Return value is 'undefined'

Question

Why is the return value 'undefined'? Sorry I am a newbie :)

// Reading file
function Reading(event, i) {
    var file = event.target.files[i];
    var reader = new FileReader();
    var txt;
    reader.onload = function(event) {
        // txt == defined
        txt = event.target.result;
    };
    reader.readAsText(file);
    // txt == undefined
    return txt;             
}

I want to do something with the text... not only show text in box.