Modeling Domain Entities With Universal JavaScript

Entities are the crown jewels of any application. All of those thousands of lines of client and server code have but one purpose: to persist, present, and allow the user to manipulate instances of domain model entities.
Give your entities the respect they deserve. Once you’ve worked out what they are for your domain, building and testing them thoroughly should be your next goal.

Link: https://dzone.com/articles/modeling-domain-entities-with-universal-javascript