Why use _this vs. this in Javascript interaction?

[chabotjeff]

I've seen Javascript used in Animate that defines _this instead of this.

For example: _this()stop; or _this.parent.gotoAndPlay

vs.

this()stop; or this.parent.gotoAndPlay

The Adobe Press Animate book actually shows screenshots with both used. And, I understand Javascript requires determining scope. Why use one or other and why not just have a standard?

[Colin Holgate]

'this' is a reserved word that means the object in which that line of code is. With JavaScript you sometimes need to convery which 'this' you mean. There are generally two ways to do that, either Bind or use a variable.

For example:

alert(this);

testfunction();

on testfunction{

alert(this);

}

The two alerts will show different values. Using Bind:

alert(this);

testfunction.bind(this);

on testfunction{

alert(this);

}

will show the same value. But, using bind everywhere to keep the level being the same may not always be what you want, and so the other way of using a variable would do:

var _this = this;

alert(_this);

on testfunction{

alert(_this);

}

would show the same value. As 'this' is a reserved word it can't be used as a variable, and a common variable to use is _this. I sometimes use 'self' too, like:

var self = this;

Then I can refer to self when I'm trying to do something on the main timeline.

Another approach that is useful is to stash your variables in the browser window level. You could say:

window.tl = this;

and then everywhere in your entire hierarchy you could get back to the main timeline by addressing window.tl.

[ClayUUID]

Polluting the global scope with variables is not great practice, BTW. At the very least I'd recommend stashing any Animate globals in exportRoot instead, which acts as the defacto Animate namespace.

[Bundaberg North5E42]

Hey Team,

I am teaching kids coding (I know right?) so I need this in a simpler way!  Why?  Even when you use the coding wizard to put simple play command onto a button the first thing it does is create var _this = this 

So I am saying something like...  this is a special word, its reserved for talking to objects on the main timeline.......... so what difference does it make if we make _this = this?? Doesnt this still mean we are referring to this special word?  Is there a super simple to explain why this is so? 

Thanks team

[JoãoCésar]

Hi.

It's because the this keyword value changes according to the context. So depending from where you use it, you may get different results. For example:

class AnotherTest
{
	doSomething()
	{
		console.log("doSomething: ", this);
	}

	static doSomethingStatic()
	{
		console.log("doSomethingStatic: ", this);
	}
}

new AnotherTest().doSomething();
AnotherTest.doSomethingStatic();

function test()
{
	console.log("test: ", this);
}

test();

stage.on("drawend", function(){ console.log("drawend: ", this); }, null, true);
stage.on("drawend", () => console.log("drawend arrow: ", this), null, true);

console.log("root: ", this);

// doSomething: AnotherTest
// doSomethingStatic:  class AnotherTest { ... }
// test: Window
// root: lib.Untitled1
// drawend: lib.Stage
// drawend arrow: lib.Untitled1

So this is why it is a good idea to save a reference to a more predictable context in a variable (eg.: var _this = this).

I hope this helps.

Regards,

JC

[Bundaberg North5E42]

Thanks JC, I think that helps

[Bundaberg North5E42]

Just to check if I understand, does that mean that if we make var _this = this - does that mean that the _this variable will refer to the timeline the object is on, even if it is called in another timeline?  e.g. If I make _this a var that equals this on the main timeline - can I use _this to refer to things from inside a MC (a separate timeline) and it will still refer to the main timeline? 

[kglad]

no.

you would need to use the correct path to the _this variable (just like every other variable). ie, using

var _this = this;

is only useful in the frame and timeline wheee it's defined..

[Bundaberg North5E42]

ok thanks 

[kglad]

you're welcome.

[kglad]

and when "this" is used on a timeline, it refers to that timeline.  ie, on the main timeline, it references the main timeline. on a movieclip timeline, it references that movieclip.

[Bundaberg North5E42]

Ok Kglad I will use what you have said here for sure, sounds good 🙂