async.js

Syntax

• Each callback must be written with this syntax:
• function callback(err, result [, arg1[, …]])
• This way, you are forced to return the error first, and can’t ignore handling them later on. null is the convention in absence of errors
• callback(null, myResult);
• Your callbacks can contain more arguments than err and result, but it’s useful only for a specific set of functions (waterfall, seq, …)
• callback(null, myResult, myCustomArgument);
• And, of course, send errors. You must do it, and handle errors (or at least log them).
• callback(err);