Example of a express JS helper function with multiple asynchronous actions that must be completed in a certain order

createUser.js

"use strict";

/** @module CreateUser */

var models = require("../wrapper").getSingleton();
var async = require("async");
var password = require("password-hash-and-salt");

/**
 * Create a user with a variable number of fields and a salted/hashed password, all users should
 * be created through this interface.
 *
 * @param {object} params - object containing desired user parameters
 * @param {string} [params.username] - optional string representing the user's username
 * @param {string} [params.phone] - optional string representing the user's phone number
 * @param {string} [params.email] - optional string representing the user's email address
 * @param {string} params.password - string representing the user's password
 * @param callback {function} - call back function that we return through asynchronously
 */

module.exports = function(params, callback){
    // validate parameters, not exhaustive (like if username is Number)
    if (!params.password || typeof params.password !== "string") { throw new Error("Invalid password parameter"); }
    if (!callback || typeof callback !== "function") { throw new Error("Invalid callback parameter"); }
    // params must include one of username, email, or phone, as a unique identifier for the user
    if (!params.username && !params.email && !params.phone){ throw new Error("Must have one of username, email, phone"); }
    
    // create a hashed password and then create a user with that password and given parameters
    async.series({
        password: function(done){
            password(params.password).hash(function(error, hash) {
                params.password = hash;
                done(error, hash);
            });
        },
        
        userInstance: function(done){
            models.User.create(params)
                .then(function(userInstance){
                    done(null, userInstance);
                })
                .catch(function(err){
                    done(err);
                });
        }
    }, callback);
};