text: class Button {
      // @param id {string} ID of button
      constructor(id) {
          if (typeof id != "string") throw new TypeError("ID is string. not " + typeof id + ".");
          this.id = id;
      }
      
      /**
       * @param callback {function} Button handler function<btn>
       * @return {Button} 
       */
      handle(callback) {
          this.callback = callback;
          return this;
      }
      
      // @param btn {object} A button object
      call(btn) {
          this.callback(btn);
          return this;
      }
  }
  
  module.exports = Button;