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Re: Puzzling JS Error

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JJ wrote:

> On Thu, 9 Apr 2015 11:49:46 -0600, Mel Smith wrote:
>> var HTTP;
>> /*
>> // problem in this next statement
>> if (HTTP && (typeof HTTP !== "object" || HTTP.NAME)) {
>>     throw new Error("Namespace 'HTTP' already exists");
>>    }
>> */
>> alert("Ready toComplete Ajax set up.") ;
>> […] 
> 
> The reason why it fails is already explained by others.

But you have misunderstood it anyway.  Again: Variables that have been 
declared but not implicitly initialized are initialized with the “undefined” 
value.  !!undefined === false.

I do not see how this code could generate a runtime error (not: exception) 
anyway.  If HTTP === undefined or HTTP === null, logical shortcut evaluation 
prevents what follows the “HTTP” expression from being evaluated, especially 
HTTP.NAME which would throw a TypeError exception if HTTP === undefined or 
HTTP === null (because those are primitive values that are not convertible 
to objects – “undefined/null has no properties”).

The only way that I can see this code to generate a runtime error is when 
“HTTP” refers to an object that type-converts to true, or a host object that 
cannot be type-converted, “typeof” on it yields "object", and it has a 
getter for a “NAME” property that throws an exception.

> As for the fix, use this.

No, do not.

> //var HTTP; //don't declare

It is *wrong* not to declare “HTTP” a variable here.  It causes a 
ReferenceError in strict mode to assign to such a pseudo-variable
(as the OP does) later.

> if (typeof HTTP !== "undefined" && HTTP.NAME) {

That should work.  However, it is pseudo-security.  Nothing prevents anyone 
else from adding a “NAME” property to their object referrable by “HTTP”.  
Because of that, I would simply write

  if (typeof HTTP != "undefined")

or in my case I have written

  if (typeof jsx.net == "undefined")
  {
    /**
     * @namespace
     */
    jsx.net = {};
  }

  // […]

  jsx.net.http = {
    // […]
  }

<http://PointedEars.de/wsvn/JSX/trunk/http.js>

It is a good idea to have one’s own namespace¹ (as much as possible) for 
one’s own objects.  “HTTP” is too simple, collisions are likely.

Note that I am not throwing exceptions if there is already a “jsx.net.http”; 
I simply try to overwrite it.  Why?  Because it could be my own.  A 
possibility to improve efficiency would be to skip initialization if already 
initialized; but it may be better to fix the problem that caused duplicate 
loading instead.

BTW, The “!==” instead of “!=” adds no efficiency here as both operands are 
of the same type, String; however, it may quiet code linters such as JSHint.

______
¹ for lack of a better word
-- 
PointedEars
FAQ: <http://PointedEars.de/faq> | SVN: <http://PointedEars.de/wsvn/>
Twitter: @PointedEars2 | ES Matrix: <http://PointedEars.de/es-matrix>
Please do not cc me. / Bitte keine Kopien per E-Mail.

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Thread

Puzzling JS Error "Mel Smith" <syntel@cox.net> - 2015-04-09 11:49 -0600
  Re: Puzzling JS Error "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2015-04-09 22:21 +0200
    Re: Puzzling JS Error "Mel Smith" <syntel@cox.net> - 2015-04-09 16:26 -0600
      Re: Puzzling JS Error "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2015-04-10 10:32 +0200
      Re: Puzzling JS Error Denis McMahon <denismfmcmahon@gmail.com> - 2015-04-10 14:51 +0000
        Re: Puzzling JS Error "Mel Smith" <syntel@cox.net> - 2015-04-10 09:48 -0600
          Re: Puzzling JS Error "Christoph M. Becker" <cmbecker69@arcor.de> - 2015-04-10 18:01 +0200
  Re: Puzzling JS Error Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-04-10 00:20 +0200
  Re: Puzzling JS Error JJ <jj4public@vfemail.net> - 2015-04-10 11:15 +0700
    Re: Puzzling JS Error Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-04-11 16:15 +0200
      Re: Puzzling JS Error Joao Rodrigues <groups_jr-1@yahoo.com.br> - 2015-04-11 18:09 -0300
        Re: Puzzling JS Error Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-04-11 23:42 +0200
          Re: Puzzling JS Error Joao Rodrigues <groups_jr-1@yahoo.com.br> - 2015-04-11 19:27 -0300
            Re: Puzzling JS Error Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-04-12 01:01 +0200
              Re: Puzzling JS Error Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-04-12 01:07 +0200
                Re: Puzzling JS Error Joao Rodrigues <groups_jr-1@yahoo.com.br> - 2015-04-11 22:07 -0300

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