TypeScript module augmentation across multiple modules

Question

I'm attempting to modify the prototype of an object inside another module (similar to as demonstrated here). However, module augmentation seems to only work when augmenting an object declared in a required file in the same module rather than another module.

For example, I have a class, TestClass:

// some-module/TestClass.ts

export class TestClass {
    someValue: string = "hi";
    someFunc(): number {
        return 5;
    }
}

And in the same module, I have this:

// some-module/index.ts

import { TestClass } from "./TestClass";

declare module "./TestClass" {
    interface TestClass {
        doSomethingElse(): void;
    }
}

TestClass.prototype.doSomethingElse = function(): void {
    console.log("Something else");
};

This works fine. However, if I move TestClass to another module (test-module/TestClass.ts) and modify the code appropriately like this, it gives me the error 'TestClass' only refers to a type, but being used as a value here. whenever I try and access TestClass.

// some-module/index.ts

import { TestClass } from "test-project";

declare module "test-project" {
    interface TestClass {
        doSomethingElse(): void;
    }
}

TestClass.prototype.doSomethingElse = function(): void {
    console.log("Something else");
};

I'm using NodeJS module resolution with CommonJS.

Any help would be much appreciated.

Answer 1

It's because you need to import the whole module with import * instead of just the class you need, see https://github.com/Microsoft/TypeScript/issues/9015