gistfile1.txt

function getData( val ){

    // return either the cached value or jqXHR object wrapped Promise
    return $.when(
        $.ajax('/foo/', {
            data: { value: val },
            dataType: 'json',
            success: function( resp ){
                
            }
        })
    );
}

getData('foo').then(function(resp){
    // do something with the response, which may
    // or may not have been retreived using an
    // XHR request.
});