A function to easily adapt mongoose models to a ES6 Promises interface (using polyfill: https://github.com/jakearchibald/ES6-Promises)

promisify.js

var Promise = require('es6-promise').Promise;

var promisify = function (obj, func) {
  return function () {
    var args = Array.prototype.slice.call(arguments, 0);
    return new Promise(function (resolve, reject) {
      args.push(function (err, result) {
        if (err) {
          reject(err);
        } else {
          resolve(result);
        }
      });
      func.apply(obj, args);
    });
  };
};

// create a rejected promise with a particular error (I feel like there must be a better way to do this)
var rejected = function(error) {
  return new Promise(function(resolve, reject) {
    reject(error);
  });
};

// Example usage: (UserModel is a mongoose model)

var createUser = promisify(UserModel, UserModel.create);
createUser({ name: "James" })
  .then(function(newuser) {
    console.log("Success!");
  }, function(err) {
    // doh
    console.log(err);
  });
  
// Slightly more complex example:
var findUser = promisify(UserModel, UserModel.findOne);
var createUser = promisify(UserModel, UserModel.create);

var user = { name:"James" };
findUser(user)
  .then(function(existing) {
    if (existing) {
      return rejected(new Error("User already exists!"));
    } else {
      return createUser(user);
    }
  })
  .then(function(newuser) {
    console.log("Success!");
  })
  .catch(function(err) {
    // an error occurred (that is, promise was rejected) either finding or creating user
    console.log(err);
  });