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24.9
So something special about arrow notation
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is that it has an implicit return, which means we don't actually
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have to write-- so we could have written the same thing as this,
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as a function that takes props.
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And what does it do?
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It returns this.
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So this actually does the exact same thing.
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So we're saying, app two is a function that
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takes a single argument called props.
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And what we do is we return this, and so arrow notation shorthand is just
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you have your arguments in arrow.
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And then if you don't have braces, which referred to it like a code block,
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it just returns whatever's next.
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And so we're saying, return this.
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What is this?
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Well, it's this div and H2.
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We're going to wrap it in a parentheses so we know that it's just one value.
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Does that make sense?
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A great question.
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Do you guys see how these are the same?
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So this is great, but we don't really have all that much power yet, right?
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If we wanted to go ahead and change these props,
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we still have to drop down to this raw JavaScript over here.
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So next we'll see exactly how we create apps that are stateful.
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What does that mean?
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Well, there's this notion of state in React,
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and state is basically an internally managed configuration
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for any component.
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And so now components become classes, and this .state is a property on that
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component's instance.
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So how do we update the state?
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Well, there's a method called this.setState,
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which is implemented in this thing called a React.Component, which
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we have to extend in order to have access to that method.
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And this goes ahead and changes that value.
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So you can pass an object to be merged or a function of the previous state.
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And so if we pass this.setState, an object,
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it will go ahead and merge that in with the existing state.
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So if we pass it in an updater function, it's
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basically a function that gets run when we want to change the state.
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And the set state calls are batched and run asynchronously.
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And of course, any change in state will also cause a re-render,
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because it would be silly if we were to change the state
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but not reflect that in the UI.
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And so how might we go about representing state over here?
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So first, let me copy this so that we can save it.
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So let's go ahead, and rather than having an app be a function,
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let's actually have it be a class.
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So we can do class app, and we want to extend React.Component.
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And within that, we want to have this method called render, which is
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automatically invoked on a re-render.
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Within render, we want to return this.
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Cool.
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So the way to now write this is rather than having
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app be a function that takes props and returns something,
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we're actually writing a class for app.
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And so as we talked about earlier, classes have instances.
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And React knows that when you want to render something
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like this, if it's a class, go ahead and create a new instance of that
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and pass in these as props.
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And notice how we don't ever take the props anywhere.
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That's because when we extend React Component,
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React Component, that base class, goes ahead and attaches the props
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to the instance.
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And so in order to get at them, rather than doing props does something,
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we do this dot props dot count.
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So again, the props that come in, in the way that React.Component
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is implemented, it automatically takes the props
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and attaches it to that instance of the class.
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And so in order for us to get them in the render method,
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we do this dot props dot count.
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Does that make sense so far, going from a function to a class?
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We'll talk about this in depth the next lecture.
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And so we talked about this thing called state,
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and how do we actually go ahead and use that?
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Well, when we want to create our state, we actually
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do that in the constructor method.
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And so the first thing that we want to do in our constructor method
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is actually called a super, which means allow
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React.Component to do stuff with the props
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that it would have done otherwise.
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And now go ahead and do what we want to do.
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What do we want to do?
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Well, we want to initialize this thing called state.
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Cool.
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So now we have this thing called state, and how are we
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going to go ahead and update it?
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Well, maybe we should have something called increase count, which
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is a method on this instance.
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And let's go ahead and increase the count here.
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So how might I do that?
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Well, I should call this dot set state and pass in count is this dot state dot
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count plus 1.
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And so now we have a method on this instance called increase count
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that we can call.
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And it should, in theory, increase that count.
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And so rather than referencing the props down here, let's reference state.
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And now it should be 0, and it's just going
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to stay at 0, because we're not doing anything.
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So we can go ahead and get rid of this interval.
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