不会的问题

<script type="text/javascript">
var person = {
    firstName: "Jhon",
    lastName : "Doe",
    id : 5566,
    fullName : function()
    {
        return this.firstName + " " + this.lastName;
    }
    
};
document.getElementById("demo").innerHTML = person.fullName();
</script>
<!-- 为什么不执行？ -->