<script>

    var name = "john";

    <jsp:setProperty name="emp" property="firstName" value=" "/>   // How do I set javascript variable(name) value here ?

    <c:set var="firstName" value=""/>  // How do I set javascript variable (name) value here ?

  </script>

<script>document.getElementById('firstName').value = 'john';</script>
<input type="hidden" id="firstName" name="firstName">

<jsp:setProperty name="emp" property="firstName" value="${param.firstName}" />

<script>
var name = "<jsp:getProperty name="emp" property="firstName" />";
</script>