This manipulates a SP list form using the DOM API

spListForm.js

/*Get a reference to the chief approve drop down box*/ 
var chiefApprove = document.querySelector("[id^='ChiefDocumentApproval']"); 

/*Attach an onchage event to drop down*/ 
chiefApprove.onchange = function(){ 

/*Grab the selected value*/ 
var aValue = chiefApprove.options[chiefApprove.selectedIndex].value; 

/*Show and hide feedback field based on the selected value*/ 
if(aValue == "Approval with Feedback" || aValue == "Further discussion needed") { 

  /*Show box*/ 
  chiefFeedback.parentNode.parentNode.parentNode.style.visibility = "table-row"; 
  }else{ 
  /*Hide box*/ 
  chiefFeedback.parentNode.parentNode.parentNode.style.visibility = "hidden"; 
 } 
}