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In the first video on Javascript primitives when we talked about the five basic data types which are

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string number Puli an undefined No we kind of lumped non undefined together at the end and I mentioned

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that we would cover them in more detail in a future video.

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So that's what we're going to do here.

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And it's still going to be a pretty quick video because there's actually not very much to know and undefined

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.

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They are both just values that a variable can be set to and they both mean nothingness in a sense.

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But there's a big difference between them.

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So the code I have here illustrates that difference.

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So let's start with undefined.

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So if we declare a variable like we have here var age but we never said it to something.

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So it's declared but not initialized to a value it's considered undefined.

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So I'll show you what that means.

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It looks like if I just run that code var age and I never set it to a value I tell javascript make some

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space for something called age but we never store anything in there.

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If I asked for age back it tells me undefined and I want to contrast that with if I just ask for something

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that doesn't exist at all.

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Like color.

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It gives me an actual error message versus age.

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It knows about age.

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It just doesn't know anything inside of age and then no.

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It's another way of expressing nothingness but it's very different because no means something is explicitly

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empty or explicitly nothingness versus undefined which just means that something doesn't have a value

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yet.

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So in this example here I'm showing a case where you might use NULL.

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So if we had a current player variable we were making a game and it's set to be a string of the name

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of the current player.

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If that player then dies there's a game over we could set current player to be No.

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To make it explicitly clear that there is no current player so direct this appear no undefined are similar

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in that they both mean nothingness.

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But there's a big difference in that no means explicitly nothing versus undefined means that something

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is empty or doesn't have a value yet.

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So that really doesn't sound like a big difference.

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But as we go throughout this course you'll run into an undefined and we'll see the difference and it

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will make a lot more sense as we keep going.

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But I just want you to be aware that there is a difference