Set-once static and instance value in subclasses / Requiring a static member / Specifying a static member in an interface

widgets.ts

abstract class Widget {
    abstract readonly widgetType: string;

    public display: { [key: string]: string; };

    constructor(data?: any) {
        this.display = {
            a: 'a' in data ? data.a : 'a-parentClass',
            b: 'b' in data ? data.b : 'b-parentClass',
            c: 'c' in data ? data.c : 'c-parentClass'
        };
    }
}

interface IWidget {
    constructor: Function & {
        readonly widgetType?: string;
    }
}

function WidgetSubClass(widgetType: string) {
    return class extends Widget implements IWidget {
        readonly widgetType: string = widgetType;
        static readonly widgetType: string = widgetType;

        constructor(data?: any) {
            super(data);

            Object.assign(this.display, {
                c: 'c' in data ? data.c : 'c-subClass'
            });
        }
    }
}

class SubClass extends WidgetSubClass('test-widgetType') {
}

const sc = new SubClass({a: 'a-instance'})

console.clear()
console.log(sc.display)
console.log(sc.widgetType)
console.log((sc as IWidget).constructor.widgetType)