code: function(options) {
      var impressionEvent = {
        httpVerb: HTTP_VERB
      };
  
      var commonParams = getCommonEventParams(options);
      impressionEvent.url = ENDPOINT;
  
      var impressionEventParams = getImpressionEventParams(options.configObj, options.experimentId, options.variationId);
      // combine Event params into visitor obj
      commonParams.visitors[0].snapshots.push(impressionEventParams);
  
      impressionEvent.params = commonParams;
  
      return impressionEvent;
    }
docs: Create impression event params to be sent to the logging endpoint
  @param  {Object} options               Object containing values needed to build impression event
  @param  {Object} options.attributes    Object representing user attributes and values which need to be recorded
  @param  {string} options.clientEngine  The client we are using: node or javascript
  @param  {string} options.clientVersion The version of the client
  @param  {Object} options.configObj     Object representing project configuration, including datafile information and mappings for quick lookup
  @param  {string} options.experimentId  Experiment for which impression needs to be recorded
  @param  {string} options.userId        ID for user
  @param  {string} options.variationId   ID for variation which would be presented to user
  @return {Object}                       Params to be used in impression event logging endpoint call