[RESOLVED] any good reason to set localVar=this?

sneakyimp

I'm working on a pretty gnarly Javascript project with all kinds of inheritance and such. Within a class, I have seen a local variables defined and pointed at this. Is there any reason to do this? I'm imagining possibly for variable scoping reasons or to isolate variables from some kind of inheritance chain, but don't know for sure. Any insights would be much appreciated. Here's some sample code:

var DirectDepositDialogue = function(myParent) {
    var self = this;
    InfoPanelX.apply(this, [new SizeObject(410, 590), 0, 0, 'rgba(240, 240, 240, 0.0)']);
    self.allowOverflow();
// blah blah lah
}

Weedpacket

A JavaScript function is an object, and a method is an object property that contains a function. Object ancestor can assign a function to a property of object baby:

ancestor.makeSomethingForBaby = function(baby)
{
    baby.doSomething = function()
    {
         .....something involving "this".
    };
};

When [font=monospace]baby.doSomething()[/font] is called, [font=monospace]this[/font] will refer to baby, not ancestor. If you want baby to hold a reference to ancestor, it has to be through another variable.

ancestor.makeSomethingForBaby = function(baby)
{
    var muggins_here = this;
    baby.doSomething = function()
    {
         .....something involving "muggins_here".
    };
};

When [font=monospace]baby.doSomething()[/font] is called, [font=monospace]muggins_here[/font] will refer to ancestor (and [font=monospace]this[/font] will still of course refer to baby).

This is called "late binding", to contrast it with "early binding" (which is what PHP would do in this situation) where [font=monospace]$this[/font] is identified with ("bound to") the object creating the function.

sneakyimp

Thanks for the clear description, Weedpacket. As it turns out, I've already encountered an issue where I was defining a function attached to an object and attempted to refer to this in said function. Fortunately this was not defined so rather than butchering some unfortunate object and getting really confused about some strange behavior, I saw a console error that said "this not defined" or some such error.

At any rate, I see that assigning this to a local var is necessary sometimes in order to manipulate the current this inside some member function where the scope may be different and "this" may refer to something else.

Weedpacket

Yes, it's what bit me when I first started writing a significant amount of JavaScript.

When I asked why my error console kept complaining that [font=monospace]this.doSomething()[/font] didn't exist ("it's right there!") and learning that this ([font=monospace]this[/font]?) was the reason, it went some way towards my first JavaScript "aha" moment.