Singleton service decorator

singleton-decorator.ts

import { ɵgetInjectableDef } from "@angular/core";

export function Harden(target: any) {
  let def = ɵgetInjectableDef(target) as any;
  const realFactory = def.factory;
  let value = undefined as any | undefined;
  def.factory = () => { value = value == undefined ? realFactory() : value; return value; };
}