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And so how are we going to change that value of 0 to be something like 1?
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We have to invoke that method, and so let's go ahead
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and do that when we click a button.
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So let's have something like div here, and within that div,
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let's create a button.
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And when we click that button, let's go ahead and invoke this thing
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called increase count.
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And let's call it increase.
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So now if we click this button, what do we expect to happen?
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So notice how we pass this thing called unclick.
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What's the value of unclick?
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Or what function should we invoke?
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Well, we should invoke this dot increase count, which should be this.
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We'll see what it is in a second.
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And then that should call this dot set state, which
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should, in theory, increase count.
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So what happens?
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Uh-oh!
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Cannot read property set state of undefined.
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So there's a bug here.
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Can anybody spot the bug?
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Cannot read property set state of undefined.
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It even tells us exactly where that was.
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So we're seeing this dot set state, which
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I told you is a method that we can use.
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But it's saying, we cannot read a property of undefined,
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which implies that this is undefined here.
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Yeah?
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What do you think is going on here?
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AUDIENCE: [INAUDIBLE]
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JORDAN HAYASHI: Yeah.
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So we have to bind that function to this.
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So we talked about this and binding this towards the end of last lecture.
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And we talked about how this is bound on whatever object it's invoked on.
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And so right now, this isn't invoked on the class, right?
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It's invoked when you click that button.
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And so we're going to have to bind this to the value that we want it to be.
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And so towards the end of the lecture, we talked about different ways
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to bind this.
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Can anybody remember those?
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Yeah?
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AUDIENCE: Bind method.
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JORDAN HAYASHI: Yeah.
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There's this thing called bind, which is a method on all functions.
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And similar to bind there is call and apply.
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And we also talked about a different way of writing functions,
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which actually lexically binds this.
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Do you guys remember how that was?
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Remember, if you use arrow notation, it automatically binds this for you.
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And so you can solve this problem in all three different ways.
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And so down here, if we wanted to do dot bind,
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we could do dot bind to this, which, since we're invoking it
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on the render method, which is a method that's invoked on the class,
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this is bound correctly.
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And so when we increase it, it will increase.
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We could also use--
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well, call and apply don't really work here.
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But we could actually use arrow notation.
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So we can say, let's have an arrow notation down here,
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which creates a new, anonymous function.
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And what does that anonymous function do?
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Well, all it does is invoke increase count.
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And so since this we wrote as an ES6 arrow notation,
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it automatically binds this to be what we want it to be.
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Does that makes sense?
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There are a couple other places that you could do this.
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We could also do it in the constructor method.
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You could do this dot increase count equals this dot increase count dot bind
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this.
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Or lastly, you could also--
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and then down here, you'd just have to do this dot increase count.
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Or alternatively, you could actually just define this in the constructor.
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And so that would be the other way.
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So you could say, this dot increase count equals an arrow notation in this.
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But let's stick with this way for now.
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Cool.
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So now we have a button that works, that increases as we expected.
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So what if instead of increasing it by 1, we wanted to increase it by 2?
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Well, of course, you could just do this.
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Let's say we want to do it by sending state twice.
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So what do we expect to happen here?
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We expect it to set state to be count as this dot state dot count plus 1 is 1.
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And then [? it does ?] set count to be 1 plus 1 to be 2, right?
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But we see it's only going up by 1.
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Why might that be?
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Well, set state calls are actually batched and run asynchronously,
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which means React is smart enough to know that if this dot set state is
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getting called a bunch of times in a row,
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rather than immediately doing that, maybe it would be better to just batch
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them and do it all in one go.
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And like I said, if you pass an object to this dot set state,
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it will just merge it into the new state.
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And so what is actually happening when you
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call this dot set state twice in a row?
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Well, they get batched together.
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And so it says, OK, now we know we need to merge
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into the old state these two objects.
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And so it's basically saying, merge the current state-- in this case,
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it's count is 0.
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And then what are we merging into it?
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