Typescript callback union parameter overload error "This overload signature is not compatible with its implementation signature."

Question

When I try to overload this function with different callback params i run into an error This overload signature is not compatible with its implementation signature.ts(2394) database.ts(41, 3): The implementation signature is declared here.

listen(onData: (data: number) => void): void
listen(onData: (data: number[]) => void): void
listen(onData: (data: number | number[]) => void): void {
  ...
}

According to this q Typescript: specifying multiple callback types in a union it appears that you can do it, so I'm not sure where I'm going wrong.

Edit: The actual implementation of the function:

  listen(
    ref:
      | firebase.firestore.DocumentReference
      | firebase.firestore.CollectionReference,
    onData: (data: ReadData<T> | ReadData<T>[]) => void
  ): () => void {
    return ref instanceof firebase.firestore.DocumentReference
      ? ref.onSnapshot((doc) => onData(this.txDocData(doc)))
      : ref.onSnapshot((snap) => onData(snap.docs.map(this.txDocData)))
  }

Answer 1

The callbacks are different types. They are not compatible. They just happen to be functions.

function listen(onData: (data: number) => void): void
function listen(onData: (data: number[]) => void): void
function listen(onData: ((data: number) => void) | ((data: number[]) => void)): void {
    onData([42]);   //Argument of type 'number[]' is not assignable to parameter of type 'number & number[]'.
                    //    Type 'number[]' is not assignable to type 'number'.(2345)
    
    onData(42);     //Argument of type 'number' is not assignable to parameter of type 'number & number[]'.
                    //    Type 'number' is not assignable to type 'number[]'.(2345)
}

Try to think the other way around: If I call listen() with a cb that expects to be called with a number[] it must not be called with a number by the implementation. I declared that cb can only be called with a number[] . The same goes the other way.

The only solution for the compiler is to enforce that onData is called with a parameter that is a number and a number[] at the same time - hence the type number & number[]