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- The next two built-in types

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of return type and parameters
both deal with functions.

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So to get started we just
have a really simple function

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that is going to showcase
all these different things.

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It's called check length,

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takes in an A property

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which is a string and a B property,

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which is a number.

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And if the length of the
string is less than the number

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we pass in it's going to
return true, otherwise false.

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And I'm just doing this

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so we have a bunch of different
types we can work with.

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We have strings, we have
numbers, and we have Booleans.

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So if we check the signature of this

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you can see the return type for it

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is set to a Boolean right here.

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Now, if we wanted to use the return type

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of this function inside
of our type, for example

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I wanted to get a type which
is return of length check.

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Well, how exactly would
I go about getting this?

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I don't have any way right now to do that.

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But with the return type function, I do.

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Return type takes in a type

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and this type must be a function

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and it'll give you whatever
that function returns

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which in our case is a Boolean.

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So you may think we just
pass in check length.

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That's not gonna work

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because check length is a
function and this expects a type.

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So we need to get the type
of our check length function.

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And now if we cover over this

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you can see the return of
it is set to a Boolean.

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The return type is really good

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if we just want to get
whatever a function returns.

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If you have a function declared like this

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as like JavaScript code

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you need to make sure
you get the type of it.

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Otherwise, if you have a type
that is an actual function

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so we'll say type func equals

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and this is just going
to return to us void.

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It's just a really simple function.

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I could just pass that in right there

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because it's just expecting a
type. And if I hover over here

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you can see it gives me
void as that return type.

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This is really useful

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if you need to make
sure you take the value

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of a return type and use it somewhere else

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in your code as like a
parameter or another type.

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Now another slightly less useful,

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but still useful built-in
type is the parameter type.

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Let's say I wanted to figure out

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what the type of the parameter
A or B is in this function.

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But what I can do is I can
use the parameters type

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and this parameters type
takes in our function.

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So we'll use check length in our case,

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we'll type of check length just like that.

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And now this is gonna be our params type.

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And if we hover over this

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you can see that it gives us an array.

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And the very first thing
in here is a string.

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And the second thing is a number.

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So it's like a two pull essentially

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because it's saying we
accept two parameters,

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the first one is a parameter
A, which is a string.

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The second one is a parameter
B, which is a number.

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So if I wanted the very first parameter

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I could do this and this
should return to me a string.

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As you can see, this is set to a string.

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While if I wanted the second
parameter, I'm over here

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you can see now this is set to a number.

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That's a really good way to
essentially get a type based

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on the parameters of a specific function.

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And if the function has
no parameters, for example

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this func right here has no parameters,

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you can see when I try to look at this

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it's just going to be an empty array

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'cause it's saying,

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"Hey, there are no parameters
at all for this function."

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Now this is definitely less useful,

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but sometimes if you're
working in a library

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where you need to pass
along things to a function

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but they don't have good
types being exposed,

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you may need to use this parameters type

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to figure out what the actual
type of that parameter is

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so you can set that type
inside of your code.

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But like I said, parameters
is much less useful.

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But return type is something

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you're going to be using all the time

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because it's very common to type something

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based on the return of another function.

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And since everything in
TypeScript is inferred by default

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this Boolean is already inferred.

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I don't need to manually
set that anywhere.

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So it's really good to be
able to continue to infer it

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by getting the return type
of that specific function

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without having to manually
say, "Okay, you know what?

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This here is a Boolean,

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and then down here, this is
also going to be a Boolean."

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I don't have to worry about that

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if I have all these
different built-in types

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that could just constantly
build on the inferring,

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which means I write less TypeScript,

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but you still get all
of the same benefits.