Arduino to Smartphone Communication

1int led = 13;
2void setup(){
3  Serial.begin(9600);
4  pinMode(led,OUTPUT);
5  digitalWrite(led,LOW);
6}
7void loop(){
8 if(Serial.available()){
9   String  value = Serial.readStringUntil('\n');
10   Serial.println(value);
11   if(value  == "on"){
12     digitalWrite(led,HIGH);
13   }
14   else if(value == "off"){
15     digitalWrite(led,LOW);
16   }}}

1int led = 13;
2void setup(){
3  Serial.begin(9600);
4  pinMode(led,OUTPUT);
5   digitalWrite(led,LOW);
6}
7void loop(){
8 if(Serial.available()){
9   String   value = Serial.readStringUntil('\
10');
11   Serial.println(value);
12   if(value   == "on"){
13     digitalWrite(led,HIGH);
14   }
15   else if(value == "off"){
16      digitalWrite(led,LOW);
17   }}}

1int led = 13;
2void setup(){
3  Serial.begin(9600);
4  pinMode(led,OUTPUT);
5
6  digitalWrite(led,LOW);
7}
8void loop(){
9 if(Serial.available()){
10   String
11  value = Serial.readStringUntil('\
12');
13   Serial.println(value);
14   if(value
15  == "on"){
16     digitalWrite(led,HIGH);
17   }
18   else if(value == "off"){
19
20     digitalWrite(led,LOW);
21   }}}