Managing concurrency

Simplifying control flows has been a concern of the Node community since the very beginning of the project. Indeed, this potential criticism was one of the very first anticipated by Ryan Dahl, who discussed it at length during the talk in which he introduced Node to the JavaScript developer community.

Because deferred code execution often requires the nesting of callbacks within callbacks, a Node program can sometimes begin to resemble a sideways pyramid, also known as The Pyramid of Doom. You've seen it: deeply nested code, 4 or 5 or even more levels deep, curly braces everywhere. Apart from syntactical annoyances, you can also imagine that tracking errors across such a call stack might be difficult—if a callback at the third level throws, who is responsible for handling that error? The second level? Even if level 2 is reading a file and level 3 is querying a database? Does that make sense? It can be hard to make sense of asynchronous program flows.