This document discusses React.js and how it changed the author's thinking about building user interfaces (UI). It describes how developing UI was previously slow, error-prone, and not fun using Handlebars templates, jQuery DOM manipulation, and redrawing on every change. React.js introduced a declarative and component-based approach that is faster, less error-prone, and more enjoyable. Components have properties (props) and state, re-render only when needed for performance, and can be reused and remain independent. This new approach to UI development freed the author to focus more on experimentation rather than fighting the framework.