Auto incrementing Job Reference

I am very new at writing code and have a limited understanding so please be gentle! I have been trying to expand on the code made on this question.

I have had limited success and am now stuck, I was hoping someone would be kind enough to point me in the right direction or tell me what I am doing wrong.

So, the scenario: A need to have an auto-incrementing "Job Reference" for booking in netbook repairs to the IT dept I work for. This booking is made via a Google Form and I can get the code in the link to work perfectly but! I was hoping to have a little more than a simple count - 1,2,3,4,5 and so on. Ideally the job ref would be displayed as JR0001, JR0002 and so on.

So, my attempt at code!

The code which user: oneself submitted which works perfectly.

function onFormSubmit(e) {
  // Get the active sheet
  var sheet = SpreadsheetApp.getActiveSheet();
  // Get the active row
  var row = sheet.getActiveCell().getRowIndex();
  // Get the next ID value.  NOTE: This cell should be set to: =MAX(A:A)+1
  var id = sheet.getRange("P1").getValue();
  // Check of ID column is empty
  if (sheet.getRange(row, 1).getValue() == "") {
    // Set new ID value
    sheet.getRange(row, 1).setValue(id);
  }
}

The first thing I tried was to simply add another variable and add it to the .setValue

function onFormSubmit(e) {
   // Get the active sheet
  var sheet = SpreadsheetApp.getActiveSheet();
  // Get the active row
  var row = sheet.getActiveCell().getRowIndex();
  // Get the next ID value.  NOTE: This cell should be set to: =MAX(A:A)+1
  var id = sheet.getRange("X1").getValue();
  // Set Job Reference
  var jobref = "JR";
  // Check of ID column is empty
  if (sheet.getRange(row, 1).getValue() == "") {
  // Set new ID value
    sheet.getRange(row, 1).setValue(jobref+id);
  }
}

This worked as far as getting "JR1" instead of 1 but the auto-increment stopped working so for every form submitted I still had "JR1" - not really auto-increment!

I then tried setting up another .getValue from the sheet as the Job Ref

function onFormSubmit(e) {
   // Get the active sheet
  var sheet = SpreadsheetApp.getActiveSheet();
  // Get the active row
  var row = sheet.getActiveCell().getRowIndex();
  // Get the next ID value.  NOTE: This cell should be set to: =MAX(A:A)+1
  var id = sheet.getRange("X1").getValue();
  // Set Job Reference
  var jobref = sheet.getRange("X2").getValue();
  // Check of ID column is empty
  if (sheet.getRange(row, 1).getValue() == "") {
  // Set new ID value
    sheet.getRange(row, 1).setValue(jobref+id);
  }
}

Same result - a non incrementing "JR1"

I then tried concatenating the working incrementing number with my job ref cell and then calling that in the script.

function onFormSubmit(e) {
  // Get the active sheet
  var sheet = SpreadsheetApp.getActiveSheet();
  // Set Job Reference
  var jobref = sheet.getRange("X2").getValue();
  // Get the active row
  var row = sheet.getActiveCell().getRowIndex();
  // Get the next ID value.  NOTE: This cell should be set to: =MAX(A:A)+1
  var id = sheet.getRange("X1").getValue();
  // Check of ID column is empty
  if (sheet.getRange(row, 1).getValue() == "") {
  // Set new ID value
    sheet.getRange(row, 1).setValue(jobref);
  }
}

Same result the value doesn't increment

I don't understand why the number stops incrementing if I add other variables and don't understand how to add the leading zeros to the incrementing number. I get the feeling that I am trying to over complicate things!

So to sum up is there a way of getting an auto-incrementing ref that is 6 characters long - in this scenario first form JR0001 second submit JR0002 and so on.

I would really like some pointers on where I am going wrong if possible, I do want to learn but I am obviously missing some key principles.

here is a working solution that uses the "brand new" Utilities.formatString() that the GAS team just added a few days ago ;-)

function OnForm(){
var sh = SpreadsheetApp.getActiveSheet()
var startcell = sh.getRange('A1').getValue();
if(! startcell){sh.getRange('A1').setValue(1);return}; // this is only to handle initial situation when A1 is empty.
var colValues = sh.getRange('A1:A').getValues();// get all the values in column A in an array
var max=0;// define the max variable to a minimal value
  for(var r in colValues){ // iterate the array
    var vv=colValues[r][0].toString().replace(/[^0-9]/g,'');// remove the letters from the string to convert to number
    if(Number(vv)>max){max=vv};// get the highest numeric value in th column, no matter what happens in the column... this runs at array level so it is very fast
    }
    max++ ; // increment to be 1 above max value
sh.getRange(sh.getLastRow()+1, 1).setValue(Utilities.formatString('JR%06d',max));// and write it back to sheet's last row.
}