This document discusses isomorphic JavaScript, which refers to code that can run both on the server during rendering and in the browser. It notes some key libraries like React that enable isomorphic code by allowing the same code to render markup on both the server and client. It also discusses pushing inconsistencies between the server and client environments up to higher levels of code or down into common libraries. The document demonstrates how to set up a basic isomorphic JavaScript project using Webpack and React, and discusses the perspectives of library authors and application developers in building isomorphic code.