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/**

 * 校验文本是否为空

 * tips：提示信息

 * 使用方法：$("#id").validate("提示文本");

 * @itmyhome

 */

$.fn.validate = function(tips){

  if($(this).val() == "" || $.trim($(this).val()).length == 0){

    alert(tips + "不能为空！");

    throw SyntaxError(); //如果验证不通过，则不执行后面

  }

}

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<script type="text/javascript" src="http://m.jb51.net/skin/mobile/js/jquery.min.js"></script>

<div><ul id="thelist2">

<li><a href="https://m.jb51.net/game/140209.html"><img src="//img.jbzj.com/do/uploads/litimg/140228/100331632c.jpg">天天飞车航哥破解版</a><em class="xj star5"></em></li>

<li><a href="https://m.jb51.net/game/143515.html"><img src="//img.jbzj.com/do/uploads/litimg/140314/0944332514F.jpg"> 节奏大师全P破解版</a><em class="xj star6"></em></li>

<li><a href="https://m.jb51.net/game/207971.html"><img src="//img.jbzj.com/do/uploads/litimg/140821/11594R51423.gif">海岛奇兵国服内购破解版</a><em class="xj star5"></em></li>

<li><a href="https://m.jb51.net/game/144709.html"><img src="//img.jbzj.com/do/uploads/litimg/140318/161504236013.gif">天天炫斗破解版</a><em class="xj star5"></em></li>

<li><a href="https://m.jb51.net/game/80896.html"><img src="//img.jbzj.com/do/uploads/litimg/130503/1J21Va46.jpg">完美女友完整版</a><em class="xj star5"></em></li>

</ul>

<div><ul id="thelist3"></ul>

<script>

alert($('#thelist2').children().length)

alert($('#thelist3').children().length)

$thelist3 = $('#thelist3');

if($thelist3.children().length==0){

//插入广告

}

/*

thel3con = $('#thelist3').html();

alert(thel3con.length);

if(thel3con=""){

alert("空");

}else{

alert("非空");

}

*/

</script>

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String.prototype.isEmpty = function () {

 var s1 = this.replace(/[\r\n]/g, '').replace(/[ ]/g, ''),

   s2 = (s1 == '') ? true : false;

 return s2;

};

$list.html().isEmpty();