Node.js Design Patterns: Master a series of patterns and techniques to create modular, scalable, and efficient applications

by Mario Casciaro

Pattern (observer): defines an object (called subject), which can notify a set of observers (or listeners), when a change in its state happens. The main difference from the callback pattern is that the subject can actually notify multiple observers, while a traditional continuation-passing style callback will usually propagate its result to only one listener, the callback.