Error Examples

errors.js

/*
Three functions:
  One if the message is hard-coded.
  One if the message is provided entirely by the user.
  One if the message needs a suffix provided by the user.

You could also create a function that used JavaScript's sprintf
and the arguments object (cast to an object by _.toArray()) for
truly flexible error messaging. That is left as an exercise for
the team.
*/
var err = function(code, msg) {
  if(!code) throw "No HTTP status code provided";
  if(!msg) throw("No error message provided for code [" + code + "]");
  return function() { return {"code":code, "message":msg}; };
};
var errCode = function(code) {
  if(!code) throw "No HTTP status code provided";
  return function(msg) {
    if(!msg) throw("Please provide an error message for code [" + code + "]");
    return {"code":code, "message":msg};
  };
};
var errCodePrefix = function(code, prefix) {
  if(!code) throw "No HTTP status code provided";
  if(!prefix) throw "No prefix provided: use errCode";
  return function(suffix) {
    if(!suffix) throw("Please provide a suffix for code [" + code "] with prefix: " + prefix);
    return {"code":code, "message":prefix + " " + suffix};
  }
};

// Use this in this way:
// var errors = require("errors.js");
// errors.UNKNOWN();
// errors.login.BAD_USERNAME(username);
module.exports = {
  UNKNOWN: err(500, "Unknown server error"),
  login: {
    BAD_USERNAME: errCodePrefix(400, "Username is not in the appropriate format: ");
  }
}

This is also a good example of precondition checking. Better would be to ensure that code is a number between 100 and 510.