向JS脚本添加淡入淡出功能？

.fade {
  opacity: 0;
  transition: opacity 0.5s ease-in-out;
}

var element = document.getElementById("element-id");

function fadeIn() {
  element.classList.add("fade");
  element.style.opacity = 1;
}

function fadeOut() {
  element.classList.remove("fade");
  element.style.opacity = 0;
}

var fadeInButton = document.getElementById("fade-in-button");
var fadeOutButton = document.getElementById("fade-out-button");

fadeInButton.addEventListener("click", fadeIn);
fadeOutButton.addEventListener("click", fadeOut);