blink a led on either an arduino or an esp8266 (with platform.io)

ledblink.ino

/*
  Blink
  Turns on an LED on for one second, then off for one second, repeatedly.

  Most Arduinos have an on-board LED you can control. On the Uno and
  Leonardo, it is attached to digital pin 13. If you're unsure what
  pin the on-board LED is connected to on your Arduino model, check
  the documentation at http://arduino.cc

  This example code is in the public domain.

  modified 8 May 2014
  by Scott Fitzgerald
 */

 // constants won't change. Used here to set a pin number :
#ifdef ARDUINO_ARCH_AVR
 const int ledPin =  3;      // the number of the LED pin
#endif

#ifdef ARDUINO_ARCH_ESP8266
 const int ledPin = D3;
#endif

 // Variables will change :
 int ledState = LOW;             // ledState used to set the LED

 // Generally, you should use "unsigned long" for variables that hold time
 // The value will quickly become too large for an int to store
 unsigned long previousMillis = 0;        // will store last time LED was updated

 // constants won't change :
 const long interval = 500;           // interval at which to blink (milliseconds)

// the setup function runs once when you press reset or power the board
void setup() {
  // initialize digital pin 13 as an output.
  pinMode(ledPin, OUTPUT);
}

// the loop function runs over and over again forever
void loop() {

  unsigned long currentMillis = millis();

  if (currentMillis - previousMillis >= interval) {

    // save the last time you blinked the LED
    previousMillis = currentMillis;

    // if the LED is off turn it on and vice-versa:
    if (ledState == LOW) {
      ledState = HIGH;
    } else {
      ledState = LOW;
    }

    // set the LED with the ledState of the variable:
    digitalWrite(ledPin, ledState);
  }
}