Example of global scope being a problem when code is converted from js.

users.coffee

###
Include database config.

@var [Object] db
@api private
###

###
Set users collection

@var [JSON] urlsSchema
@api private
###

###
Get and set users collection

@var [Object] urls
@api private
###

###
Export urls modules for use in main app

@var [Object] module.exports.add
@var [Object] module.exports.findUrl
@return [Function] add
@return [Function] findUser
@api private
###

###
Add a doc to collection

@method add
@param [String] username
@param [String] email
@param [Number] id
@param [Function] callback
@return [Function][String] err
@return [Function][String] instance
@api public
###
add = (username, email, id, callback) ->
  instance = new users()
  instance.username = username
  instance.email = email
  instance.id = id
  instance.save (err) ->
    if err
      callback err
    else
      callback null, instance


###
Find users list

@method usersList
@param [String] id
@param [Function] callback
@return [Function][String] err
@return [Function][String] data
@api public
###
usersList = (callback) ->
  users = db.mongoose.model("users")
  users.find (err, data) ->
    if err
      callback err
    else
      callback null, data

db = require("./db.js")
usersSchema = new db.Schema(
  username: String
  email: String
  id: Number
)
users = db.mongoose.model("users", usersSchema)
module.exports.add = add
module.exports.usersList = usersList

users.js

/**
 * Include database config.
 *
 * @var [Object] db
 * @api private
*/
var db = require('./db.js');

/**
 * Set users collection
 *
 * @var [JSON] urlsSchema
 * @api private
*/
var usersSchema = new db.Schema({
    username : String,
    email : String,
    id : Number
});

/**
 * Get and set users collection
 *
 * @var [Object] urls
 * @api private
*/
var users = db.mongoose.model('users', usersSchema);

/**
 * Export urls modules for use in main app
 *
 * @var [Object] module.exports.add
 * @var [Object] module.exports.findUrl
 * @return [Function] add
 * @return [Function] findUser
 * @api private
*/
module.exports.add = add;
module.exports.usersList = usersList;

/**
 * Add a doc to collection
 *
 * @method add
 * @param [String] username
 * @param [String] email
 * @param [Number] id
 * @param [Function] callback
 * @return [Function][String] err
 * @return [Function][String] instance
 * @api public
*/
function add(username, email, id, callback) {
    var instance = new users();
    instance.username = username;
    instance.email = email;
    instance.id = id;

    instance.save(function (err) {
        if (err) {
            callback(err);
        }
        else {
            callback(null, instance);
        }
    });
}

/**
 * Find users list
 *
 * @method usersList
 * @param [String] id
 * @param [Function] callback
 * @return [Function][String] err
 * @return [Function][String] data
 * @api public
*/
function usersList(callback) {
    var users = db.mongoose.model('users');
    users.find(function(err, data) {
        if(err) {
            callback(err);
        }
        else {
            callback(null, data);
        }
    });
}