code: function(cb) {
        if (!has(fileReloaders, localHmr.updateMode)) {
          cb(new Error("updateMode "+localHmr.updateMode+" not implemented"));
          return;
        }
        var reloader = fileReloaders[localHmr.updateMode];
        reloader(function(err, data) {
          if (err || !data || lastScriptData === data) {
            cb(err, false);
            return;
          }
          lastScriptData = data;
          localHmr.newLoad = null;
          try {
            if (bundle__filename || bundle__dirname) {
              new Function('require', '__filename', '__dirname', data)(require, bundle__filename, bundle__dirname);
            } else {
              new Function('require', data)(require);
            }
            // running the file sets _hmr.newLoad
          } catch (err2) {
            localHmr.newLoad = null;
            cb(err2);
            return;
          }
          if (!localHmr.newLoad) {
            cb(new Error("Reloaded script did not set hot module reload data"));
            return;
          }
          cb(null, true);
        });
      }
docs: cb(err, expectUpdate)