How to fade-in and fade-out an LED?

I ended up wanting to do something like this too. I actually ended flashing with a sin wave instead to simplify things and it gives it a nice glow IMO. You can change how long the flash cycle is by adjusting the constant to multiply getTime() by. I liked a 2 second cycle, hence the code below.

To ensure it always starts by fading in you could use a new Date object when you start the fade instead.

function fadeLED() {
  analogWrite(ledPin, (Math.sin(getTime()*3)+1)/2);
}

setInterval(fadeLED, 10);