js原生手写一个拖拽小功能

先上效果图

附上代码

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8" />
  <meta http-equiv="X-UA-Compatible" content="IE=edge" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Document</title>
  <style>
    .four {
      width: 200px;
      height: 100px;
      background-color: #00f;
      position: relative;
    }
  </style>
</head>

<body>
  <div class="four" style="top:0;left:0"></div>
  <script>
    var div = document.getElementsByTagName('div')[0];
    function drag() {
      var disX,
        disY;
      div.addEventListener('mousedown', function (e) {
        var e = e || window.event;
        disX = e.pageX - parseInt(div.style.left);
        disY = e.pageY - parseInt(div.style.top);
        document.addEventListener('mousemove', move);
        document.addEventListener('mouseup', function (e) {
          document.removeEventListener('mousemove', move);
        })
      })
      function move(e) {
        var e = e || window.event;
        div.style.left = (e.pageX - disX) + 'px';
        div.style.top = e.pageY - disY + 'px';
      }
    }
    drag()
  </script>
</body>

</html>