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();
}