if($("#id")){
}else{}

if($("#id").length>0){}else{}

if($("#id")[0]){} else {}

if(document.getElementById("id")){} else {}

<p>one</p>
<div id=”ch”>
<span>two</span>
</div>
jQuery代碼及功能：
function jq(){
alert($(“#ch”).children().html());
}
$(“#ch”).children()得到對象[ <span>two</span> ].所以.html()的結果是”two”
[code]
三、根據子節點查找父節點
[code]
<div id=”ch”>
<span>two</span>
<span id=”sp”>three</span>
</div>

Jquery.ready ({
alert($(“#ch”).children(“#sp”).html());
});
$(“#ch”).children()得到對象[<span>two</span><span id="sp">three</span> ].
$(“#ch”).children(“#sp”)過濾得到[<span id="sp">three</span> ]