Module src/PrimeFaces

class MyWidget extends PrimeFaces.widget.BaseWidget {
  init(cfg){
    // ...
  }
}

Note to TypeScript users: You will need to specify the type parameters explicitly. The best way to do so is by defining the interfaces for the classes separately:

interface BaseWidgetCfg {
 prop1: string;
}
interface AccordionPanelCfg extends BaseWidgetCfg {
  prop2: boolean;
}
interface BaseWidget {
  init(cfg: BaseWidgetCfg): void;
  method1(x: number): boolean;
}
interface Accordion extends BaseWidget {
  init(cfg: AccordionPanelCfg): void;
  method1(): boolean;
  method2(): boolean;
}

Now you can use it normally:

const BaseWidget = Class.extend<BaseWidget, [BaseWidgetCfg]>({
  init(cfg: BaseWidgetCfg) {
    // ...
  },
  method1(x: number): boolean {
    return x > 0;
  },
});
const Accordion = BaseWidget.extend<Accordion, [AccordionCfg]>({
  init(cfg: AccordionCfg) {
    this._super(cfg);
  },
  method1() {
    return !this._super(42);
  },
  method2() {
    return true;
  }
});
const base: BaseWidget = new BaseWidget({prop1: ""});
const accordion: Accordion = new Accordion({prop1: "", prop2: true});
base.method1(2);
accordion.method1();
accordion.method2();