怎么取标签选中的value值或title值？

[细雨531]

<form name="f1">
a:<input type="checkbox" name="n1" value="a" title="1"><br/>
b:<input type="checkbox" name="n1" value="b" title="1"><br/>
c:<input type="checkbox" name="n1" value="c" title="1"><br/>
<input type="button" id="b" value="tijiao">
</form>



写到这里我就不会了，哪位大神帮帮忙，直接alert出来就行。
<script type="text/javascript">
var b = document.getElementById("b");
b.onclick = function(){
xxxxxxxx
xxxxxxxxxxx
}
</script>

[街头小贩]

<form name="f1">
a:<input type="checkbox" name="n1" value="a" title="1"><br/>
b:<input type="checkbox" name="n1" value="b" title="1"><br/>
c:<input type="checkbox" name="n1" value="c" title="1"><br/>
<input type="button" id="b" value="tijiao">
</form>

<script type="text/javascript">  	
  	var b = document.getElementById("b");
  	b.onclick = function(){
		var chV="";
		var inputElements =document.getElementsByTagName("input");
		for(var i=0; inputElements[i]; ++i){
	      	if(inputElements[i].name==="n1" && inputElements[i].checked){
           		chV+=inputElements[i].value+",";
			}
		}
		alert(chV.substring(0,chV.length-1));
  	};
</script>

[细雨531]

<script type="text/javascript"> var b = document.getElementById("b"); b.onclick = function(){ var sign = 0; var inputs = document.getElementsByTagName('input');//获取所有的input标签对象。 for(var i=0;i<inputs.length;i++){ var obj = inputs[i]; if(obj.type=='checkbox'){ if(obj.checked==true){ sign=1; alert(obj.value); } } } if(sign==0)//没有被选择项 { alert("未选择"); } } </script>