How to convert file into encoded data and concatenate with other javacsript variables?

hey i want to convert the file which has been uploaded by users from html front end. i prefer to convert bse64 format and converted file data should concatenate with other form data eg: firstName, lastName, email id. i want all those field values in single string firstName+lastName+emailID+fileData(base64encode or anyother encoding format to store in database) Please help me out from this,. I have tried fileReader to get base64 format but i don't know how to concatenate encoded data and filedata.
Thanks in advance .

code:

var gbyte = [];    

function handleFileSelect(evt) {
    var files = evt.target.files[0]; // FileList object

    var reader = new FileReader();
    reader.onload = (function (reader) {
        return function () {
            gbyte = reader.result;
        }
    })(reader);

    reader.readAsDataURL(files);
}

var x = gybte[] (i need the fileReader data here );