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;">測試文字</div>
<script>
function drag(obj){
  obj.onmousedown = function(e){
    e = e || event;
    //獲取元素距離定位父級的x軸及y軸距離
    var x0 = this.offsetLeft;
    var y0 = this.offsetTop;
    //獲取此時(shí)鼠標(biāo)距離視口左上角的x軸及y軸距離
    var x1 = e.clientX;
    var y1 = e.clientY;
    //鼠標(biāo)按下時(shí)，獲得此時(shí)的頁面區(qū)域
    var L0 = 0;
    var R0 = document.documentElement.clientWidth;
    var T0 = 0;
    var B0 = document.documentElement.clientHeight;
    //鼠標(biāo)按下時(shí)，獲得此時(shí)的元素寬高
    var EH = obj.offsetHeight;
    var EW = obj.offsetWidth;
    document.onmousemove = function(e){
      e = e || event;
      //獲取此時(shí)鼠標(biāo)距離視口左上角的x軸及y軸距離
      x2 = e.clientX;
      y2 = e.clientY;  
      //計(jì)算此時(shí)元素應(yīng)該距離視口左上角的x軸及y軸距離
      var X = x0 + (x2 - x1);
      var Y = y0 + (y2 - y1);
      /******范圍限定*******/
      //獲取鼠標(biāo)移動時(shí)元素四邊的瞬時(shí)值
      var L = X;
      var R = X + EW;
      var T = Y;
      var B = Y + EH;
      //在將X和Y賦值給left和top之前，進(jìn)行范圍限定
      //只有在范圍內(nèi)時(shí)，才進(jìn)行相應(yīng)的移動
      //如果脫離左側(cè)范圍，則left置L0
      if(L < L0){X = L0;}
      //如果脫離右側(cè)范圍，則left置為R0
      if(R > R0){X = R0 - EW;}
      //如果脫離上側(cè)范圍，則top置T0
      if(T < T0){Y = T0;}
      //如果脫離下側(cè)范圍，則top置為B0
      if(B > B0){Y = B0 - EH;}
      obj.style.left = X + 'px';
      obj.style.top = Y + 'px';
    }
    document.onmouseup = function(e){
      //當(dāng)鼠標(biāo)抬起時(shí)，拖拽結(jié)束，則將onmousemove賦值為null即可
      document.onmousemove = null;
      //釋放全局捕獲
      if(obj.releaseCapture){
        obj.releaseCapture();
      }
    }
    //阻止默認(rèn)行為
    return false;
    //IE8-瀏覽器阻止默認(rèn)行為
    if(obj.setCapture){
      obj.setCapture();
    }
  }  
}
drag(test);
</script>

<div id="test" style="height: 100px;width: 100px;background:pink;position:absolute;top:0;left:0;">測試文字</div>
<script>
function drag(obj){
  obj.onmousedown = function(e){
    e = e || event;
    //獲取元素距離定位父級的x軸及y軸距離
    var x0 = this.offsetLeft;
    var y0 = this.offsetTop;
    //獲取此時(shí)鼠標(biāo)距離視口左上角的x軸及y軸距離
    var x1 = e.clientX;
    var y1 = e.clientY;
    //鼠標(biāo)按下時(shí)，獲得此時(shí)的頁面區(qū)域
    var L0 = 0;
    var R0 = document.documentElement.clientWidth;
    var T0 = 0;
    var B0 = document.documentElement.clientHeight;
    //鼠標(biāo)按下時(shí)，獲得此時(shí)的元素寬高
    var EH = obj.offsetHeight;
    var EW = obj.offsetWidth;
    document.onmousemove = function(e){
      e = e || event;
      //獲取此時(shí)鼠標(biāo)距離視口左上角的x軸及y軸距離
      x2 = e.clientX;
      y2 = e.clientY;  
      //計(jì)算此時(shí)元素應(yīng)該距離視口左上角的x軸及y軸距離
      var X = x0 + (x2 - x1);
      var Y = y0 + (y2 - y1);
      /******磁性吸附*******/
      //獲取鼠標(biāo)移動時(shí)元素四邊的瞬時(shí)值
      var L = X;
      var R = X + EW;
      var T = Y;
      var B = Y + EH;
      //在將X和Y賦值給left和top之前，進(jìn)行范圍限定
      //只有在范圍內(nèi)時(shí)，才進(jìn)行相應(yīng)的移動
      //如果到達(dá)左側(cè)的吸附范圍，則left置L0
      if(L - L0 < 50){X = L0;}
      //如果到達(dá)右側(cè)的吸附范圍，則left置為R0
      if(R0 - R < 50){X = R0 - EW;}
      //如果到達(dá)上側(cè)的吸附范圍，則top置T0
      if(T - T0 < 50){Y = T0;}
      //如果到達(dá)右側(cè)的吸附范圍，則top置為B0
      if(B0 - B < 50){Y = B0 - EH;}
      obj.style.left = X + 'px';
      obj.style.top = Y + 'px';
    }
    document.onmouseup = function(e){
      //當(dāng)鼠標(biāo)抬起時(shí)，拖拽結(jié)束，則將onmousemove賦值為null即可
      document.onmousemove = null;
      //釋放全局捕獲
      if(obj.releaseCapture){
        obj.releaseCapture();
      }
    }
    //阻止默認(rèn)行為
    return false;
    //IE8-瀏覽器阻止默認(rèn)行為
    if(obj.setCapture){
      obj.setCapture();
    }
  }  
}
drag(test);
</script>