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In this video, we're going to look at the final case in which we will make use of type annotations,

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so this will be whenever we have a variable where it's type cannot be reasonably inferred, even if

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the declaration and initialization are on the same line.

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This is really going to be a very big court case right here.

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Nonetheless, I just want you to understand when we want to use type annotations.

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So let's try to get through this example really quickly.

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I'm going to flip back over to my editor.

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And at the very bottom, once again, I'm going to add in a new section down here.

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And I'll say this is the third case.

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This is when we have a variable who is type cannot be inferred correctly.

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For this example, we're going to create a array of numbers called numbers, I'm going to give it values

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like negative ten, negative one and 12.

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I'll then declare a variable called number above zero.

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So here's the idea.

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I want to iterate through the numbers array.

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If we find a number that is greater than zero, I want to assign it to number above zero.

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Otherwise, I want to assign false to number above zero.

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So that means that we're going to have two different types of values being assigned to this variable.

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Now, this is some kind of bad code or writing right here.

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I really don't recommend taking this approach in general.

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However, there are scenarios where you might need to do something like this.

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Doing it with a number and a boolean doesn't make a lot of sense.

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But there might be other cases like maybe you're trying to find a user's favorite piece of media and

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maybe possible choices for media are like a blog post or an image or a book or a movie or something

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like that.

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So if we had something like, you know, users favorite media, it would possibly make sense to assign

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some different type of values to this variable.

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So this is kind of where the scenario is going to come up.

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All right, so we're going to say that by default, number above zero is going to be false.

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And then if we ever find something above zero inside this array, we need to assign that number to number

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above zero.

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So let's write this code out.

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Let's say four let equals zero, i.e. less than numbers that length.

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Plus, plus, and then if numbers that eye is greater than zero, we'll say no above zero is going to

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be that number.

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So let's give me numbers at I.

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As soon as we put that in, we're going to see an error message.

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And I definitely would expect to see this.

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So type inference already kicked in right here on this line of code we assign false to number above

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zero.

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So TypeScript very correctly thinks that this is supposed to be of type boolean.

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Then on this line of code right here, we're trying to assign something that is definitely a number

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to that variable.

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So the error is that we're trying to assign a number to a variable that is supposed to reference a boolean.

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So this is one of the downsides of type, inference, type inference kicked in here, it did its job,

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but it didn't really understand the intent of our code or what we're trying to do.

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So because type in France is marking this variable, as always, being a boolean, our code won't work

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as is.

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So to fix this, we could add in a type annotation so we could add in something that says Kullen.

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And then Boolean.

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Or no, like so this character right here is the pipe.

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It is not an L. It's not an eye is a character right above the return key on your keyboard.

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You can picture this as being like an understatement, so this is saying that number above zero is going

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to be either a boolean value or it might be a number one of the two.

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So any time that we need to express kind of like an overstatement with our types, we will have to use

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a type annotation type.

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Inference won't work correctly.

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So by using this type annotation, TypeScript now understands that we can assign a value of type boolean

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or a value of type number to its variable, and if we mouseover that variable, it'll be reflected right

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there as well.

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All right.

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Like I said, this is kind of a bad code example, but we could kind of easily imagine some other scenarios

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where we might have to express a single variable that has possibly different types.

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And we definitely are going to add some code that looks like this in a future project in this course.

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So we'll see this kind of scenario of where we have to add in a type annotation, but with a more practical

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approach.

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All right.

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So that's pretty much it.

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So we now have taken a look at the three different cases where we definitely will have to use a type

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annotation.

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So let's now take a quick pause.

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We're going to move on in the next video and talk about type annotations and inference around functions

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and objects as well.

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Where's that diagram?

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Right here, functions and objects.

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And we're going to get through these other approaches really quickly because we just had to do some,

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like, really original work in this variable section.

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