TypeScript type extension with methods

> And I want to add a method bar() to class A globally, how would I do that in TypeScript?

Assuming class A is yours, then just add bar() - there's no point to doing anything else because any .ts or .js file that could manipulate the A.prototype will already have to import it anyway.

export class A {
    a: string;
    b: number;

    foo() { }
    bar() { }

}

Whereas if class A is not an exported type, but is an old-school "global"-scoped class or Constructor-function, then you will still need to use A.prototype manipulation (probably using any in TypeScript) but you can still codify your changes with a declare declaration:

In (modern) TypeScript you'll probably only encounter declare if you peer inside .d.ts files from @types libraries, or the lib.dom.d.ts in your TypeScript install folder - but it's also used to allow TypeScript to work with uncooperative libraries...

You can also have multiple declare statements for the exact same type: that works because TypeScript will merge them together for you.

Module Augmentation:

Also, doing this for patching external module code is known as "module augmentation":

Uncooperative.js:

export class A {
    foo() { }
}

You.ts:

// First, import it:
import { A } from "./uncooperative";

// Second: extend the interface of `class A` with a declaration:
// Yes, you use an `interface` type to extend a `class` type.
declare module "./uncooperative" {
    
    interface A {
        bar(): void;
    }
}

// Third: you do the actual monkey-patching, with the `any` type to avoid TS errors that we don't care about here:
( A as any ).prototype.bar = function() {
    window.location = 'https://leekspin.com/';
};

Global Augmentation:

If you're working with a .js file that doesn't use Modules: good ol' fashioned global-scoped files, just like they were back in 1996, then you need to use the declare global statement:

Uncooperative.js:

function MacGuffin() {
    this.fieldA = 'a';
}
MacGuffin.prototype.runTurboEncabulator = function( arg1 ) {
    
};

You.ts:

// Nothing to import, just make your declarations:

declare global {
    interface MacGuffin {
        doSomethingElse(): void;
    }
}

// Then monkey-patch it:

MacGuffin.prototype.doSomethingElse = function() {
    window.location = 'https://www.youtube.com/watch?v=uKfKtXYLG78';
}