js遇到的有关问题，引号的有关问题

<html>
<script>
function set(){
var s;
var a=1;
var b='aaa';
s="<input type='button' onclick='add("+a+",\""+b+"\",2)' value='click' />";
document.getElementById("content").innerHTML=s;
}
function add(id,name,num){
alert(id);
}
</script>
<body>
<input type="button" value="touch" onclick="set()" />
<div id="content"></div>
</body>
</html>