What's the first argument passed to a Node Js callback handler?

In Node Js all core modules, as well as most of the community-published modules, follows a pattern where the first argument to any callback handler is an error object. this object is optional, if there is no error then in that case null or undefined is passed to the callback.

Example of the callback function

function callback(err, results) {
	// usually we'll check for the error before handling results
	if(err) {
		// handle error somehow and return
	}
		// no error, perform standard callback handling
}