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Re: Two versions of code - advantages and differences

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On 2013-01-06 22:30, Cezary Tomczyk wrote:
> I've just wrote something like this:
> 
> var main = {
>      [...]
> 
>      apply : (function(){
>          var tempImg = document.createElement('img'),
>              t, img;
>          tempImg.width = '10';
>          tempImg.height = '10';
> 
>          return function(o){
>              img = tempImg.cloneNode(false);
>              img.alt = o.alt;
>              img.src = o.src;
>              t = document.createElement('span');
>              t.appendChild(document.createTextNode('\u00a0'));
>              t.appendChild(img);
>              t.appendChild(document.createTextNode('\u00a0'));
>              window.setTimeout(function(){ examplefn(t); }, 100);
>          };
>      }

       )(),

> 
>      [...]
> };
> 
> As I understand (correct me if I am wrong) this is inefficient because:
> 
> * closure need extra memory and will be always in memory because 
> returned function refers to variables that are outside of returned function.

In your example, the returned function closes over tempImg. If this
element is actually needed, you will have to store it _somewhere_. It
won't take up more memory just because it's in a closure. If it's not
needed, and you're concerned about memory usage, then don't create it.

Keeping a value around in a closure when it's not strictly needed can be
convenient (from a programmer's perspective), and it can speed up
execution later on, at the cost of some memory. You could look at it as
an optimization measure that favors speed over memory usage.

In another message, you asked about the performance penalty of closures
compared to non-closure* functions. I was about to reply that the
difference is insignificant, but thought I'd better provide a benchmark
for support. That's when I ran into my little benchmark problem with
Chrome, and I forgot to reply :-/  Anyway, having a closure does not by
itself cause a noticable difference in performance. You'd need millions
of calls to see any difference at all, and then the result is mixed. In
some cases/implementations, the closure variant was actually executed
faster than the function+variable variant. You'll have to take my word
for that or test it yourself, because I'm done with benchmarks for today :)

(* technically, every function call creates a closure, but that
definition of a closure is so broad that it becomes almost useless)

> * "apply" is parsed immediately which is not needed always

Nitpick: the whole script is parsed before anything can be executed. You
probably meant to say that the anonymous function is executed
immediately, and a tempImg element is created even though it may not be
needed.

Creating a handful of elements when a script is loaded won't make much
of a difference. If you still want to reduce the overhead, you could
initialize such elements lazily (ie, only when they are needed):

   apply: (function () {

       var tempImg;

       function prepareImg() {
           if (!tempImg) {
               tempImg = document.createElement("img");
               tempImg.width = 10;
               tempImg.height = 10;
           }
       }

       return function (o) {
           prepareImg();    // (or inlined)
           ...
       };

   })(),

To be honest, I don't see the need for a tempImg (or a closure) at all
in this method. The element just gets cloned every time apply() is
called. Why not simply create the actual image element when you need it?

> Anything else inefficient or wrong?

Nothing wrong, just more from the nitpick department:

I would avoid the name "apply" for a method. It's not wrong, but it can
lead to confusion for others who read your code.

The 't' and 'img' variables should be declared where they are used, not
in the surrounding function.

\u00a0: ten years ago, a typical HTML document was littered with
non-breaking spaces, but they are rarely needed today (except when you
actually need a non-breaking space between words). Manually creating DOM
text nodes for them is a waste. Use CSS to create fillers and margins
instead.

tempImg.width = '10';  - should be an number, not a string.


HTH,

- stefan

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Two versions of code - advantages and differences Cezary Tomczyk <cezary.tomczyk@gmail.com> - 2013-01-03 22:58 +0100
  Re: Two versions of code - advantages and differences Stefan Weiss <krewecherl@gmail.com> - 2013-01-04 00:41 +0100
    Re: Two versions of code - advantages and differences Cezary Tomczyk <cezary.tomczyk@gmail.com> - 2013-01-04 09:15 +0100
      Re: Two versions of code - advantages and differences Gregor Kofler <usenet@gregorkofler.com> - 2013-01-04 11:26 +0100
        Re: Two versions of code - advantages and differences Cezary Tomczyk <cezary.tomczyk@gmail.com> - 2013-01-04 14:29 +0100
          Re: Two versions of code - advantages and differences Gregor Kofler <usenet@gregorkofler.com> - 2013-01-04 14:56 +0100
            Re: Two versions of code - advantages and differences Cezary Tomczyk <cezary.tomczyk@gmail.com> - 2013-01-04 16:52 +0100
    Re: Two versions of code - advantages and differences Cezary Tomczyk <cezary.tomczyk@gmail.com> - 2013-01-04 09:19 +0100
  Re: Two versions of code - advantages and differences Gregor Kofler <usenet@gregorkofler.com> - 2013-01-04 11:25 +0100
  Re: Two versions of code - advantages and differences Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-01-04 15:31 +0100
    Re: Two versions of code - advantages and differences David Mark <dmark.cinsoft@gmail.com> - 2013-01-05 08:37 -0800
      Re: Two versions of code - advantages and differences Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-01-05 18:14 +0100
        Re: Two versions of code - advantages and differences David Mark <dmark.cinsoft@gmail.com> - 2013-01-05 10:51 -0800
          Re: Two versions of code - advantages and differences Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-01-05 20:46 +0100
            Re: Two versions of code - advantages and differences David Mark <dmark.cinsoft@gmail.com> - 2013-01-05 17:12 -0800
              Re: Two versions of code - advantages and differences David Mark <dmark.cinsoft@gmail.com> - 2013-01-06 17:43 -0800
              Re: Two versions of code - advantages and differences Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-01-07 03:58 +0100
                Re: Two versions of code - advantages and differences David Mark <dmark.cinsoft@gmail.com> - 2013-01-06 19:23 -0800
                Re: Two versions of code - advantages and differences Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-01-07 05:19 +0100
    Re: Two versions of code - advantages and differences Cezary Tomczyk <cezary.tomczyk@gmail.com> - 2013-01-06 22:30 +0100
      Re: Two versions of code - advantages and differences Stefan Weiss <krewecherl@gmail.com> - 2013-01-07 02:35 +0100
        Re: Two versions of code - advantages and differences Cezary Tomczyk <cezary.tomczyk@gmail.com> - 2013-01-07 22:45 +0100
      Re: Two versions of code - advantages and differences Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-01-07 04:36 +0100
        Re: Two versions of code - advantages and differences Cezary Tomczyk <cezary.tomczyk@gmail.com> - 2013-01-07 23:06 +0100
          Re: Two versions of code - advantages and differences Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-01-08 12:35 +0100
            Re: Two versions of code - advantages and differences Cezary Tomczyk <cezary.tomczyk@gmail.com> - 2013-01-09 08:34 +0100
  Re: Two versions of code - advantages and differences David Mark <dmark.cinsoft@gmail.com> - 2013-01-05 09:13 -0800
    Re: Two versions of code - advantages and differences Cezary Tomczyk <cezary.tomczyk@gmail.com> - 2013-01-06 11:09 +0100
      Re: Two versions of code - advantages and differences David Mark <dmark.cinsoft@gmail.com> - 2013-01-06 12:56 -0800
        Re: Two versions of code - advantages and differences Cezary Tomczyk <cezary.tomczyk@gmail.com> - 2013-01-07 22:53 +0100
  Re: Two versions of code - advantages and differences Luc Yen <luc@goal.tw> - 2013-01-05 12:54 -0800
    Re: Two versions of code - advantages and differences Cezary Tomczyk <cezary.tomczyk@gmail.com> - 2013-01-06 21:38 +0100

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