NodeJS wait asynchronous

Question

I've been reading about NodeJS and callbacks but I've been unable to get the grasp of it. Wait for function Here they explain why this happens but I don't understand how to do it with 2 nested functions.

I have a function that calls another function that will select a row from PostgreSQL Simple as that. Problem is, my function that looks up this row always returns before the select finishes so it doesn't work.

function testing() {
  var p=getTrackerId("123");
  console.log(p);
}



function getTrackerId(id) {
  var tracker_id="TEST";
  pg.connect(conString, function(err, client, done) {
    if(err) {
      return console.error('error fetching client from pool', err);
    }   
    tracker_id="THIS1";
    var q = squel.select()
      .from("access")
      .where("unique_id = ?",id)
      .toString();

    client.query(q, function(err, result) {
      if(result.rows[0]!=undefined){
        tracker_id=result.rows[0].tracker_id;
      }

      done();
      tracker_id="THIS3";
      if(err) {
        return console.error('error running query', err);
      }
    });


  });
  return tracker_id;
}  

This function will always return "TEST" and, therefore, p will always be TEST. Any ideas on how to solve this issue?