js 判断js函数、变量是否存在

<script type="text/javascript">
try {
if(typeof(eval(funcName)) == "function") {
funcName();
}
}catch(e) {
//alert("not function");
}
</script>

<script type="text/javascript">
function check() {
if (typeof(myvalue) == "undefined") {
alert("value is undefined");
}else {
alert("value is true");
}
}
</script>