由于绝对位置，如何使Modal位于屏幕中心？

.modal {
  position: fixed;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

function centerModal() {
  var modal = document.getElementById("modal");
  var modalWidth = modal.offsetWidth;
  var modalHeight = modal.offsetHeight;
  var screenWidth = window.innerWidth;
  var screenHeight = window.innerHeight;

  modal.style.left = (screenWidth - modalWidth) / 2 + "px";
  modal.style.top = (screenHeight - modalHeight) / 2 + "px";
}

// 在Modal打开时调用centerModal函数