$('html').append('<div id="overlay-loader"><img src="gfx/overlay-loader.gif" /><br />Downloading data...</div>');

$.ajax({
    url: 'someurl',
    dataType: 'jsonp',
    cache: false,
    success: function (data) {
        $.when(test()).done(function() {
            rows = parseData(data);
            $('#overlay-loader').remove();
        });
    },
    error: function (jqXHR, errorStatus, message) {
        alert(message);
    }
});
function test() {
    var dfd = $.Deferred();
    $('#overlay-loader').html('<img src="gfx/overlay-loader.gif" /><br />Parsing...');
    // If I uncomment the alert the alertbox will appear and when I click ok the loader text has changed to "Parsing...". But without the alertbox the text won't change at all.
    //alert('wait');
    dfd.resolve();
    return dfd.promise();
}