Answers for "change input name on dropdown select"

dropdown option selection change event in jquery

$("select [name='element_name']").change(function(){
	// condition goes here
});

jquery detect change in textarea content

$('#textareaID').bind('input propertychange', function() {

      $("#yourBtnID").hide();

      if(this.value.length){
        $("#yourBtnID").show();
      }
});