"Default" Getter/Setter

Hi, in the Back End Engineer path I learned to use the below syntax for defining a field with “default” getter and setter, and used this to complete the “Find Your Hat” mini project. However, now when trying to use this syntax off site, I am getting an error ReferenceError: get is not defined. I’ve tried to look back through the lessons, but cannot find where this syntax was introduced, and cannot figure out how to make it work off site. Is there a module that I need to require in? Or should I just be defining the get and set methods explicitly? I would prefer to be able to use the simplified syntax.

class ClassName {
  Field() { get; set; }