HTML5 FileReader how to return result?

I use JS FileReader. I want to get result after file reading operation and work with this data. FileReader is asynchronous and I don't know when result is ready. How to get it done right?

$(document).ready(function(){
    $('#file_input').on('change', function(e){
        var res = readFile(this.files[0]);

        //my some manipulate with res

        $('#output_field').text(res);
    });
});

function readFile(file){
    var reader = new FileReader(),
        result = 'empty';

    reader.onload = function(e)
    {
        result = e.target.result;
    };

    reader.readAsText(file);

    //waiting until result is empty?

    return result;
}

http://jsfiddle.net/ub22m/4/

It's show "empty".

How to wait until "result" is empty? Another way?