Adding methods to external class via module augmentation : error

Question

This seems so trivial, yet I fail to make it work. I have a class Collection in an external npm dependency (say packagename). In my current project, I would like to add some methods to the prototype of that class, and informing TS of that fact.

In the external module, the class is defined like :

export class Collection<T extends Document> {
  // ...
}

So here is what I wrote in an external.d.ts file in my project :

import { Document } from "packagename";

declare module "packagename" {
  export interface Collection<T extends Document> {
    newMethod(): Promise<T | null>;
  }
}

But then in another file :

import { Collection } from "packagename"

Collection.prototype.newMethod= function<T extends Document>(this: Collection<T>) {
  // ...
};

fails with the following error :

TS2693: 'Collection' only refers to a type, but is being used as a value here.

Same goes if I import Collection with import * as everything from 'packagename' then everything.Collection.

What am I missing here ? Thanks.

strblr · Accepted Answer · 2021-04-13 22:30:44Z

2

So it seems that it doesn't work because imports from packagename are re-exported imports in a index.ts file that only aggregates other files. Targeting the exact source file of the class I wanted to augment seems to work :

import { Document } from "packagename";

declare module "packagename/dist/collection" {
  export interface Collection<T extends Document> {
    newMethod(): Promise<T | null>;
  }
}

This comment gave me the hint.

answered Apr 13, 2021 at 22:30

strblr

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Comments

Vasiliy vvscode Vanchuk · Accepted Answer · 2021-04-13 20:38:07Z

0

Typescript allows you to extend the interface (what you did), but the interface is just a type (no implementation as you see).
I do not understand you need to extend the class in that way. Why don't you just create a child class based on the existing one? It seems like a right way to extend class instead of monkey patching via prototype extending

import { Collection } from "packagename"

export class ExtendedCollection extends Collection {
  newMethod() {

  }
}

and after that in places, you need just import and use that new class

do you have strong reasons to not do it in such a way?

answered Apr 13, 2021 at 20:38

Vasiliy vvscode Vanchuk

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1 Comment

strblr Over a year ago

Because then I wouldn't be able to use newMethod for example on a Collection returned by the external package (which is a type that I don't control). Module augmentation is supposed to work around that. Example : stackoverflow.com/a/61485387/11153160

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