HTML Options Looping

var aa = document.getElementById("Criteria_FlightNo");
var str = "{";
for(var i=0; i < aa.options.length; i++){
  str += "\"" + aa.options[i].value + "\"" +  ":" + "\"" + aa.options[i].innerHTML + "\"";
  if(i!= aa.options.length-1){
    str += ",";
  }
}
str += "}";
console.log(str);