How to use getreference() with callback function in client Script | #ServiceNow

What is the use of getReference() ?

getReference() gets the record used in another reference field. For example you have the requested_for (reference to the sys_user table) and with getReference, you will retrieve the User record and save this as a gliderecord into a variable.

Why we use callback function with getReference? 

callback function allow you to use asynchronous processing with your getReference call.

Example: FetchCompany&phoneNo

function onChange(control, oldValue, newValue, isLoading) {

   if (isLoading || newValue == '') {

      return;    }

 var requester_user = g_form.getReference('requested_for', callback);

function callback(requester_user) {

g_form.setValue('email_id',requester_user.email);

g_form.setValue('manager', requester_user.manager);

}

}