JQuery plugin typescript definition mixin

I am writing a JQuery plugin in TypeScript.

I have an implementation of a class Foo.ts

//Foo.ts
export class Foo {}

I have installed @types/jquery and the completions for that work fine.

Now I would like to declare a function onto the JQuery type, which I can do with a types.d.ts file like so:

//types.d.ts
interface JQuery {
     bar(): string;
}

This works to allow me to do

//Baz.ts
let result: string = $('#Baz').bar();

However, the return type of bar() is not string but should be Foo.

//types.d.ts
//import {Foo} from './Foo';

interface JQuery {
     bar(): Foo;
}

Gives me a red line under Foo [ts] Cannot find name 'Foo'. If I uncomment the import line, types.d.ts is happy, but then I then get an error in

//Baz.ts
let result: string = $('#Baz').bar();
// [ts] Property 'bar' does not exist on type 'JQuery<HTMLElement>'.

It seems the definitions for JQuery are no longer merging. I understand that a types declaration file is not global if it has imports or exports, but then I don't know how to reference types in other files.

How can I declare an additional function on the JQuery type whos definition references other types?
Additionally, can this work so that when I import this module into another project the types for JQuery are merged correctly?

It certainly seems to me like this should be possible.

If you are using a module system then then you must place the extension of JQuery in the global namespace:

import {Foo} from './Foo';

declare global {
    interface JQuery {
        bar(): Foo;
    }
}

If you are not using a module system then you don't need to use export on the Foo class and is should work.