Function object constructor

hi
i saw this function constructor and i dont know what u can get with this function constructor

can some1 explain that to me what this construct means?
what is purpose for this type of construct
thnx

 

this what i get somewhere else

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for example, it can save ram when a bit ends, something like:

function heavyRamUsage(){
  var ReallyBigFile = 'someFile.txt';

  var line94 = (function(file){
    var contents = ajaxText(file);
    var arr = contents.split("\n");
    return arr[93];
  )(ReallyBigFile);
  // do something with line 94 here...
};

Code (markup):

in this way the contents of the text file after it has been loaded, and the extra array we generated is dropped when the function ends, this can be a lot better on ram usage than keeping the huge array around if you are not using it.

The main usage you will see though, is to pass a reference to something that would normally not be linking there later, for example a number in a loop:

var eles = document.getElementsByTagName('img');
for(var i = eles.length-1; i >= 0; i--){
  eles[i].onclick = function(){
    alert(i);// doesn't work, I changes on the next loop, so the code doesn't work as we want.
  }

  eles[i].onclick = (function(x){
       return function(){
         alert(x);// this does work, since x comes from a function called now.
       };
     }(i);
  }
}

Code (markup):

I hope I haven't confused you here, I'm not very good at explaining things.

 

this is a nice article on anonymous functions, closures and scoping of variables.
http://www.jibbering.com/faq/faq_notes/closures.html#clEncap