The problem being addressed is that asynchronous coding in JavaScript results in the Pyramid of Doom. To explain, let's reiterate the example Ryan Dahl gave as the primary Node.js idiom:
db.query('SELECT ..etc..', function(err, resultSet) {
if (err) {
// Instead, errors arrive here
} else {
// Instead, results arrive here
}
});
// We WANT the errors or results to arrive here
The goal was to avoid blocking the event loop with a long operation. Deferring the processing of results or errors using callback functions was an excellent solution and is the founding idiom of Node.js. The implementation of callback functions led to this pyramid-shaped problem. Namely, that results and errors land in the callback. ...
