The power of Typescript in utilizing generics and overloading functions

Currently, I am in the process of implementing some functions in Typescript that I plan to overload, and they involve the usage of generics. However, I am encountering a confusing issue that has left me puzzled:

*Please disregard the irrelevant code snippets that have been excluded, as they do not contribute to the problem at hand.

The following code snippet works:

export function persistContactData<T>(
  contactData: T,
  callback?: () => void
): void;
export function persistContactData<T>(
  { contactData }: { contactData: T },
  callback?: () => void
): void;
export function persistContactData<T>(
  { contactData }: { contactData: T },
  callback?: () => void
) {
  // implementation
}

However, the following snippet encounters an error on the first function:


// *** Compile error: "This overload signature is not compatible with its implementation"
//
export function getPersistedContactData<T>(
  contactId: string,
  callback: (result?: T) => void
): void;

// There are no more errors when I remove the above signature.
export function getPersistedContactData<T>(
  { contactId }: { contactId: string },
  callback: (result?: T) => void
): void;
export function getPersistedContactData<T>(
  { contactId }: { contactId: string },
  callback: (result?: T) => void
) {
  // implementation
}

I am struggling to pinpoint the exact issue here, but I suspect it has something to do with how the generic parameter is utilized. However, in theory, this should not be causing any problems.

(The Typescript version I am using is 3.9.3)

Edit: I made updates to remove any unknown types

Answer №1

The problem arises due to the use of a generic parameter, which causes the compiler to assume that the shape could potentially define the expected type. However, the actual implementation will define the actual parameter shapes.

This distinction becomes clear when generics are replaced with actual types.

An Alternate Approach to Consider:

export function persistContactData<T>(
  contactData: T,  
  callback?: () => void
): void;
export function persistContactData<T>(
  { contactData }: { contactData: T },
  callback?: () => void
): void;
export function persistContactData<T>(
  { contactData }: { contactData: T },
  callback?: () => void
) {
  // implementation
}

A Less Ideal Situation:

// Error
export function persistContactData(
  contactData: string,  
  callback?: () => void
): void;
export function persistContactData(
  { contactData }: { contactData: string },
  callback?: () => void
): void;
export function persistContactData(
  { contactData }: { contactData: string },
  callback?: () => void
) {
  // implementation
}

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