how to get user's uuid by user id in java script for RTC?

This JavaScript will do the job for you - you need to supply the Team Area id, and the name of the role as text. If you wanted to use the name of the Team Area instead you could modify the script accordingly.

Note that this script will run asynchronously so that it doesn't affect the performance of your browser, which means that the results may not be available directly in your calling function

dojo.require("jazz.client");

  var jazzClient = jazz.client;

  var getTeamMembersWithRole = function(team, role) {
    if (!team || !role) {
      console.log('getTeamMembersWithRole: Error! team = ' + team + ', role = '
          + role);
      return null;
    }

    var xhrArgs = {
      url : '/ccm/rpt/repository/foundation?fields=foundation/teamArea[itemId='
          + team
          + ']/roleAssignments/(contributor/itemId|contributorRoles/name)',
      headers : {
        'Accept' : 'application/xml'
      },
      handleAs : 'xml'
    };

    return jazzClient.xhrGet(xhrArgs).then(
        function(data) {
          var memberIds = data.evaluate(
              'foundation/teamArea/roleAssignments[contributorRoles/name/text()="'
                  + role + '"]/contributor/itemId', data, null,
              XPathResult.ANY_TYPE, null);

          var members = [];
          var id = memberIds.iterateNext();

          while (id !== null) {
            members.push(id.textContent);
            id = memberIds.iterateNext();
          }

          return members;
        });
  };

See https://rsjazz.wordpress.com/2016/07/15/rtc-process-customization-what-you-can-and-cannot-do/ for a good overview what is possible and what not.

In the Web UI it would be possible to use RESt APIs to communicate to the server and to get such information.

However, this is not supported and customers have managed to break performance render the UI unusable by doing so. See https://rsjazz.wordpress.com/2019/03/07/registering-custom-resource-intensive-scenarios-to-clm-applications/ for some anecdotal information and suggestions for a minimal performance monitoring.