<div id="panel">
  <div id="dragIcon"></div>
</div>

<style>
    #panel{
      position: absolute;
      width: 200px;height: 200px;
      background: green;
    }
    #dragIcon{
      position: absolute;bottom: 0;right: 0;
      width: 20px;height: 20px;
      background: yellow;
    }
</style>

<script>
  window.onload = function () {
    // 1. 获取两个大小div
    var oPanel = document.getElementById('panel');
    var oDragIcon = document.getElementById('dragIcon');
    // 定义4个变量
    var disX = 0;//鼠标按下时光标的X值
    var disY = 0;//鼠标按下时光标的Y值
    var disW = 0; //拖拽前div的宽
    var disH = 0; // 拖拽前div的高
    //3. 给小div加点击事件
    oDragIcon.onmousedown = function (ev) {
      var ev = ev || window.event;
      disX = ev.clientX; // 获取鼠标按下时光标x的值
      disY = ev.clientY; // 获取鼠标按下时光标Y的值
      disW = oPanel.offsetWidth; // 获取拖拽前div的宽
      disH = oPanel.offsetHeight; // 获取拖拽前div的高
      document.onmousemove = function (ev) {
        var ev = ev || window.event;
        //拖拽时为了对宽和高 限制一下范围，定义两个变量
        var W = ev.clientX - disX + disW;
        var H = ev.clientY - disY + disH;
        if(W<100){
          W = 100;
        }
        if(W>800){
          W =800;
        }
        if(H<100){
          H = 100;
        }
        if(H>500){
          H = 500;
        }
        oPanel.style.width =W +'px';// 拖拽后物体的宽
        oPanel.style.height = H +'px';// 拖拽后物体的高
      }
      document.onmouseup = function () {
        document.onmousemove = null;
        document.onmouseup = null;
      }
    }
  }
</script>