When first confronted with node.js, you are not only presented with a completely new programming environment. You also encounter what is often referred to as callback hell accompanied by weird unfamiliar programming patterns. One of these is the way node treats callback functions.
The following post explains the conventions that node.js uses for its callback patterns (referred to as Continuation-passing style) and how you should implement them in order to comply.
First argument is an error object pattern
Node expects - almost - all callback functions to accept an Error object as the first argument. If no error occurred, the first argument should be null. If you use inline anonymous functions, this is a typical code snippet that you will encounter using node:
// include the filesystem module
var fs = require('fs');