Trying to write to JSON

gistfile1.txt

var data = {
    "customers": [{
        "firstName": "John",
        "lastName": "Smith",
        "age": 25,
        "address": {
            "streetAddress": "20 Broadway St",
            "city": "New York",
            "state": "NY",
            "postalCode": "10021",
            "phoneNumber": "212 555-1234"
    }
    }]
};
var customersrepo = JSON.parse(data);

function returnOrder() {
    for (i = 0; i < 4; i++) {
        if (orderForm.orderCheckbox[i].checked) {
            confirm("Hi " + orderForm.firstName.value + " " +
            orderForm.lastName.value + ", you've ordered " +
            orderForm.orderCheckbox[i].value);
            customersrepo.customers[0].firstName = orderForm.firstName.value;
            customersrepo.customers[0].lastName = orderForm.lasvName.value;
        }
    }
}