Having trouble making optional parameters overload in Typescript?

Question

Having trouble making optional parameters overload in Typescript?

Let's consider a simplified instance (although overloading may not be necessary in this case due to the simplification)

UPDATE: The initial example provided did not fully illustrate the issue, so here is an improved version:

function fn <T>( // Overload signature is not compatible with function implementation.ts(2394)
  fn: (item: T) => T,
): (idx: number) => (src: T[]) => T[]
function fn <T>(
  fn: (item: T) => T,
  idx: number,
): (src: T[]) => T[]
function fn(fn: (x: any) => any, idx?: number) {

}

How should the Return type be qualified in the implementation in this scenario?

I'm encountering a ts(2394) error on the first definition and I'm unsure of what mistake I'm making.

Appreciate any help, Seb

typescript overloading

Answer 1

Answer №1

If you want to resolve the issue at hand, it's crucial to have a return type specified within the actual function logic, not just in the definitions.

Considering that each definition overload may produce a different outcome, the logic itself must be equipped to handle this variability. Taking the easy route would involve simply returning any, but a more effective approach would be to return all definitions separated by a pipe symbol |. In the event that the return types become overly lengthy, it's advisable to create a declare type definition and declare each one individually.

declare type A<T> = (idx: number) => (src: T[]) => T[]
declare type B<T> =  (src: T[]) => T[]

function fn<T>(fn: (item: T) => T): A<T>
function fn<T>(fn: (item: T) => T, idx: number): B<T>
function fn<T>(fn: (x: any) => any, idx?: number): A<T> | B<T> {

}

Answer 2

If you want to resolve the issue at hand, it's crucial to have a return type specified within the actual function logic, not just in the definitions.

Considering that each definition overload may produce a different outcome, the logic itself must be equipped to handle this variability. Taking the easy route would involve simply returning any, but a more effective approach would be to return all definitions separated by a pipe symbol |. In the event that the return types become overly lengthy, it's advisable to create a declare type definition and declare each one individually.

declare type A<T> = (idx: number) => (src: T[]) => T[]
declare type B<T> =  (src: T[]) => T[]

function fn<T>(fn: (item: T) => T): A<T>
function fn<T>(fn: (item: T) => T, idx: number): B<T>
function fn<T>(fn: (x: any) => any, idx?: number): A<T> | B<T> {

}

Having trouble making optional parameters overload in Typescript?

Answer №1

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