Node.js Design Patterns

The observer pattern

Another important and fundamental pattern used in Node.js is the observer pattern. Together with reactor, callbacks, and modules, this is one of the pillars of the platform and an absolute prerequisite for using many node-core and userland modules.

Observer is an ideal solution for modeling the reactive nature of Node.js, and a perfect complement for callbacks. Let's give a formal definition as follows:

Note

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 ...