javascript - 为什么调用函数后它的对象属性值为undefined?

var contacts = [
    {
        "firstName": "Akira",
        "lastName": "Laine",
        "number": "0543236543",
        "likes": ["Pizza", "Coding", "Brownie Points"]
    },
    {
        "firstName": "Harry",
        "lastName": "Potter",
        "number": "0994372684",
        "likes": ["Hogwarts", "Magic", "Hagrid"]
    }
    ];
function lookUp(p){
    alert(contacts[0].p);
}
lookUp("number");

上面的函数调用后，显示的结果是 undefined ,为什么不是 "0543236543" ？