The building blocks of our JavaScript applications are components. The glue that holds them together is the communication model used. At a low level, inter-component communication consists of one component passing a message to another, through a broker mechanism of some sort. This is often abstracted and simplified as an event system.

We looked at what actually gets passed around from one component to the next in the form of event data. This data needs to be consistent, predictable, and meaningful. We also looked at traceable events. That is, can we globally log events as they're triggered from the event triggering mechanism?

The boundaries of our JavaScript code are communication endpoints. We looked at the various components with responsibilities of communication with external systems, like the DOM, Ajax calls, or local storage. We need to insulate our smart components from the edges of our system.

Substitutability and layers are crucial concepts for scaling. Replacing components...