How to use Callback function ServiceNow using client script?

function onChange(control, oldValue, newValue, isLoading) {
g_form.getReference('caller_id', CheckVip); // doAlert is our callback function
}

function CheckVip(caller) { // reference is passed into callback as first arguments
if (caller.getValue('vip') == 'true') {
alert('Caller is a VIP!');
}
}