JavaScript applications are getting bigger. That's because we can do more with the language—more than most thought possible. After all, JavaScript was conceived as a means to activate otherwise static web pages. A means by which to fill gaps in HTML, as it were. Year after year, more and more web sites started developing JavaScript code to improve the functionality of their pages.

Despite the frustrations of certain language idiosyncrasies, JavaScript popularity gained critical mass—today it's the most popular programming language on GitHub (http://githut.info/). From then onward, web sites started looking more like applications that a user would install on their desktop. Libraries and frameworks started popping up left right and center. Why? Because frontend JavaScript applications are large and complex.

In the present day frontend development profession, there's a lot of tools at our disposal. The JavaScript language has evolved into something that's usable on its own; it's becoming less dependent on libraries to perform the most fundamental and basic programming tasks. This is especially true of the next iteration of the ECMAScript specification, where problems that have plagued developers for years are at least partially addressed by constructs added to the language. This, of course, doesn't negate the need for application frameworks. The frontend development environment and its supporting web standards are far from perfect, but they're improving.

Something that's been missing from the frontend development picture for a long time is architecture. Frontend architectures have become prevalent in recent years due to the complexity of what's being implemented. Sophisticated tools, allow frontend developers to design an architecture that's able to scale with the problems we're trying to solve. And that's the crux of this book—JavaScript architectures that scale. But scale to what exactly? It's not your traditional scaling problem in computing, where you need to handle more load in a distributed server environment. Scaling in the frontend presents its own unique challenges and constraints. This chapter will define some of the scaling issues faced by JavaScript architectures.