如何阻止img溢出？

.container {
  width: 200px;
  height: 200px;
  overflow: hidden;
}

.container img {
  width: 100%;
  height: auto;
}

window.addEventListener('resize', function() {
  var container = document.querySelector('.container');
  var img = container.querySelector('img');
  var containerWidth = container.offsetWidth;
  var containerHeight = container.offsetHeight;
  var imgWidth = img.naturalWidth;
  var imgHeight = img.naturalHeight;
  var containerRatio = containerWidth / containerHeight;
  var imgRatio = imgWidth / imgHeight;

  if (containerRatio > imgRatio) {
    img.style.width = '100%';
    img.style.height = 'auto';
  } else {
    img.style.width = 'auto';
    img.style.height = '100%';
  }
});