code: function (url, callback) {
          // INIT
          const name = '_maptalks_jsonp_' + UID();
          if (url.match(/\?/)) url += '&callback=' + name;
          else url += '?callback=' + name;
  
          // Create script
          let script = document.createElement('script');
          script.type = 'text/javascript';
          script.src = url;
  
          // Setup handler
          window[name] = function (data) {
              callback(null, data);
              document.getElementsByTagName('head')[0].removeChild(script);
              script = null;
              delete window[name];
          };
  
          // Load JSON
          document.getElementsByTagName('head')[0].appendChild(script);
          return this;
      }
docs: Get JSON data by jsonp
  from https://gist.github.com/gf3/132080/110d1b68d7328d7bfe7e36617f7df85679a08968
  @param  {String}   url - resource url
  @param  {Function} cb  - callback function when completed