TypeScript: How to write definition when I extend a module?

I extend Chai with a helper in my TypeScript test.

import * as chai from 'chai';

chai.use((_chai) => {
  let Assertion = _chai.Assertion;
  Assertion.addMethod('sortedBy', function(property) {
    // ...
  });
});

const expect = chai.expect;

In the same file test case makes use of this method:

expect(tasks).to.have.been.sortedBy('from');

Compiler gives error that "Property 'sortedBy' does not exist on type 'Assertion'".

How can I add declaration of sortedBy to Chai.Assertion?

I've tried to add module declaration, just like other Chai plugin modules do, but it doesn't work.

declare module Chai {
  interface Assertion {
    sortedBy(property: string): void;
  }
}

I don't want to make the helper an individual module, because it's trivial.

Try the following:

Extend chai in the chaiExt.ts like this:

declare module Chai 
{
    export interface Assertion 
    {
        sortedBy(property: string): void;
    }
}

Consume in chaiConsumer.ts:

import * as chai from 'chai';
//...
chai.expect(tasks).to.have.been.sortedBy('from');

[EDIT]

If you are using 'import' - you turn your file into the external module and declaration merging is not supported: link