Fixing the JavaScript typeof operator

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<a href="https://javascriptweblog.wordpress.com/2011/08/08/fixing-the-javascript-typeof-operator/" target="_blank" rel="noopener nofollow">https://javascriptweblog.wordpress.com/2011/08/08/fixing-the-javascript-typeof-operator/</a><br>javascript 类型判断函数<br>var toType = function(obj) {   return ({}).toString.call(obj).match(/\s([a-zA-Z]+)/)[1].toLowerCase() }<br><br><br>

A Better Way?

[[Class]]

Every JavaScript object has an internal property known as [[Class]] (The ES5 spec uses the double square bracket notation to represent internal properties, i.e. abstract properties used to specify the behavior of JavaScript engines). According to ES5, [[Class]] is “a String value indicating a specification defined classification of objects”. To you and me, that means each built-in object type has a unique non-editable, standards-enforced value for its [[Class]] property. This could be really useful if only we could get at the [[Class]] property…

Object.prototype.toString

…and it turns out we can. Take a look at the ES 5 specification for Object.prototype.toString…

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let class be the value of the [[Class]] internal property of O.
3. Return the String value that is the result of concatenating the three Strings "[object ", class, and "]".

In short, the default toString function of Object returns a string with the following format…

[object [[Class]]]

…where [[Class]] is the class property of the object.

Unfortunately, the specialized built-in objects mostly overwrite Object.prototype.toStringwith toString methods of their own…

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[1,2,3].toString(); //"1, 2, 3"   (new Date).toString(); //"Sat Aug 06 2011 16:29:13 GMT-0700 (PDT)"   /a-z/.toString(); //"/a-z/"

 
…fortunately we can use the call function to force the generic toString function upon them…

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Object.prototype.toString.call([1,2,3]); //"[object Array]"   Object.prototype.toString.call(new Date); //"[object Date]"   Object.prototype.toString.call(/a-z/); //"[object RegExp]"

 
Introducing the toType function

We can take this technique, add a drop of regEx, and create a tiny function – a new and improved version of the typeOf operator…