Typesafely extending Object prototype in TS

December 6, 2024


const via = Symbol('via')

declare global {

  interface Object {
    [via]<R>(cont:(self:this)=>R):R
  }
}

const test = [1,2,3][via](x=>x) // test is inferred as Object, it should be inferred as number[]

Is there a way to type annotate this correctly?

>Solution :

Use a generic parameter in the interface that TypeScript correctly infers the type of the caller and applies it to the function argument and return type:

const via = Symbol('via');

declare global {
  interface Object {
    [via]<R, T = this>(this: T, cont: (self: T) => R): R;
  }
}

Object.prototype[via] = function<R, T = this>(this: T, cont: (self: T) => R): R {
  return cont(this);
};

const test = [1, 2, 3][via](x => x);