Firebase+Appinventor.ino

#include <ESP8266WiFi.h>
#include<FirebaseArduino.h>

const int Ledpin = D0;

#define FIREBASE_HOST "xxxxxxxxxxxxxxxxxx.firebaseio.com" //Your Firebase Project URL goes here without "http:" , "\" and "/"
#define FIREBASE_AUTH "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx" //Your Firebase Database Secret goes here
#define WIFI_SSID "xxxxxxxxxx" //your WiFi SSID for which yout NodeMCU connects
#define WIFI_PASSWORD "xxxxxxxxx"//Password of your wifi network 
void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200); //baud rate ,if you want to see the process in the serial monitor ,same baud rate should be set.
  pinMode(Ledpin,OUTPUT);
  WiFi.begin(WIFI_SSID,WIFI_PASSWORD);
  Serial.print("connecting");
  while (WiFi.status()!=WL_CONNECTED){
    Serial.print(".");
    delay(500);
  }
  Serial.println();
  Serial.print("connected:");
  Serial.println(WiFi.localIP());

  Firebase.begin(FIREBASE_HOST,FIREBASE_AUTH);
  Firebase.setString("S","0");//the variable in which is used in our Firebase and MIT App Inventor
}
void firebasereconnect()
{
  Serial.println("Trying to reconnect");
    Firebase.begin(FIREBASE_HOST, FIREBASE_AUTH);
  }
int l;
void loop() {
  if (Firebase.failed()) {
      Serial.print("setting number failed:");
      Serial.println(Firebase.error());
      firebasereconnect();
      return;
  }
  l=Firebase.getString("S").toInt();//The value read from the firebase is read in the form of String and is converted into Integer
  if(l==1){
      digitalWrite(Ledpin,HIGH);
      Serial.println("lightOFF");
    }
    else if(l==0){
      digitalWrite(Ledpin,LOW);
      Serial.println("lightON");
    }
}