如何让不断变化的视图居中？

.container {
  display: flex;
  justify-content: center;
  align-items: center;
}

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

var container = document.querySelector('.container');
var child = document.querySelector('.child');

var containerWidth = container.offsetWidth;
var containerHeight = container.offsetHeight;
var childWidth = child.offsetWidth;
var childHeight = child.offsetHeight;

child.style.left = (containerWidth - childWidth) / 2 + 'px';
child.style.top = (containerHeight - childHeight) / 2 + 'px';