Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.javascript > #25914

Re: eventDispath how that work

Show key headers only | View raw

JJ wrote:

> On Wed, 15 Oct 2014 15:24:48 +0200, Thomas 'PointedEars' Lahn wrote:
>>>   e = document.querySelector("#elem");
>> 
>> Avoid this newer, less compatible, less efficient pattern, and replace it
>> with
>> 
>>   e = document.getElementById("elem");
> 
> Well, in this case getElementById() would be much more efficient.

AISB.
 
>> You also need to declare “e”.  Otherwise – often discussed here before –
>> the code will not run in strict mode and will cause side-effects in
>> non-strict mode:
>> 
>>   var e;
>> 
>> Declaration and initialization can be combined:
>> 
>>   var e = document.getElementById("elem");
> 
> Since my code was meant to be placed within sendTest(), i.e.:
> 
>    function sendTest(e) {
>      console.log("sendTest(e) e:");
>      console.dir(e);
>      event = new CustomEvent('build', { 'detail': e.target.dataset.date
>      });
> 
>      e = document.querySelector("#elem");
>      e.dispatchEvent(event);
>    }
> 
> This is fine I presume?

No, given just this code, “event” is neither declared a variable in 
sendTest(), nor is it the identifier of a formal parameter.  It would 
therefore refer to the “event” property of the next object in the scope 
chain that has it, it would be considered an attempt to augment the global 
object with an “event” property in non-strict mode, or, because no object in 
the scope chain had such a property, it would throw a ReferenceError 
exception in strict mode.

You would probably see a kind of runtime error caused by the inability to 
overwrite a property of a host object in MSHTML-based runtime environments 
(e.g., Internet Explorer), and a ReferenceError exception in strict mode 
elsewhere.

Also, one should avoid reusing formal parameters as if they were local 
variables.  In this case, there is the additional caveat of accessing host 
objects in perhaps unforeseen ways: “e” would refer to one, an event object 
(as indicated by “e.target”; I am not sure about any browser-based host 
object having a built-in “dataset” property either, so there is potentially 
an unforeseen *write* access to avoid before).

Incidentally, at work today I encountered, essentially, this piece of code 
written by someone else:

  if (window.toString.call(…) == "[object Array]") {
    // …
  }

(Obviously an attempt to detect whether an value would be a reference to an 
Array instance, which is in many cases a misguided one [see “duck typing” 
for details].)

It failed reproducibly in Internet Explorer 8 (with the infamous “error code 
0”) on Windows XP¹ because there was no “window.toString”, and that caused 
*other* code (a progress indicator) to not function (a JScript/MSHTML 
peculiarity).

For now, I can see three possible explanations how this code came about 
(I will investigate this tomorrow morning):

  A. Someone declared a global “toString” function or variable, and
     attempted to access the global object through “window”.

  B. Someone attempted to assign a reference to a Function instance to a 
     previously non-existing “toString” property of the object referred to
     by “window”.

  C. Someone thought “window” would refer to the native ECMAScript global
     object and would therefore have a built-in “toString” method.

Either way, that someone *must* have been unaware² that the (*host-defined*) 
“window” property (of the ECMAScript global object) does _not_, in general, 
refer to the ECMAScript global object, but to a *host* object that already 
has properties (some with setters), and neither needs to admit augmentation 
nor does it need to inherit native methods like Object.prototype.toString().

______
¹ the oldest version we need to support for this application
² probably blissfully, likely through bad books and/or bad tutorials like
  w3schools.com

> Cause

Non sequitur.

> AFAIK, the x variables of these two functions are both local.
> 
>   function f1() {
>     var x;
>     x = 1;
>   }
>   function f2(x) {
>     x = 1; //this won't refer to any scope-inherited x variable
>   }

Correct, except that the latter “x” is the identifier of a formal parameter, 
_not_ a local variable.
 
> Or is there a potential problem with this?

Only that these examples do not fully apply to sendTest().
 
>> (Likewise, when you are tempted to write
>> 
>>   document.querySelector(".foo")
>> 
>> then write
>> 
>>   document.getElementsByClassName("foo")

Correction: The latter would be equivalent to

  document.querySelectorAll(".foo")

Instead,

  document.getElementsByClassName("foo")[0]

would be equivalent to the former, provided that the method returned a 
reference to an object implementing the NodeList or HTMLCollection interface 
(which would need to be tested before the “0” property read access).

-- 
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.

Back to comp.lang.javascript | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

eventDispath how that work Une Bévue <unbewusst.sein@fai.invalid> - 2014-10-14 16:23 +0200
  Re: eventDispath how that work JJ <jj4public@vfemail.net> - 2014-10-15 03:24 +0700
    Re: eventDispath how that work Une Bévue <unbewusst.sein@fai.invalid> - 2014-10-15 10:49 +0200
    Re: eventDispath how that work Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-10-15 15:24 +0200
      Re: eventDispath how that work JJ <jj4public@vfemail.net> - 2014-10-16 02:10 +0700
        Re: eventDispath how that work Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-10-15 23:02 +0200
          Re: eventDispath how that work Joao Rodrigues <groups_jr-1@yahoo.com.br> - 2014-10-15 18:59 -0300
            Re: eventDispath how that work Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-10-16 02:48 +0200
          Re: eventDispath how that work Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-10-16 18:11 +0200
      Re: eventDispath how that work Une Bévue <unbewusst.sein@fai.invalid> - 2014-10-15 22:06 +0200
        Re: eventDispath how that work Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-10-15 23:39 +0200
          Re: eventDispath how that work Une Bévue <unbewusst.sein@fai.invalid> - 2014-10-16 14:26 +0200
            Re: eventDispath how that work Joao Rodrigues <groups_jr-1@yahoo.com.br> - 2014-10-16 14:39 -0300
              Re: eventDispath how that work Joao Rodrigues <groups_jr-1@yahoo.com.br> - 2014-10-16 19:44 -0300
                Re: eventDispath how that work Une Bévue <unbewusst.sein@fai.invalid> - 2014-10-17 09:00 +0200
                Re: eventDispath how that work Joao Rodrigues <groups_jr-1@yahoo.com.br> - 2014-10-17 10:16 -0300
                Re: eventDispath how that work Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-10-17 21:40 +0200
                Re: eventDispath how that work Joao Rodrigues <groups_jr-1@yahoo.com.br> - 2014-10-17 17:03 -0300
                Re: eventDispath how that work Une Bévue <unbewusst.sein@fai.invalid> - 2014-10-18 15:46 +0200
              Re: eventDispath how that work Une Bévue <unbewusst.sein@fai.invalid> - 2014-10-17 08:26 +0200

csiph-web