fadeWindowIn.js

const fadeWindowIn = (
    browserWindowToFadeIn,
    step = 0.1,
    fadeEveryXSeconds = 10
  )  => {
  
    // Get the opacity of the window.
    let opacity = browserWindowToFadeIn.getOpacity();
  
    // Increase the opacity of the window by `step` every `fadeEveryXSeconds`
    // seconds.
    const interval = setInterval(() => {
      
      // Stop fading if window's opacity is 1 or greater.
      if (opacity >= 1) window.clearInterval(interval);
      browserWindowToFadeIn.setOpacity(opacity);
      opacity += step;
    }, fadeEveryXSeconds);
  
    // Return the interval. Useful if we want to stop fading at will.
    return interval;
  }