code: function jobCallback(job, worker, index) {
  
      // as long as we have urls that we want to crawl we execute the job
      var url = pages[index];
      if (index < pages.length) {
  
          // the first argument contains the data which is passed to the worker
          // the second argument is a callback which is called when the job is executed
          job({
              url : url,
              id : index
          }, function(err) {
              // Lets log if it worked
              if (err) {
                  console.log('There were some problems for url ' + url + ': ' + err.message);
              } else {
                  console.log('DONE: ' + url + '(' + index + ')');
              }
          });
      } else {
          // if we have no more jobs, we call the function job with null
          job(null);
      }
  }
docs: Called when a worker is ready for a new job job is the function that needs to be called to execute the job index contains a number (starting at 0) that is increased with each jobCallback call