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- This next built-in
type is really simple.

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It's very similar to required and partial

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in that it just takes
an entire object or type

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and it just manipulates the entire type

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to be one specific thing.

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Imagine we wanted to
have like a final to do

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and this to do is one
that cannot be changed.

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It's the final to do.

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It's like saved as is.

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Essentially, it's read only.

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Well, we can use a built-in
read only type to do that.

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So we can say read only
and pass it in our type,

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which is to do.

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And now, when we hover over here,

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you can see it marks every
single property as read only.

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This is very similar
to how as const works,

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but as const can only be
used on JavaScript code

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and it cannot be used on TypeScript code.

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So I could say here that I have a to do,

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which is going to be of type to do

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I'm gonna set that equal to
a to to do here real quick.

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And our completed is false.

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And now, if I hover over this,

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you can see it has that type of to do.

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Well, what I could do is
I could set that as const,

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and now essentially,
that's going to give me

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a read only version of my to do.

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I can just remove this
typing here from it.

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And you can see, we get a read
only version of this to do

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by using this as const property.

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It's the exact same thing as what happens

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when I use read only on the type itself.

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So if you wanna get the type of to do,

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I can even say here, type
test equals type of to do,

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you can see that this test type right here

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is exactly the same as this final to do.

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They both do the exact same thing,

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as const is just what you use
when you have JavaScript code

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and read only is what you have

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when you want to create a type

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from another type that you
want to make read only.

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I'm gonna be honest,

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this isn't one that I find
myself using too terribly often,

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but for example, there's
a method in objects

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called object.freeze,

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and essentially, this
makes an object immutable,

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so you can't change anything inside of it.

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And this essentially just returns to you

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a read only version of your type,

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so it's using that read only type

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to essentially define this
if we can look at this.

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And if I scroll down a little ways

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to this freeze method here,

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here's a really simple version of it.

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You can see right here,

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it just returns a read only
version of your object,

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so this is a really good use case

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for where that would be something
that you would want to do.

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So again, it's not something
you use all the time,

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but if you want to convert one
type to a read only version

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of that same exact type,

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the read only built-in type is perfect.