get the value of last row in a sheet in google docs

lastValue.js

function lastValue(column) {
  var parts = column.split("!");
  if (parts.length == 2) {
    var sheetName = parts[0];
    var column = parts[1];
    var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName(sheetName);
  } else {
    var column = parts[0];
    var sheet = SpreadsheetApp.getActiveSheet();
  }
  
  var lastRow = sheet.getMaxRows();
  var values = sheet.getRange(column + "1:" + column + lastRow).getValues();

  for (; values[lastRow - 1] == "" && lastRow > 0; lastRow--) {}
  return values[lastRow - 1];
}

Your function produces an error. This works.

// ignore zeros at end too

function lastNoZero(myRange) {
    lastRow = myRange.length;
    for (; myRange[lastRow - 1] == "" || myRange[lastRow - 1] == 0 && lastRow > 0 ; lastRow--)  { 
       /*nothing to do*/ 
    }
    return myRange[lastRow - 1];
}