString的基准：

You could see the benchmark for underscore typeof vs toString here :

http://jsperf.com/underscore- js-istype-alternatives

还有一些github问题有更好的解释：

Also there are some github issues with better explaination :

< a href =https://github.com/documentcloud/underscore/pull/332\"rel =noreferrer> https://github.com/documentcloud/underscore/pull/332

https://github.com/documentcloud/underscore/pull / 321

编辑1：

你可以als o查看这篇好文章：

You could also check this great article :

http://javascriptweblog.wordpress.com/2011/08/08/fixing-the-javascript-typeof-operator/

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