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

Hello,
I am new to Espruino and Javascript and am struggling with the asynchronous nature of it. For my first shot at Espruino, I tried to fade in and out an LED. I have done this before in Arduino, but struggling here. What's the best way to do it?

Here is what I did, but I realize this is wrong as both the fadeIn and fadeOut methods a running in parallel. Thanks in advance.

var ledPower = 0;
var ledPin = D15;

function fadeIn(){
    var result = setInterval(function(){
      if(ledPower >= 1){
        clearInterval(result);
      }
      ledPower+= 0.001;
      analogWrite(ledPin, ledPower);
  }, 50);
}

function fadeOut(){
   result = setInterval(function(){
      if(ledPower <= 0){
        clearInterval(result);
      }
      ledPower-= 0.001;
      analogWrite(ledPin, ledPower);
  }, 50);
}

while(true){
  ledPower =0;
  fadeIn();
  fadeOut();
}

I have got it to work(sort of), but I am not happy with it. It feels like there is/should be a better way to do it than below.

While googling I found this jhonny-five example, which is quite nice. https://github.com/rwaldron/johnny-five/­blob/master/docs/led-fade.md

A LED class might make it easier for a beginner like me to start with Espruino. In all, loving Espruino so far. Need to learn Javascript to fully appreciate it though.

/* This example works by dividing 1 second into 10 ms time intervals and increasing/decreasing the led's analogWrite output by .01 per interval, to produce the fade-in and fade-out effects.
*/

var ledPower =0;
var ledPin = D15;

function fadeIn(callback){
  //start fadeIn effect should take 1 second to finish.
    var result = setInterval(function(){
      analogWrite(ledPin, ledPower);
      ledPower+= 0.01;
       if(ledPower >= 1){
        clearInterval(result);
        return;
      }
  }, 10);
  
  //set the fade-out effect to start after 1 second.
  setTimeout(callback, 1000);
}

function fadeOut(){
    var result = setInterval(function(){
      analogWrite(ledPin, ledPower);
      ledPower-= 0.01;
       if(ledPower <= 0){
        clearInterval(result);
        return;
      }
  }, 10);
}


setInterval(function(){
  fadeIn(fadeOut);
} , 2000);

I wouldn't be happy with that code either...

TBH - what I'd do for something like this is have a single function getting called every 10 milliseconds with setinterval, set up like kind of like a state-machine, which determines if it needs to brighten, dim, or do nothing. Which feels slightly un-Espruino-ey... but I think it'll probably work better. I don't like a solution that's relying on two intervals staying in sync - that's not guaranteed.

You are right, I was unnecessarily complicating things. Thanks a lot for your suggestion @DrAzzy

This is what I have got now

let ledBrightness = 0;
let ledPin = D15;
let fadeIn = false;

function fadeLED() {
 if((ledBrightness <= 0) || (ledBrightness >= 1)){
   fadeIn = !fadeIn;
 }
  
  setLedBrightness(fadeIn);

}

function setLedBrightness(fadeIn){
    if(fadeIn){
    ledBrightness += 0.01;
  }
  else {
    ledBrightness -=0.01;
  }
  
  analogWrite(ledPin, ledBrightness);
}

setInterval(fadeLED , 10);

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);