Interface Class<TBase>

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

Note for TypeScript users: Normally you should just write a widget as a class that extends from the appropriate base class. If you must use this method 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 AccordionCfg extends BaseWidgetCfg {
  prop2: boolean;
}
interface BaseWidget {
  init(cfg: BaseWidgetCfg): void;
  method1(x: number): boolean;
}
interface Accordion extends BaseWidget {
  init(cfg: AccordionCfg): 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();