The Callback pattern
Callbacks are the materialization of the handlers of the Reactor pattern (introduced in the previous chapter). They are one of those imprints that give Node.js its distinctive programming style.
Callbacks are functions that are invoked to propagate the result of an operation, and this is exactly what we need when dealing with asynchronous operations. In the asynchronous world, they replace the use of the return instruction, which, in turn, always executes synchronously. JavaScript is the ideal language for callbacks because functions are first-class objects and can be easily assigned to variables, passed as arguments, returned from another function invocation, or stored in data structures. Another ideal construct for implementing callbacks is closures. With closures, we can reference the environment in which a function was created; this way, we can always maintain the context in which the asynchronous operation was requested, no matter...