docs: function sn_api_node_availableSources(urlHelper, jsonClient, callback) {
      if (callback === undefined) {
        callback = jsonClient;
        jsonClient = d3.json;
      }
      if (!(urlHelper && urlHelper.availableSourcesURL && callback)) {
        return;
      }
      var url = urlHelper.availableSourcesURL();
      jsonClient(url, function(error, json) {
        var sources;
        if (error) {
          callback(error);
        } else if (!json) {
          callback("No data returned from " + url);
        } else if (json.success !== true) {
          callback(json.message ? json.message : "Query not successful.");
        } else {
          sources = Array.isArray(json.data) ? json.data.sort() : [];
          sources.sort();
          callback(null, sources);
        }
      });
    }