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- So to get started talking
about the never type

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I'm actually gonna be using the exact code

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from the last video

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because it ties in perfectly

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with what the never type
is meant to be used for

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and the never type in TypeScript,

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it's just a type that means

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that it is never possible
for it to be anything.

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Essentially, the never type is saying

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that you should never be able

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to get to this point in the code

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and this variable should never even exist

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'cause it never has any type to it at all.

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The perfect example is a switch statement

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where you have essentially
three values here.

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It could be any number of values,

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but we have three values

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and we're accounting for
all those different values.

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So when I come into here

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and I try to create a default case

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and I try to counsel log,

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just like I've been doing before,

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what my priority is

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you'll notice here that the
priority has a type of never.

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And the reason for that

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is because we've checked
for high, normal and low

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which are the only three
options that our type can be.

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So we know if we've checked for all three

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of these different types,

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that it is impossible for
it to be anything else.

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So it's impossible for it

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to reach this default value section.

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This type may seem useless to you,

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other than the fact that,

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"Hey, I know that this
can never be anything",

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so when I try to do something on it,

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you'll notice I get no
auto complete, no nothing

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and I'm always gonna get an error

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because the never type cannot be used

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with anything, obviously.

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This is actually slightly more useful

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than you may think.

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One thing that you can
do with the never type

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is you can actually check to see

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if something is the never type

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to know if you are actually checking

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all your possible options.

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What we can do down here

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is we can just essentially
create a variable

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to determine if we're
exhaustively checking this.

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It doesn't matter what you call this

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we'll call this exhaustive check,

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just like that.

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And we're gonna say that
the type of this is never

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and then we're gonna set that equal

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to our todo.priority.

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Essentially, we're just gonna set this

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to whatever the thing up here is

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that we're switching over.

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And then we can do whatever we want.

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It really doesn't matter.

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We can just return it or do whatever.

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It should never get to this point.

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So it doesn't really matter.

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And let me just make sure
I spell this correctly,

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so it's a little bit better.

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There we go.

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So it should never ever get to this point.

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And right now all of
our code is working fine

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but what happens if we
add a brand new priority?

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Let's come in here and we'll
say like super low priority.

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Now when I save and come down here

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you're gonna notice I get an error

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and it's essentially saying the type

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of string is not assignable
to the type of never.

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And that's 'cause this priority here

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could still be super low.

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We've checked for high, normal and low

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but we haven't checked for super low.

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This is gonna throw a
TypeScript error for me

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letting me know,

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"Hey, you haven't checked
for this thing yet."

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This is one use case where the
never type is really useful.

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And what I can do is add
in my case for super low,

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break out of it, do whatever we want.

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And now you can see that
this is working again.

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So if you want to create
a switch statement

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that requires you to check for
every single possible option

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adding in some simple code

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like this where you just have a variable

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it doesn't matter what the name is,

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I'll just change it to A for simplicity,

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that you manually set to the type of never

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and you assign it to whatever

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the thing inside of
your switch statement is

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what you're checking.

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That's gonna make sure you always check

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for every single possibility,

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which is really useful
for union types like this.