JavaScript实现Checkbox最多选中6个的限制功能

<SCRIPT LANGUAGE="JavaScript"> 
var c=0,limit=6; 
function doCheck(obj) { 
obj.checked?c++:c--; 
if(c>limit){ 
obj.checked=false; 
alert("over"); 
c--; 
} 
} 
</SCRIPT> 
<html> 
<body> 
<input type="CHECKBOX" name="sample1" onClick="doCheck(this)">A 
<input type="CHECKBOX" name="sample2" onClick="doCheck(this)">B 
<input type="CHECKBOX" name="sample3" onClick="doCheck(this)">C 
<input type="CHECKBOX" name="sample4" onClick="doCheck(this)">D 
<input type="CHECKBOX" name="sample5" onClick="doCheck(this)">E 
<input type="CHECKBOX" name="sample6" onClick="doCheck(this)">F 
<input type="CHECKBOX" name="sample7" onClick="doCheck(this)">G 
<input type="CHECKBOX" name="sample8" onClick="doCheck(this)">H 
<input type="CHECKBOX" name="sample9" onClick="doCheck(this)">I 
<input type="CHECKBOX" name="sample10" onClick="doCheck(this)">J 
</body> 
</html>