How to extend an ES6 class in a TypeScript class

I have a JavaScript class written in ES6:

// MyClass.js
export default class MyClass {
  myfunction() {
    console.log('myfunction called');
  }
}

I'd like to extend this class with a TypeScript class:

// MyTSClass.ts
import MyClass from './MyClass';

class MyTSClass extends MyClass {
  public initialize() {
    this.myfunction();
  }
}

This produces a TypeScript compiler error:

TS2339: Property 'myfunction' does not exist on type 'MyTSClass'

I have esModuleInterop set to true in my tsconfig.

I attempted to make a declaration file, thinking this might help:

// MyClass.d.ts
export = MyClass;

declare class MyClass {
  myfunction: void;
}

This makes the compiler happy, but in the generated code in the browser I see something unexpected, and the console log does not print:

class MyTSClass extends WEBPACK_IMPORTED_MODULE_1__["MyClass"]...

What am I doing wrong?