Arduino to Smartphone Communication

int led = 13;
void setup(){
  Serial.begin(9600);
  pinMode(led,OUTPUT);
  digitalWrite(led,LOW);
}
void loop(){
 if(Serial.available()){
   String value = Serial.readStringUntil('\n');
   Serial.println(value);
   if(value == "on"){
     digitalWrite(led,HIGH);
   }
   else if(value == "off"){
     digitalWrite(led,LOW);
   }}}