Can you divide and repurpose several function overloads in TypeScript?

I've put together this sandbox of code

In my project, there are 2 functions named slice in separate classes. They both contain a lengthy and repetitive set of overrides:

export class Atom<S>  {
  constructor(private initial: S) {
  }
  
  // Several more slice overrides go here...
  

If I have to modify these overrides, it will require changing the code in 2 different places.

Is there a way to reuse a single function across two different areas?

I attempted to create a generic slice function


export function slice<A, S, Key extends keyof A>(atom: A, key: Key): Slice<A[Key], S>;
export function slice<A, S, Key1 extends keyof A, Key2 extends keyof A[Key1]>(atom: A, key1: Key1, key2: Key2): Slice<A[Key1][Key2], S>;
// More slice function overrides...

And then calling it like this:

  slice<T>(...keys: Key[]): Slice<T, S> {
    // eslint-disable-next-line @typescript-eslint/no-explicit-any
    return slice(this, keys as any);
  }

However, the typing information is lost without the overloads specified on each function.

Answer №1

After experimenting with interface merging without success, I found a workaround by using arrow function properties instead of traditional methods. This approach allows you to define a function type with multiple overloads:

type complexSignature<S> = {
    <Key extends keyof S>(key: Key): Slice<S[Key], S>;
    <Key1 extends keyof S, Key2 extends keyof S[Key1]>(key1: Key1, key2: Key2): Slice<S[Key1][Key2], S>;
    // Additional overloads...
}
export class Atom<S> {
    constructor(private initial: S) {
    }
    slice: complexSignature<S> = (...keys: string[]): any => {
      return new Slice(this, keys);
    }
}

Although not perfect, this workaround is preferable to repeating the overload declarations.

Answer №2

Once, I came across a comment mentioning the use of TypeScript 4.0 beta for variadic tuples. I couldn't find the comment later, but it's a valid option: (playground link for the equivalent definition of NestedIndex with a less elegant approach.

type _N<T> = NonNullable<T>
// Additional code snippet provided for a different implementation of NestedIndex
// Testing section not included for clarity

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