fadeIn & fadeOut in vanilla js

fade.js

function fadeOut(_target){
  let el = document.querySelector(_target);
  el.style.opacity = 1;

  (function fade() {
    if ((el.style.opacity -= 0.02) < 0) {
      el.style.display = "none";
    } else {
      requestAnimationFrame(fade);
    }
  })();
};

function fadeIn(_target, display){
  let el = document.querySelector(_target);
  el.style.opacity = 0;
  el.style.display = display || "block";

  (function fade() {
    var val = parseFloat(el.style.opacity);
    if (!((val += 0.02) > 1)) {
      el.style.opacity = val;
      requestAnimationFrame(fade);
    }
  })();
};

I made 2 minor adjustments to extend the duration of the effect.