Push button on pin 12 toggles the LED on pin 9, on an Arduino Uno

// A push button toggles an LED: press once for on, again for off.

//   pin 9 -> 220 ohm -> LED -> GND
//   10k from 5V -> pin 12; the push button from pin 12 -> GND
//
// The 10k pull-up holds pin 12 HIGH while the button is up; pressing it
// joins the pin to ground and it reads LOW.

// A contact bounces for a few milliseconds as it closes, so one press can
// look like several. The sketch reads the button, waits 10 ms and reads it
// again; only a press that is still there counts. Then it waits for the
// button to come back up before it will count another.

const int BUTTON = 12;
const int LED_PIN = 9;
bool lit = false;

void setup() {
  pinMode(BUTTON, INPUT);
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: press the button on pin 12 to toggle the LED");
}

void loop() {
  if (digitalRead(BUTTON) == LOW) {
    delay(10);
    if (digitalRead(BUTTON) == LOW) {
      lit = !lit;
      digitalWrite(LED_PIN, lit ? HIGH : LOW);
      Serial.println(lit ? "LED on" : "LED off");
      while (digitalRead(BUTTON) == LOW) {
      }
      delay(10);
    }
  }
}