[SOLVED] Function overload with varying return values not accepted

Issue

I do not understand why the following implementation is not accepted. I hope you can help.

type F = {
    (a: number): string
    (a: string, b: number): number
}
const func: F = (numberOrString: string | number, b?: number): string | number => {
    if (typeof numberOrString === 'number')
        return numberOrString.toString();
    else if (b !== undefined)
        return numberOrString.length + b;
    throw new Error("Can't happen");
};

The error message is:

Type '(numberOrString: string | number, b?: number | undefined) => string | number' is not assignable to type 'F'.
  Type 'string | number' is not assignable to type 'string'.
    Type 'number' is not assignable to type 'string'.ts(2322)

Solution

It seems like it should be, right? But as it says in the function overloads section

The implementation signature must also be compatible with the overload signatures.

(Their emphasis)

A return type of string | number isn’t compatible with a return type of string (your first overload) or number (your second).

In practice, this often means that your implementation signature — particularly the return type — is much broader than you would otherwise write. That’s okay, because the code using the function doesn’t see the implementation signature, just the overload signatures. While you can use unions for the parameter types (as you have), you can’t use them for the return type because it won’t be compatible.

In your example, we can solve the problem by changing the return type to any (as you probably already know):

type F = {
    (a: number): string;
    (a: string, b: number): number;
};
const func: F = (numberOrString: string | number, b?: number): any => {
    if (typeof numberOrString === 'number')
        return numberOrString.toString();
    else if (b !== undefined)
        return numberOrString.length + b;
    throw new Error("Can't happen");
};

Playground link

Normally any is something to avoid, but in a function overload implementation signature, it’s okay and sometimes necessary.

Answered By – T.J. Crowder

Answer Checked By – Mildred Charles (BugsFixing Admin)

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