For https://medium.com/good-eggs-product/synchronize-ajax-calls-for-backbone-models-and-collections-8d18300a6ce8

withSerializedSync.js

withSerializedSync = function(cls) {
  var sync = cls.prototype.sync || Backbone.sync;
  cls.prototype.sync = function() {
    var args = arguments.length ? Array.prototype.slice.call(arguments,0) : [];
    if (!this._lastSync) {
      this._lastSync = sync.apply(this, args);
    } else {
      var _this = this;
      this._lastSync = this._lastSync.then(function() {
        return sync.apply(_this, args);
      });
    }
    return this._lastSync;
  };
}