esp8266/libraries/esp8266/examples/BlinkWithoutDelay/BlinkWithoutDelay.ino

/*
  ESP8266 BlinkWithoutDelay by Simon Peter
  Blink the blue LED on the ESP-01 module
  Based on the Arduino Blink without Delay example
  This example code is in the public domain

  The blue LED on the ESP-01 module is connected to GPIO1
  (which is also the TXD pin; so we cannot use Serial.print() at the same time)

  Note that this sketch uses LED_BUILTIN to find the pin with the internal LED
*/

int ledState = LOW;

unsigned long previousMillis = 0;
const long interval = 1000;

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    if (ledState == LOW) {
      ledState = HIGH;  // Note that this switches the LED *off*
    } else {
      ledState = LOW;  // Note that this switches the LED *on*
    }
    digitalWrite(LED_BUILTIN, ledState);
  }
}