I accomplished to fade a line with this code

function fadeOut(ctx, canvas) {
  ctx.fillStyle = "rgba(255,255,255,0.1)";
  ctx.fillRect(0, 0, canvas.width, canvas.height);
  setTimeout(() => fadeOut(ctx, canvas), 50);
}

But the problem with this type of fading is, it fills the transparent background with a color and this cause the background div to fade as well.

The type of fading I would like to achieve is on gartic.io's background canvas.

You can check gartic.io