Convert vietnamese string to unsigned string by JavaScript

function showUnsignedString() {
    var signedChars     = "àảãáạăằẳẵắặâầẩẫấậđèẻẽéẹêềểễếệìỉĩíịòỏõóọôồổỗốộơờởỡớợùủũúụưừửữứựỳỷỹýỵÀẢÃÁẠĂẰẲẴẮẶÂẦẨẪẤẬĐÈẺẼÉẸÊỀỂỄẾỆÌỈĨÍỊÒỎÕÓỌÔỒỔỖỐỘƠỜỞỠỚỢÙỦŨÚỤƯỪỬỮỨỰỲỶỸÝỴ";
    var unsignedChars   = "aaaaaaaaaaaaaaaaadeeeeeeeeeeeiiiiiooooooooooooooooouuuuuuuuuuuyyyyyAAAAAAAAAAAAAAAAADEEEEEEEEEEEIIIIIOOOOOOOOOOOOOOOOOUUUUUUUUUUUYYYYY";
    var input = document.getElementById('input').value;
    var pattern = new RegExp("[" + signedChars + "]", "g");
    var output = input.replace(pattern, function (m, key, value) {
        return unsignedChars.charAt(signedChars.indexOf(m));
    });
    document.getElementById('output').value = output;
}