function limitedRange(obj,fn){
  var L0 = 0;
  var R0 = document.documentElement.clientWidth;
  var T0 = 0;
  var B0 = document.documentElement.clientHeight;
  var L = obj.offsetLeft;
  var R = obj.offsetLeft + obj.offsetWidth;
  var T = obj.offsetTop;
  var B = obj.offsetTop + obj.offsetHeight;
  if(L >= L0 && R <= R0 && T >= T0 && B <= B0){
    fn(obj);
  }
}

<div id="test" style="height: 100px;width: 100px;background:pink;position:absolute;top:0;left:0;">测试文字</div>
<script>
function drag(obj){
  obj.onmousedown = function(e){
    e = e || event;
    //获取元素距离定位父级的x轴及y轴距离
    var x0 = this.offsetLeft;
    var y0 = this.offsetTop;
    //获取此时鼠标距离视口左上角的x轴及y轴距离
    var x1 = e.clientX;
    var y1 = e.clientY;
    //鼠标按下时，获得此时的页面区域
    var L0 = 0;
    var R0 = document.documentElement.clientWidth;
    var T0 = 0;
    var B0 = document.documentElement.clientHeight;
    //鼠标按下时，获得此时的元素宽高
    var EH = obj.offsetHeight;
    var EW = obj.offsetWidth;
    document.onmousemove = function(e){
      e = e || event;
      //获取此时鼠标距离视口左上角的x轴及y轴距离
      x2 = e.clientX;
      y2 = e.clientY;  
      //计算此时元素应该距离视口左上角的x轴及y轴距离
      var X = x0 + (x2 - x1);
      var Y = y0 + (y2 - y1);
      /******范围限定*******/
      //获取鼠标移动时元素四边的瞬时值
      var L = X;
      var R = X + EW;
      var T = Y;
      var B = Y + EH;
      //在将X和Y赋值给left和top之前，进行范围限定
      //只有在范围内时，才进行相应的移动
      //如果脱离左侧范围，则left置L0
      if(L < L0){X = L0;}
      //如果脱离右侧范围，则left置为R0
      if(R > R0){X = R0 - EW;}
      //如果脱离上侧范围，则top置T0
      if(T < T0){Y = T0;}
      //如果脱离下侧范围，则top置为B0
      if(B > B0){Y = B0 - EH;}
      obj.style.left = X + 'px';
      obj.style.top = Y + 'px';
    }
    document.onmouseup = function(e){
      //当鼠标抬起时，拖拽结束，则将onmousemove赋值为null即可
      document.onmousemove = null;
      //释放全局捕获
      if(obj.releaseCapture){
        obj.releaseCapture();
      }
    }
    //阻止默认行为
    return false;
    //IE8-浏览器阻止默认行为
    if(obj.setCapture){
      obj.setCapture();
    }
  }  
}
drag(test);
</script>

<div id="test" style="height: 100px;width: 100px;background:pink;position:absolute;top:0;left:0;">测试文字</div>
<script>
function drag(obj){
  obj.onmousedown = function(e){
    e = e || event;
    //获取元素距离定位父级的x轴及y轴距离
    var x0 = this.offsetLeft;
    var y0 = this.offsetTop;
    //获取此时鼠标距离视口左上角的x轴及y轴距离
    var x1 = e.clientX;
    var y1 = e.clientY;
    //鼠标按下时，获得此时的页面区域
    var L0 = 0;
    var R0 = document.documentElement.clientWidth;
    var T0 = 0;
    var B0 = document.documentElement.clientHeight;
    //鼠标按下时，获得此时的元素宽高
    var EH = obj.offsetHeight;
    var EW = obj.offsetWidth;
    document.onmousemove = function(e){
      e = e || event;
      //获取此时鼠标距离视口左上角的x轴及y轴距离
      x2 = e.clientX;
      y2 = e.clientY;  
      //计算此时元素应该距离视口左上角的x轴及y轴距离
      var X = x0 + (x2 - x1);
      var Y = y0 + (y2 - y1);
      /******磁性吸附*******/
      //获取鼠标移动时元素四边的瞬时值
      var L = X;
      var R = X + EW;
      var T = Y;
      var B = Y + EH;
      //在将X和Y赋值给left和top之前，进行范围限定
      //只有在范围内时，才进行相应的移动
      //如果到达左侧的吸附范围，则left置L0
      if(L - L0 < 50){X = L0;}
      //如果到达右侧的吸附范围，则left置为R0
      if(R0 - R < 50){X = R0 - EW;}
      //如果到达上侧的吸附范围，则top置T0
      if(T - T0 < 50){Y = T0;}
      //如果到达右侧的吸附范围，则top置为B0
      if(B0 - B < 50){Y = B0 - EH;}
      obj.style.left = X + 'px';
      obj.style.top = Y + 'px';
    }
    document.onmouseup = function(e){
      //当鼠标抬起时，拖拽结束，则将onmousemove赋值为null即可
      document.onmousemove = null;
      //释放全局捕获
      if(obj.releaseCapture){
        obj.releaseCapture();
      }
    }
    //阻止默认行为
    return false;
    //IE8-浏览器阻止默认行为
    if(obj.setCapture){
      obj.setCapture();
    }
  }  
}
drag(test);
</script>