Book excerpt: Reactive programming is revolutionary. It makes asynchronous programming cleaner, intuitive, and robust. Discover how to use the RxJS library to write programs in a simpler way, unifying asynchronous mechanisms such as callbacks and promises into a single, powerful construct. Learn to think about your programs as streams of data that you can transform by expressing what should happen, instead of having to painstakingly program how it should happen. You'll be able to handle real-world concurrency and write complex flows of events in your applications with ease. Create highly concurrent applications while writing simpler code using the Reactive Extensions for JavaScript (RxJS), a set of libraries for event composition. Programming real-world applications in JavaScript involves lots of asynchronous events--you might end up spending more time coordinating asynchronous code in your program than writing its actual functionality. This book introduces concepts and tools from reactive programming that will help you write cleaner and more robust asynchronous programs. Find out about Observable sequences, a unifying data type to write highly concurrent code. Discover Schedulers and change the concept of time in your applications, making asynchronous testing sane again. Along the way you'll see real-world examples for the browser and Node.js, including a real-time earthquake visualization in 20 lines of code and a frantic shoot-'em-up space videogame. You'll also use Cycle.js - a modern, fully reactive web framework - to make a whole new breed of web applications. By the end of the book, you'll know how to apply reactive programming to solve complex problems, build efficient programs with reactive user interfaces, and write your code more declaratively. What You Need: A modern web browser, Node.JS installed in your computer, and the RxJS JavaScript library.