conditional function arguments in typescript

Question

Is it possible to have a conidial required argument type based on the first argument type:

type ConditionalRequiredArg<T, P> = T extends string ? P | undefined : P;

function func<T, P>(_arg1: string | number, _arg2: ConditionalRequiredArg<T, P>) {}

func('foo'); // Error: Expected 2 arguments, but got 1.

In theory, the above function should not require a second argument, but it does!

Edit: I am aware of '?' for optional argument types, but I would like to make it conditional to leverage the compiler error and suggestions.

Answer 1

You can always use the signature overloading to screen users from directly using the implementation signature:

function func<P>(_arg1: string, _arg2?: P)
function func<P>(_arg1: number, _arg2: P)
function func<P>(_arg1: string | number, _arg2?: P) { }

This code forces user to choose either the first signature or the second to use, and not the third signature of the implementation.

func('foo', 1); // success
func('foo'); // success
func(1, 1); // success
func(1); // Error: Argument of type 'number' is not assignable to parameter of type 'string'

Answer 2

I think you are mixing up two different things.

func<T, P>(_arg1: string | number, _arg2: ConditionalRequiredArg<T, P>)

means that the function expects 2 arguments, where the second one might have the type undefined, meaning:

_arg2 = undefined, where the field _arg2 is already declared.

But you want the field to be not declared.

_arg2 = undefined is not equivalent of saying:

func<T, P>(_arg1: string | number, undefined)

For this use case, you can use ? as follows:

func<T, P>(_arg1: string | number, _arg2?: ConditionalRequiredArg<T, P>)

Edit:

Types and arguments are two different things. The type defines the type of the value, whilst an argument is a field.

Although it is not a good practice, you could also initialize the field manually with undefined as follows:

func<T, P>(_arg1: string | number, _arg2: string = undefined)

But this is in the end same as _arg2?

The type undefined is like saying "The field does not contain anything" or better say, "The field contains undefined", whilst the field undefined is "The field does not exist"