Is it possible to deduce Typescript argument types for a specific implementation that has multiple overloaded signatures?

My call method has two signatures and a single implementation:

call<T extends CallChannel, TArgs extends CallParameters[T]>(channel: T, ...args: TArgs): ReturnType<CallListener<T>>;
call<T extends SharedChannel, TArgs extends SharedParameters[T]>(channel: T, ...args: TArgs): ReturnType<SharedListener<T>>;
call(channel, ...args) { ... }

The issue is that Typescript is not typechecking properly because the arguments for call are automatically typed as any. These arguments should actually be either of type CallChannel or SharedChannel for the first argument, and one of two specific types for the second argument.

The ideal implementation would look like this:

call<T extends CallChannel | SharedChannel, TArgs extends CallParameters[T] | SharedParameters[T]> (channel: T, ...args: TArgs) {

Unfortunately, this doesn't work because the type CallParameters[T] cannot resolve when T is a SharedChannel (and vice versa).

I have already provided all necessary type information to define call, so how can I satisfy Typescript with the implementation signature?

Answer №1

To achieve this, you can employ conditional types effectively.

Create a conditional type for the arguments and another one for the return value.

    type Args<T extends CallChannel | SharedChannel> = 
        T extends CallChannel 
        ? CallParameters[T] 
        : T extends SharedChannel 
        ? SharedParameters[T] 
        : never

Then, adjust the function to utilize it:

 TArgs extends Args<T>

The same principle can be applied to determine the return type.

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