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:

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;
}

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?

Abhishek Prakash · Accepted Answer · 2020-03-02 06:21:34Z

21

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;
}

answered Mar 2, 2020 at 6:21

Abhishek Prakash

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1 Comment

im_tsm Over a year ago

TypeScript doc link: typescriptlang.org/docs/handbook/…

Explosion Pills · Accepted Answer · 2018-01-12 18:02:35Z

12

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;
}

edited Jan 12, 2018 at 18:02

answered Jan 12, 2018 at 17:49

Explosion Pills

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3 Comments

artem Over a year ago

You actually can modify (or augment) type declaration in TypeScript, it's called declaration merging, but how to do that depends on how exactly library types are defined and exported, see ths question for one example

Explosion Pills Over a year ago

@artem thanks! This is really good to know. Unfortunately it doesn't look like declaration merging will work in this case: "Non-function members of the interfaces should be unique. If they are not unique, they must be of the same type."

artem Over a year ago

Ah yes thanks, I did not even notice that restriction, because I always use it only to add properties.

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