int buttonPin = 12;
int ledPin = 9;
boolean isLighting = false;

void setup() {
  // put your setup code here, to run once:
  pinMode(buttonPin, INPUT);
  pinMode(ledPin, OUTPUT);

}

void loop() {
  // put your main code here, to run repeatedly:
  if(digitalRead(buttonPin) == LOW){ //if LED is off
    delay(10); // skip the phenomenon of 'bounce'
    if(digitalRead(buttonPin) == LOW){ // if LED is off
      reverseLED(); // call funcipn that turns on LED
      while(digitalRead(buttonPin) == LOW){
        delay(10); // keep delaying 10ms to avoid 'bounce' while LED is off
      }
    }
  } 
}

void reverseLED(){
  if(isLighting){ // if LED is on
    digitalWrite(ledPin, LOW);
    isLighting = false; // store state of led
  }
  else{
    digitalWrite(ledPin, HIGH);
    isLighting = true; // store state of LED
  }
}