Extending @types - delete field from interface, add type to field in interface

Question

I have javascript library with types from npm/@types.

I need to make two fixes to @types which applies only in case of my application, so I can't merge them into DefinitelyTyped repository.

I need to:

1. remove one of fields from interface. Example:

   // before changes:
   interface A {
           a?:string;
           b?:string;
           c?:string;
   }
   
   // after changes:
   interface A {
           a?:string;
           c?:string;
   }
   

2. add more types to one field in interface. Example:

   // before changes:
   interface B {
           a?: C;
   }
   
   // after changes:
   interface B {
           a?: C | D;
   }
   

Also I still want to download main @types definitions from external repository.

What is the best way to achieve this?

Answer 1

This can be solved using the following method.

import { A as AContract, B as BContract, C, D } from './contracts.ts';

// Removes 'b' property from A interface.
interface A extends Omit<AContract, 'b'> { }

interface B extends BContract {
  a?: C | D;
}

Answer 2

You cannot override type declarations of existing properties of interfaces in TypeScript but you could do this by extending the type interfaces since you can override property types:

interface afterA extends A {
  b?: never;
}

interface afterB extends B {
  a?: C | D;
}