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Well, we want to merge this dot state dot count plus 1.
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And then we also want to merge that same thing again.
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And so what happens when you merge these three things together?
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Well, these two get merged together, and it actually turns into this.
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Well, you go left to right, so count gets merged,
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gets replaced by this dot state dot count,
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which then gets replaced by this dot state dot count plus 1.
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And so that is actually just 0 plus 1, which is why we end up getting a 1
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here.
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Does that make sense?
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And so we can prove that it runs asynchronously by logging.
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So what do we expect this dot state dot count to be at line 21?
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AUDIENCE: [INAUDIBLE]
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JORDAN HAYASHI: It actually logs 0, because when you call this dot set
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state, all it does is add it to that queue
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that we know it's eventually going to execute.
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Same with this dot set state.
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It gets added to a queue as well.
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And so when we get to line 21 here, what has actually executed?
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Well, nothing.
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And so this dot state dot count still has a value of 0,
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and then only after that do those batch set states run.
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So what if we actually did care about the previous count before?
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Say we wanted to add 1 to the previous count,
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but we actually really wanted that state to exist beforehand.
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Well, we can pass it what's called an updater function,
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and that's a function that takes the previous state
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and returns some new state.
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In this case, we want it to be the previous state, dot count plus 1.
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So now we have two set states, where rather
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than just passing an object to emerge, we're saying,
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actually run this function.
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This function takes the old state and returns
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a new object, which is where the count is the previous state's count plus 1.
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And so now when it batches it, it says, oh, we actually
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need to run this function twice.
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And so now when we click, it actually goes up by two.
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So any questions on React, props, or state thus far?
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Cool.
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Let's go ahead and take a quick break.
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And then when we come back, we can play with React a little bit more.
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Hello, and welcome back.
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So before the break, we were talking about React and props and state.
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And now with all three, we can actually go ahead and start
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building pretty powerful apps.
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And so for homework, for the project zero,
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you guys have been working on a to do app, which you've
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been writing in all vanilla JavaScript.
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And today for the rest of the class, we're
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going to go ahead and together implement that in all React.
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So what are some strategies you may go around doing
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your thing in vanilla JavaScript?
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Well, say we had to dos created as list items, where within each list item,
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we have input, which has a checkbox.
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Maybe you're doing the challenge and you want to take care of the deletes
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as well.
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And then, of course, you're going to have some span
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or some way of displaying text.
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And so maybe have a couple different functions.
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One is to create a to do.
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One is to delete a to do.
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And what might you do in those functions in order to create those to dos?
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Well, first, maybe you'll get the text.
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Then what?
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Maybe go ahead and using document dot create element,
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maybe you want to create a list item.
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Then what?
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You'll probably have to create the input.
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Then probably create a button.
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Create the span.
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And maybe at the end, you hook those all up together
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and append those to the list.
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And so how might you take care of delete?
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Well, find the to do.
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Maybe delete that.
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Then what?
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Make sure to update the counts.
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And maybe we have to do that over here as well.
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So you see how we're doing this in a very imperative manner.
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Using JavaScript, we tell the browser exactly what we want to do.
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Well, we know that our to dos are shaped like this.
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So first go ahead and get the text.
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Maybe create that [INAUDIBLE],, create the checkbox,
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create all these other things.
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Append them to each other, and maybe append them to the list.
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And so we go ahead, and we created a new to do, but we had to do a lot of steps
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in order to get there.
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And so what might be an easier way to do this?
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Well, in to do one, maybe we wanted to abstract out the creation of the
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to do itself.
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So maybe in that new create to do function,
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we go ahead and make the list item, make the input, make the button,
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make the span.
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Hook those all up together.
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But that's one, little, discrete part of the UI, right?
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We're going to start to componentize these things.
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Maybe abstract out a function for creating the to do, and then in the new
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to do, we can still get the text, update the counts, and append to the list.
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But rather than doing all of this work in the new to do function,
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maybe we just invoke create to do.
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