// Function

function extend(obj) {
  class Extended extends obj.constructor {
    constructor(obj) {
      super(...(Array.isArray(obj) ? obj : [obj]));
      this.$ = 'something';
    }
  }
  return new Extended(obj);
}

// Examples

const originalString = 'test';
const extendedString = extend(originalString);

String(extendedString) === originalString

extendedString.toString()
extendedString.valueOf()
extendedString.$
extendedString.constructor.prototype

const originalArray = [4, 5, 6];
const extendedArray = extend(originalArray);

JSON.stringify(extendedArray) === JSON.stringify(originalArray)

extendedArray.push(7)
extendedArray.valueOf()
extendedArray.toString()
extendedArray.$