Simple fade in/out function without any library. (pur JavaScript)

fade.js

// source: https://chrisbuttery.com/articles/fade-in-fade-out-with-javascript/

// fadeOut (fade.js)
function fadeOut(el) {
	el.style.opacity = 1;
	(function fade() {
		var val = parseFloat(el.style.opacity);
		if (!((val -= .1) < 0)) {
			el.style.opacity = val;
			requestAnimationFrame(fade);
		} else {
			el.style.display = "none";
		}
	})();
	return true;
}

// fadeIn (fade.js)
function fadeIn(el, display) {
	el.style.opacity = 0;
	el.style.display = display || "block";
	(function fade() {
		var val = parseFloat(el.style.opacity);
		if (!((val += .1) > 1)) {
			el.style.opacity = val;
			requestAnimationFrame(fade);
		}
	})();
}