Mixin Composition via Abstract Constructor Signature Typing

type AbstractConstructor<T = {}> = abstract new (...args: any[]) => T;
 
function WithTimestamp<TBase extends AbstractConstructor>(Base: TBase) {
  abstract class Timestamped extends Base {
    readonly createdAt: Date = new Date();
    abstract getMetadata(): Record<string, unknown>;
  }
  return Timestamped;
}
 
abstract class BaseEntity {
  abstract readonly id: string;
}
 
class UserEntity extends WithTimestamp(BaseEntity) {
  constructor(public readonly id: string, public readonly name: string) {
    super();
  }
  getMetadata() {
    return { id: this.id, name: this.name };
  }
}