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From: Cezary Tomczyk <cezary.tomczyk@gmail.com>
Newsgroups: comp.lang.javascript
Subject: Re: Built-in native method
Date: Sun, 21 Oct 2012 18:46:27 +0200
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W dniu 2012-10-21 08:44, JJ pisze:
> Cezary Tomczyk <cezary.tomczyk@gmail.com> wrote:
>> So, when I'm doing:
>>
>> if (String.prototype.trim)
>>
>> and its passed then this means that trim method exists. The only problem
>> is that I cannot be sure if this works correctly or if the native
>> built-in method is not overwritten by someone else.
>>
>> Is there a way to check if (for example: String.prototype.trim) is a
>> real built-in method?
>
> The code inside native functions are syntatically incorrect. You can use
> this to check them. For example, the "escape" function (any function will
> do):
>
> var a=escape.toString();
> console.log(a);
> //Firefox/Safari: "function escape() {\n    [native code]\n}"
> //Chrome/MSIE/Opera: "function escape() { [native code] }"
>
> You see that the code contains only an expression, which is an array. The
> array contains only one expression, which is "native code". i.e.: two
> references to identifiers which are neither reserved word, variable,
> function nor object. This expression is not correct and will cause a syntax
> error exception.

I completely forgot about "[native code]".

> The easiest way to check it is to use the "eval" function to check whether
> an existing function is syntatically correct or not.
>
> function isNativeFunc(f) {
>    var a, z;
>    if (typeof f == 'function') {
>      //is a function
>      try {
>        a = f.toString();
>        eval('(function(){'+a+'})()');
>        return false;
>      } catch(z) {
>        //syntax error
>        return true;
>      }
>    } else {
>      //not a function. assume false
>      return false;
>    }
> }
>
> console.log(isNativeFunc(escape));
> //shows: true
> console.log(isNativeFunc(isNativeFunc));
> //shows: false
>
> Note that the usage of "eval" is considered as unsafe. So, since there's no
> direct alternative for that method, parsing is neccessary. A regular
> expression is used for this method.
>
> function isNativeFunc(f) {
>    if (typeof f == 'function') {
>      //is a function
>      var a = f.toString();
>      //reg-exp for native function code block format
>      var r = /[^{]+\{[\n ]+\[[a-z]+ [a-z]+\][\n ]+\}$/im;
>      return a.match(r) != null;
>    } else {
>      //not a function. assume false
>      return false;
>    }
> }
>
> console.log(isNativeFunc(escape));
> //shows: true
> console.log(isNativeFunc(isNativeFunc));
> //shows: false

Thank you for explanation. Make sense. I have no more questions in this 
topic.

-- 
Cezary Tomczyk
http://www.ctomczyk.pl/