fadeWindowOut.js

const fadeWindowOut =  (
    browserWindowToFadeOut,
    step = 0.1,
    fadeEveryXSeconds = 10
  ) =>  {
  
    // Get the opacity of the window.
    let opacity = browserWindowToFadeOut.getOpacity();
  
    // Reduce the opacity of the window by `step` every `fadeEveryXSeconds`.
    // seconds.
    const interval = setInterval(() => {
      
      // Stop fading if window's opacity is 0 or lesser.
      if (opacity <= 0) window.clearInterval(interval);
      
      browserWindowToFadeOut.setOpacity(opacity);
      opacity -= step;
    }, fadeEveryXSeconds);
  
  // Return the interval. Useful if we want to stop fading at will.
    return interval;
  }