I wanted 3 possibilities and experimented a bit, so in case someone is ever interested in this:
$.messager.prompt('My Title', 'Please type something', function(r){
if (r) {
console.log('ok clicked, r = '+r);
} else if (r == undefined) {
console.log('cancel clicked');
} else if (r.localeCompare("")==0) {
console.log('ok clicked, but r is EMPTY');
}
});