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In this video we're going to write out a couple of code examples to understand how type annotations

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work specifically with variables.

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Remember a type annotation is going to be a little bit of code that we write to help typescript along

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the type annotation system is kind of at odds with type inference.

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So we're going to first focus on type annotations and then come along and understand how type inference

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comes into play.

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All right.

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So going to change on over to my editor and inside my project directory I'm going to make a new folder

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called annotations.

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We're going to eventually create separate files inside this directory that are all related to type annotations.

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So we're going to organize them all inside of here inside this file during went to make a new file called

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variables dot t s and then inside this file I'm going to write out my first variable declaration that

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has a type annotation.

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So I'm going to write out const apples is going to be a number I'm going to set that to five like so

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so the colon and the word number right after it is our type annotation you're going to see syntax that

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looks exactly like this in the vast majority of typescript code that you work with this type annotation

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is trying to tell typescript that we are only ever going to assign a value of type number to the variable

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of apples if I ever tried to assign some other type of value besides a number all very quickly see an

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error message.

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So for example if I update the five right here to true which is a boolean I'll see an error right away

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and it tells me that the type of true is not a sizable to a type of number.

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Now this is also going to work if I tried to update the value assigned to that variable as well.

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So if I update the cons keyword to let.

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Which will allow me to reassign apples.

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I can then reassign apples down here a value of 10 without any issue however if I tried to assign it

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some other type of value like a string.

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Once again I'll very quickly see an error and once again I see a message that says essentially you have

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a string and you're trying to assign it to a variable that's supposed to be a number not for the rest

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of this file.

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I want to show you some reassignments all over the place.

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So I'm going to continue using the left keyword even though in just about all the declarations we're

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going to add here.

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Usually I would be making use of the concert keyword instead type invitations like this right here can

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be used with any type of value.

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Remember we just spoke about some of the different types of values inside of typescript.

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So we've got no strings billions no void no undefined and functions arrays and so on so with every one

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of these different types of values we can use a type invitation let's write out a couple of examples

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just to understand how type notation works with each of these different types.

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All right.

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So in every case we'll write outlet and then a variable name.

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In this case I going to call my variable speed then a colon then the type of value.

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In this case I going to say it's going to be a string and I'm going to assign it a value of fast like

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so once again if I tried to assign this a value that is not a string.

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So in this case maybe a number I'll see an error and that's pretty much it.

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We're going to see that same pattern repeating every time we tried to assign it a different type of

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value.

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Let's take a look at a boolean as well so let's say maybe let has name is going to be a boolean of true.

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How about no.

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I'll make a variable called nothing much and I'll make that null and that's going to be assigned a value

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of null.

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Now this is kind of interesting right here it's our first example of where we have a value that has

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a name identical to its type.

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We're actually going to see that a couple of times in typescript.

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That's something that's a little bit difficult to grasp.

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Once you start getting into some more complicated topics we're gonna have several lectures later on

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to understand when we're working with something that's a value.

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And when we're working with something that is a type we could also try a new variable called something

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like say nothing and make it undefined like so now we can also use type notation with every other type

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of value as well like we just said.

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So I can use this with an instance of a class or with a built in object or just about anything you can

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imagine.

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So let's say that we're looking at maybe a Date object.

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I'll say I'll put a little section header here and I'll say built in objects so I can make a new variable

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called now I'll say that's going to be a type date and then I'll make a new date and assign it on the

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other side.

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So in this case we're once again doing the same thing we're doing up here we're saying that we're going

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to have a variable called now and we can only assign it a value of type date and then on the other side

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we're going to assign it an instance of a date object.

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All right.

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So now we're kind of getting the idea here let's take a quick pause when we come back the next section

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we'll take a look at some of the more complicated syntax that comes up when we start working with objects

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arrays and functions.

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So quick posit I'll see you in just a minute.