this
What is this? Reconnecting with scope and looking at 'this' for the first time.
Video Transcript
00:01The next thing I want to talk about is that this what is this and we're kind of
00:06reconnecting and retouching with this topic of scope which is going to follow
00:10us throughout a lot of courses in the future as well. And scope is basically
00:15how does flash know or how does our code know where is what. And as we
00:22create different variables and different objects and different
00:25properties, it's becoming even more critical to understand what is
00:30our scope. And to sample this out is what I want to do is I want to create a
00:34property for our hello which will be instead of Ben let's say here we're going
00:38to create a property which will be called message. And in our message we're
00:42going to type here hello which is our message. Now in my trace method right
00:48here I could have just said o.message right but then I'm linking my
00:55function with that specific object which in our scenario is not the end of the
00:59world but if we run our application again we're going to see hello. Now what
01:03would happen if we didn't know that we could well not only we didn't know we
01:07usually try to avoid inside of functions inside of methods to call
01:12something explicitly the name of its parent. And the reason is or its
01:17definer and the reason is is very simple. We want our functions to be able to be
01:22anonymous and reusable and we don't really want to reference the things
01:27outside of those functions if we could avoid it. And Flash has a few ways to help
01:32us avoid using explicitly the word o inside of our function. And just to make
01:37sure that we understand the scope issue if I create a new variable here which I
01:42will call message as well and I could do that because oops not a stack frame
01:47it's a string. Now we're going to type here no no no. And if I just try to trace
01:57out from my function message what my what my function now is first of all it's
02:02checking to see is there a definition of message inside of this function? No. Is
02:06there inside of this function? No. Thus I'm going to look outside outside of my
02:11scope. I'm going to look out into the bigger area and in this bigger area I'm
02:16going to find a message so I'm going to output message. So if I run this app
02:19again no no no and it's finding my string. Now if I would comment that out it
02:24would generate an error right because it could not find it. How could I let Flash
02:29know from inside here without saying without actually saying it without
02:33actually using the o as my reference? How can I make my function know or my
02:38message know that it belongs to whatever container it's in? So the
02:43container is our object and the and the keyword and I've said in the beginning
02:47is actually this this dot message. What does this mean? This is kind of like a
02:54dynamic value. It's an it's a dynamic object that basically this is always
02:59what defined it. So in inside of our function our definer is our o right so
03:06this is o when we're saying it inside of our function inside of our method
03:10because methods methods belong to something. So our this is basically it's
03:16like saying what do I belong to? So what whoever whoever I belong to dot
03:21message and this dot message should output a low and it does so but if I
03:27would grab this trace and trace it out outside right here what do you think
03:32would happen? If I trace this out right here let's see what happens bam there's
03:36an error. Why is there an error? You might be thinking but I defined here a
03:41variable you definitely did but that does not belong to there are this are
03:46this what is our this? This variable is not part of a global scope this variable
03:53belongs to our function but our function is not an object it's just a function
03:57this function belongs to a bigger object which is called main and thus our main
04:04does not have a property called message our function does but not our not our
04:11function not our main class not our big object now this could get confusing but
04:17let's see how this works again let me just so this we say we can't we well
04:21this won't work won't work as there is no message there is no very there is no
04:29property called message but only a variable right there's a variable called
04:37and here we're gonna we kind of see it more explicitly that although we do have
04:42a variable here but this variable is not a property because it doesn't belong to
04:46an object same deal here if I would create a variable right inside here it
04:50would be a variable it would not be a property and thus when we would use our
04:54this our this really could only reference different things inside of a
04:58specific object so if our this was here the long-term object if our this
05:03is outside it belongs to the bigger object and in our scenario it's
05:06actually our application it's our main which we didn't dig into and it's fine
05:11but let's see let's see what that is so if I just trace out right here this
05:16before we even get into our message if I just trace out this and let me just put
05:20here tracing from the main method right now we know it's a method it's not a
05:29function anymore and we trace out this we're gonna see that we're getting back
05:34an object of a main object right which is our main object and if we trace out
05:40from inside of our hello function from our hello method and you'll notice there's
05:48a lot of people can get confused with it because it's fine calling a method a
05:52function is not a sin calling a function a method could get you confused but
05:57functions are functions and methods are type of functions only methods belong to
06:02something they have a this they have a belonging they belong to something so
06:08now if I run up my application one more time I should be getting that our hello
06:13method belongs to the object object now let's just see one more thing if I create
06:18here another function one of those traditional functions that we've created
06:21before function it's kind of like a function inside of a function right let
06:25me just make a screen a bit bigger so I'm gonna create a function here that
06:30it's gonna be test this and it's not gonna return anything and we're just
06:38gonna trace and trace this is not a method right it's a function I'm gonna
06:50set that as a function and I'm just gonna call test this and let's see what
06:54what does this return and try to think about it what do you think is the
06:57logical value that would return as this well this test is not a function
07:04right where do you think it's gonna find the value of our this and there you
07:10go and basically it's telling us that it doesn't really have a this and it's a
07:15global this and this our global this is basically there is no scope our scope
07:21so if we try to use this inside of a function that actually is not a method
07:26we will get confused although we're getting something back here this
07:30something that we're getting back is not exactly what we would expect right so
07:36it's very important to notice that when you're fun when you're trying to trace
07:39that this out of a function you're gonna get ran well not random things we're
07:44gonna get things you're not expecting and if you work as you work with this
07:47and you get more comfortable with this it's gonna be much easier but it's in
07:51the beginning time and it's again with these are one of the most most common
07:56bugs and flesh or especially in an earlier versions of as to and it
08:01definitely now as well is that people get confused by what is this and that
08:07this is always for what we're concerned right now are this is always the
08:11connection of our this between our our method and who it belongs to and thus
08:17because this is not really a method we're getting something unexpected but
08:22this is a method then we're getting what we expect and this is definitely a
08:27method which is our class definition and we're getting the value that we would
08:31expect as well okay so that's my that's my notes about this and don't worry too
08:37much about this we're gonna definitely meet a lot a lot more and we're gonna
08:40see how powerful this is especially as we work with multiple things at the
08:45same time when we don't have to explicitly say what this is