deepclone.ts

export class cloneable {
  public static deepCopy<T>(source: T): T {
    return Array.isArray(source)
    ? source.map(item => this.deepCopy(item))
    : source instanceof Date
    ? new Date(source.getTime())
    : source && typeof source === 'object'
          ? Object.getOwnPropertyNames(source).reduce((o, prop) => {
             Object.defineProperty(o, prop, Object.getOwnPropertyDescriptor(source, prop)!);
             o[prop] = this.deepCopy((source as { [key: string]: any })[prop]);
             return o;
          }, Object.create(Object.getPrototypeOf(source)))
    : source as T;
  }
}

I got some TS errors (due to the version, i guess), which can be fixed like that
- TS2345 on Object.defineProperty(o, prop, Object.getOwnPropertyDescriptor(source, prop))
==> Object.defineProperty(o, prop, Object.getOwnPropertyDescriptor(source, prop)!)
- TS7053 on o[prop] = this.deepCopy(source[prop])
==> o[prop] = this.deepCopy((source as { [key: string]: any })[prop])

I got some TS errors (due to the version, i guess), which can be fixed like that - TS2345 on Object.defineProperty(o, prop, Object.getOwnPropertyDescriptor(source, prop)) ==> Object.defineProperty(o, prop, Object.getOwnPropertyDescriptor(source, prop)!) - TS7053 on o[prop] = this.deepCopy(source[prop]) ==> o[prop] = this.deepCopy((source as { [key: string]: any })[prop])

You are right @Neander64, it is due to typescript version 4.1, the type checking is stricker....