Stripe.js promise based async client loader

stripe.js

var Promise = require('bluebird');

var CLIENT_URL = 'https://js.stripe.com/v2/';
var PUBLISHABLE_KEY = '...';

var _promise;

function loadClient() {
  // If we haven't started loading yet, create a promise
  if (!_promise) {
    _promise = new Promise(function(resolve) {
      var script = document.createElement('script');
      script.async = true;
      script.type = 'text/javascript';
      script.onload = function() {
        var client = window.Stripe;
        client.setPublishableKey(PUBLISHABLE_KEY);

        resolve(client);
      };

      script.src = CLIENT_URL;

      document.getElementsByTagName('head')[0].appendChild(script);
    });
  }

  return _promise;
}

function createToken(cardNumber, expirationMonth, expirationYear, verificationCode) {
  return loadClient().then(function(client) {
    return new Promise(function(resolve, reject) {
      client.createToken({
        number: cardNumber,
        exp_month: expirationMonth,
        exp_year: expirationYear,
        cvc: verificationCode
      }, function(status, response) {
        if (response.error) {
          return reject(response.error);
        }

        resolve(response.id);
      });
    });
  });
}