application.js

function valididate_dependent(select, textarea, hidden) {  
  if (select.val() == 'No' || textarea.val() != "" ) {
    hidden.val('N/A');
    textarea.removeClass('error');
  } else {
    hidden.val('');
    textarea.addClass('error');
  }
}

function make_dependent(select, textarea, hidden) {
  var select = $(select);
  var textarea = $(textarea);
  var hidden = $(hidden);
  
  select.change(function() {
     valididate_dependent(select, textarea, hidden)
  });
  
  textarea.blur(function() {
     valididate_dependent(select, textarea, hidden)
  });
}