Why document.getElementById(“something”).value not working for with not working after Locker Service enabled?

The document.getElementById("something").value in my controller on HTML tag  is not working after Locker Service is enabled. I did console.log() and it's returning nothing.

Any idea how to overcome this issue? Any idea why document.getelementbyid not working?

Help is very much appreciated.

HTML markup :

 <select class="form-control picklist" onchange="{!c.filteredProducts}" name="Discount__c" id="discount"> Discount 10 20 30 40 </select> 

And the javascript function:

filteredProducts: function(component,event,helper) { var discount =document.getElementById("discount").value; }