Interrupt Test Ver 2 (Flashing without delay)

const byte interruptLed = 13; // LED controlled by interrupt pin
const byte blinkLed = 4; // LED to flash for interrupt testing
int delayTime = 1000; // Delay time for blinking LED
const byte interruptPin = 2; // Button connected between here and GND

//long unsigned int previousMillis = 0;
const long int interval = 5000;

// Set to 'volatile' to allow the ISR to change the variable
volatile byte state = LOW; // State of interrupt LED.

void setup()
{
  Serial.begin(9600);
  pinMode(blinkLed, OUTPUT);
  pinMode(interruptLed, OUTPUT);
  pinMode(interruptPin, INPUT_PULLUP);
  // Mode: CHANGE -> on when button pushed. RISING -> Latches On
  attachInterrupt(digitalPinToInterrupt(interruptPin), blink, RISING);
}

void loop()
 {
  flash(blinkLed);
  digitalWrite(interruptLed, state);
 }

//-------------------------------------------------------------------------

// LED flash function
void flash(byte pin)
{
  //static variables will only be created and initialized the first time a function is called.
  static boolean ledState = LOW;
  static long previousMillis = 0;
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval)
  {
    previousMillis = currentMillis;
    ledState = !ledState;
    digitalWrite(pin,ledState);
  }
}

//-------------------------------------------------------------------------

// Interrupt Function (ISR)
void blink()
  {
  state = !state;
  }