When to use TypeScript's `never` type

https://twitter.com/kentcdodds/status/1381453907730128900?s=20

Hey friends So last night I tweeted this TypeScript question. When have you

used never in a production codebase links to examples welcome? And I failed

to mute the conversation and so I ended up waking up the next morning with

tons of notifications, which was great. So many people were happy to share

and I just wanted to share with you a couple or the like basic themes of

the different examples that people shared.

So the first is discriminating union. Oh and by the way, if you've never

heard of never before the basic idea is it's a way for you to say.Hey the

type script type checker as it's flowing through the types of my

application, it should never be able to hit this type.

So it's like those situations where you like this should never happen.

That's basically when you use never. So anyway, the first is discriminating

unions and a common example for this was if you had a component that could

take a variety of different props. So an example is a component that could

like draw a shape so you could be rectangle or a square or a circle and

depending on the type of shape you want to draw you're going.

To have different props And so square will have a width Rectangle will also

have a height in addition to that. And then circle won't have either of

those it'll have like a diameter something. So the you know, the

discriminating union's use case for never would be when you union those

props together you want to be able to say hey when it's a circle I never

want to have a height or a width and when it it's a square.

I never want to have a height because I just get it from the width and I

never want to have a diameter because it's not a circle. In my

mind,Actually most of the examples were similar to that and in my mind most

of the time you're actually probably better off just making two separate

components or yeah different components entirely that are responsible for

doing that specific thing.

However, sometimes there are definitely use cases for you know, if if

you're going to provide me this prop, you also must provide me this prop,

but if you provide this other one, then you must not provide this this

other. So like aria described by versus already described stuff like that.

So that was like the primary use case in addition.Switch out ifs and else

statements this is where you can make a function that is basically they

would call it a certain ever and it would return never it would throw an

error. And basically your time typescript well by calling that function you

get a type error.

And so if you put this as the default case or something and you're not

handling every case then type script is going to cause a problem for you.

And then the last one was conditional types and that one still pretty hazy

for me. I can't really explain it. So anyway, hopefully some of that was

interesting.

And I hope you have a wonderful day. Bye.