Google Apps Script Undo function

code.gs

/**
 * Test function for onEdit. Passes an event object to simulate an edit to
 * a cell in a spreadsheet.
 * Check for updates: https://stackoverflow.com/a/16089067/1677912
 */
function test_onEdit() {
  onEdit({
    user : Session.getActiveUser().getEmail(),
    source : SpreadsheetApp.getActiveSpreadsheet(),
    range : SpreadsheetApp.getActiveSpreadsheet().getActiveCell(),
    value : SpreadsheetApp.getActiveSpreadsheet().getActiveCell().getValue(),
    authMode : "LIMITED"
  });
}


function onEdit() {
  // This script prevents cells from being updated. When a user edits a cell on the master sheet,
  // it is checked against the same cell on a helper sheet. If the value on the helper sheet is
  // empty, the new value is stored on both sheets.
  // If the value on the helper sheet is not empty, it is copied to the cell on the master sheet,
  // effectively undoing the change.
  // The exception is that the first few rows and the first few columns can be left free to edit by
  // changing the firstDataRow and firstDataColumn variables below to greater than 1.
  // To create the helper sheet, go to the master sheet and click the arrow in the sheet's tab at
  // the tab bar at the bottom of the browser window and choose Duplicate, then rename the new sheet
  // to Helper.
  // To change a value that was entered previously, empty the corresponding cell on the helper sheet,
  // then edit the cell on the master sheet.
  // You can hide the helper sheet by clicking the arrow in the sheet's tab at the tab bar at the
  // bottom of the browser window and choosing Hide Sheet from the pop-up menu, and when necessary,
  // unhide it by choosing View > Hidden sheets > Helper.
  // See https://productforums.google.com/d/topic/docs/gnrD6_XtZT0/discussion

  // modify these variables per your requirements
  var masterSheetName = "Master" // sheet where the cells are protected from updates
  var helperSheetName = "Backup" // sheet where the values are copied for later checking

  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var masterSheet = ss.getActiveSheet();
  if (masterSheet.getName() != masterSheetName) return;

  var masterRange = masterSheet.getActiveRange();

  var helperSheet = ss.getSheetByName(helperSheetName);
  var helperRange = helperSheet.getRange(masterRange.getA1Notation());
  var newValue = masterRange.getValues();
  var oldValue = helperRange.getValues();
  Logger.log("newValue " + newValue);
  Logger.log("oldValue " + oldValue);
      Logger.log(typeof(oldValue));
  if (oldValue == "" || isEmptyArrays(oldValue)) {
    helperRange.setValues(newValue);
  } else {
    Logger.log(oldValue);
    masterRange.setValues(oldValue);

  }
}

// In case the user pasted multiple cells this will be checked
function isEmptyArrays(oldValues) {
  if(oldValues.constructor === Array && oldValues.length > 0) {
    for(var i=0;i<oldValues.length;i++) {
      if(oldValues[i].length > 0 && (oldValues[i][0] != "")) {
          return false; 
      }
    }
  }
  return true;
}

Hi,
Just can across your script and i am wondering how to get this to work with formulas? I have a Google spreadsheet which is formula heavy and is shared with staff. I don't want them to change cells that have formula in. Thank You