Blink without delay

const int ledPin =  LED_BUILTIN;
int ledstatus = LOW;
unsigned long previousMillis = 0;
const long interval = 500;

void setup() {

  pinMode(ledPin, OUTPUT);
}

void loop() {

  unsigned long currentMillis = millis();

  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    if (ledstatus == LOW) {
      ledstatus = HIGH;
    } else {
      ledstatus = LOW;
    }
    digitalWrite(ledPin, ledstatus);
  }
}