How can I add extension method to existing class

Question

I want to add method extension to existing class, but I don't know how. I tried this:

But I always receive Property 'addAssembler' doesnt exist on type 'Container' error.

Answer 1

You can just just expant class members by defining interface with same name and adding new methods to it:

foo2.ts:

class Foo {
    spam?: string;
    bar() {
        console.log('Foo#bar');
    }
}

app.ts:

import { Foo } from './foo2';

declare module './foo2' {
    interface Foo {
        foo(): void;
    }
}

Foo.prototype.foo = function(this: Foo) {
    console.log('Foo#foo', this.spam);
}

const f = new Foo();
f.spam = 'eggs';
f.bar();
f.foo();

Answer 2

You need to declare a module augmentation:

declare module 'inversify' {
    export interface Container {
        addAssembler(): void
    }
}

You can add the declaration above in your ts file where you add the method to the Container prototype.

Edit

Unfortunately the way Container is defined prevents augmentation. We can merge classes from modules with interfaces, but they have to be defined as export class Container {}. In this case container is exported as class Container {} export { Container }. This prevents augmentation unfortunately.