SOURCE

// 快速排序
function quickSort(array) {
  var len = array.length
  var smallerArr = []
  var base = [array[0]]
  var biggerArr = []
  for (var i = 1; i < len; i++) {
    if (array[i] < base[0]) {
      smallerArr.push(array[i])
    }
    else {
      biggerArr.push(array[i])
    }
  }
  return quickSort(smallerArr).concat(base.concat(quickSort(biggerArr)))
}

<div id="viewport">

  <div id="container">
      <div class="middleAlignDiv"></div>
  </div>
  
</div>

html, body {
  margin: 0;
  padding: 0;
}

* {
  box-sizing: border-box;
}

#viewport {
  width: 360px;
  height: 500px;
  margin: 0 auto;
  background-color: white;
}

#container {
  display: flex;
  justify-content: center;
  align-items: center;
  background-color: red;
  height: 300px;
}

.middleAlignDiv {
  width: 50px;
  height: 50px;
  margin: 0 auto;
  background-color: blue;
  position: relative;
  top: 50%;
  transform: translateY(-50%);
}