ArcGIS JSAPI: Query Return Distinct - Precallback Request (V < 3.11)

gistfile1.js

require(["esri/request"], function (esriRequest) {
  esriRequest.setRequestPreCallback(function(ioArgs){
    // test if we're sending the request to a mapservice query
    if(/query\/?$/.test(ioArgs.url)){
      // if not requesting ObjectIds, count, or querying with a list of ObjectIDs, return distinct values.
      if (ioArgs.content && !ioArgs.content.returnIdsOnly && !ioArgs.content.returnCountOnly && !ioArgs.content.objectIds) {
        ioArgs.content.returnDistinctValues = true;
      }
    }
    return ioArgs;
  });
});