Question

161

How to change the context of a function in javascript

rated 0 times [ 163] [ 2] / answers: 1 / hits: 24855 / 15 Years ago, thu, october 8, 2009, 12:00:00

I'm trying to understand why in javascript, you might want to change the context of a function. I'm looking for a real world example or something which will help me understand how / why this technique is used and what its significance is.

The technique is illustrated using this example (from http://ejohn.org/apps/learn/#25)

var object = {}; 

function fn(){ 

  return this; 

} 

assert( fn() == this, The context is the global object. ); 

assert( fn.call(object) == object, The context is changed to a specific object. );

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answered 15 Years ago jalyn · Accepted Answer

jQuery makes use of it to good effect:

$('a').each(function() {

    // this is an a element - very useful

});

The actual jQuery code looks like this:

for ( name in object ) {

    if ( callback.call( object[ name ], name, object[ name ] ) === false ) {

        break;

    }

}

If it just did callback( name, object[ name ] ) then this wouldn't be set to the current object in your iterator and you'd have to use the parameter instead. Basically it just makes things easier.