Non-standard
 
      This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
The Navigator.productSub read-only property returns the build number of the current browser.
Syntax
prodSub = window.navigator.productSub
- prodSubis a string.
Example
<script> 
function prodsub() {
  var dt = document.getElementById("d").childNodes[0];
  dt.data = window.navigator.productSub;
} 
</script>
<button onclick="prodsub();">productSub</button>
// returns: 20010725
Notes
On IE, this property returns undefined.
On Apple Safari and Google Chrome this property always returns 20030107.
Specification
This property is not part of any specification.