File.prototype.convertToBase64 = function(callback){

        var FR= new FileReader();
        FR.onload = function(e) {
             callback(e.target.result)
        };       
        FR.readAsDataURL(this);
}

and later call this function using this

var selectedFile = this.files[0];

 selectedFile.convertToBase64(function(base64)

you get your base64 code

File.prototype.convertToBase64 = function(callback){

    var FR= new FileReader();
    FR.onload = function(e) {
         callback(e.target.result)
    };       
    FR.readAsDataURL(this);
}

and later call this function using this

var selectedFile = this.files[0];
selectedFile.convertToBase64(function(base64)

you get your base64 code.