Thread by @TitianCernicova

As a follow on to yesterday’s post (inspired by @dagda1's question), what is the difference between using any[], unknown[] or never[] as constraints to the function rest parameter in #TypeScript ?

1/6🧵

https://twitter.com/TitianCernicova/status/1501237698991898624?s=20&t=Wxgfs-ir1lftwseHGHeASQ

The answer is that if you use a type parameter for the args, there isn’t much difference. You can’t call the function with anything that isn’t typed as A, since you can’t assign a concrete type to a type parameter.
2/6🧵
typescriptlang.org/play?#code/GYV…

If the rest parameter is in a function type constraint (the way I recommend you don’t do in the previous post), then we get differences.
3/6🧵

F extends (...a: unknown[]) => void is a function that should be able to handle any arguments, so we can call it with anything. This means if we try to pass in, for example, a function taking a string we get an error.

F extends (...a: never[]) => void is a function we can’t call, since nothing will ever be compatible with never. But since we can’t call it, it is safe to pass in any function

F extends (...a: any[]) => void is the more unsafe version (any is unsafe, what a surprise!). We can both pass in any function AND call the function with any arguments, which is probably not a good idea.

typescriptlang.org/play?#code/FAM…