There's a subtle difference between TS' question mark and union. The question mark means "this argument is optional", which can be different than "this argument can be undefined".

The following code is valid:

  function foo1(value?: number) {}
  
  foo1()

But the following code will raise a type error:

  function foo2(value: number | undefined) {}
  
  foo2()

In practice, that rarely becomes problematic. But it's good to know the difference

I really think the second one shouldn't raise a type error.

This is already invalid:

  function foo1(arg1?: number, arg2: number) {}

In that case you have to use

  function foo1(arg1: number | undefined, arg2: number) {}

  foo1(undefined,1);

But other than that what is the use case?

The second one raises an error:

https://www.typescriptlang.org/play?#code/AQMwrgdgxgLglgewqB...

This conflation of "optional" and "undefined-able" is more obvious in interfaces. This is why TypeScript added the `exactOptionalPropertyTypes` option:

https://www.typescriptlang.org/tsconfig#exactOptionalPropert...

Sometimes "is not defined" needs to be treated differently than "is set to undefined"