title: Onchange of a input field changed by Javascript
body: <p>I have a input field <code>type="hidden"</code> which is changed by javascript itself.</p>
  
  <p>When this field changes i want an event/function to be triggered.</p>
  
  <p>Tried this:</p>
  
  <pre><code>$(".product_id").on("change paste keyup input", function(event){alert('1');});
  $(".product_id").blur(function(event){alert('1');});
  $(".product_id").keypress(function(event){alert('1');});
  </code></pre>
  
  <p>But it does not seem to work as the value is changed by other JavaScript file itself.</p>
  
  <p>I cannot change or add to the existing JavaScript.
  I cannot add anything to the input field.</p>