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it's just mutated it.
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On the other side, if we did something like let thisIsALet A Let equal to 51,
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and then want to change that later, we go 50, that's totally OK.
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But if I actually tried to reassign-- so let thisIsALet.
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If I tried to do this again, it's going to yell at me because it's saying, hey,
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you already declared something called thisIsALet.
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You cannot declare that again.
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And so const and let actually protect you
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from declaring something with the same variable name twice, which is something
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that var does not do.
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Cool.
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What do you guys think would happen if I tried to do this?
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Anybody care to guess?
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Undefined?
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So let's try to run it.
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Error.
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So since these things are block scoped, it
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means the variable is declared at the line that it is written.
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And if we try to use it before then, it actually does not even exist at all.
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So if I tried to console.log something called thisIsAConst here, remember,
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the JavaScript interpreter is just reading down
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and it won't see like, hey, what the heck is thisIsAConst?
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I have no idea what that is so I'm just going to Error here.
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Same thing if I tried to do this as a let here.
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That will also Error with the same error.
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AUDIENCE: So what's line 14 error on?
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JORDAN: Why does it error?
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Because we're trying to declare two variables with the same variable
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name twice.
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So we have let thisIsALet equal 51 here, and we try to do it again
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and it's saying, hey, you already have a variable called thisIsALet,
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so I'm not going to allow you to do that again.
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So it's going to be this error right here.
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AUDIENCE: So it's the instantiation of it?
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JORDAN: Yeah.
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It's saying like-- basically I'm saying, hey, give me a variable to call,
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this is a let here, and then a couple lines later I'm saying,
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hey, give me a variable called thisIsALet,
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and JavaScript says, hey, like, you already
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have a block scope variable called thisIsALet,
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so I can't give you another one.
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However, if I tried to just update it like this, that's totally OK.
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Any other questions here?
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So the other thing we said, we can have these things called a var.
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If I did var thisIsAVar equal to 50, that's fine, I can update it.
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That's also fine.
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I can also do this, and that won't yell at me.
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So vars are older ways to declare variables
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and they don't have the same protection that let and const do.
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You can override them like this and it's totally OK.
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And you can actually declare a whole new variable with the same variable name
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and that's also OK.
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And something-- yeah?
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It just-- yeah, so that's--
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So if I try to run this, it will update the old one.
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It'll just replace that value.
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It's actually called shadowing, where you create a new variable
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with the same variable name which basically just overshadows
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that old one.
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So it's as if the other one didn't exist.
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Cool, so check this out.
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So this errors and we know this errors.
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But say I were to do this.
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You'd expect this to also error, but actually it does not.
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It returns undefined.
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So this is another weird thing about JavaScript.
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This is called hoisting.
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And so certain things are hoisted, which it
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means basically it takes the definition of something
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and hoists it to the very top of the file and does that first.
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Get
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And a few things are hoisted.
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Var the-- actually, the declaration of the creation of an empty variable
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are hoisted.
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Function definitions are hoisted as well, but const and let
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are not, as we saw if we tried to access the variable name of the const or let,
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then it errors.
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But with a var, the declaration of that variable is actually hoisted.
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And we can talk a little bit more about how that works in a second,
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but let's actually play with it a little bit more.
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So function definitions are hoisted.
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So let's clean up this file a little bit.
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So let's call this new function, called--
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so this is console.log.
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So I defined this function called thisIsHoisted
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at the very bottom of the file.
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And all it does is it console.logs, there's
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a function declared at the bottom of the file.
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But something very interesting is at the very top of this file, I can call it.
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And that will actually work.
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And so that is what's called function hoisting.
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The behavior whereby a function definition
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declared at the very bottom of a file is actually available for use
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at the very top of the file as well.
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But this does not work in other cases.
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So say I were to do something like this.
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So who can tell me what the difference is between line 21 and line 25?
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They look pretty similar, right?
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Yeah?
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OK.
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