JavaScript scalability includes the load time of the application, and the responsiveness of the application when the user interacts with it. Collectively, we refer to these two architectural qualities as performance. Performance is the prominent indicator of quality in the eyes of a user—it's important to get it right.

As our applications acquire new features and as the user base grows, we must find a way to avoid the associated performance degradation. The initial load is affected by things such as the JavaScript artifact payload size. The responsiveness of our UI has more to do with the runtime characteristics of our code.

Throughout this chapter, we'll address these two dimensions of performance, and how the various trade-offs we'll make will impact other areas of the system.