js中base64編碼問題

最近遇到了一個比較奇葩的問題，字符串-->base64，但是里邊還涉及到了gbk編碼問題，代碼如下：

var content_base64 = "CgkJCgkJCQk8Y29uZGl0aW9ucyB0eXBlPSJBTkQiPgoJCQkJCTxzeXNDb25kaXRpb25zIHR5cGU9Inx8Ij4KCQkJCQk8c3lzQ29uZGl0aW9uIG5hbWU9ItDCzsS1tSIgZXhwcmVzc2lvbj0iS0dselRtVjNSRzlqS0NrcElBPT0iLz4KCQkJCQk8c3lzQ29uZGl0aW9uIG5hbWU9IrWxx7C0psDtyMsiIGV4cHJlc3Npb249IktHbHpRM1Z5VlhObGNpZ3BLU0E9Ii8+CgkJCQkJPC9zeXNDb25kaXRpb25zPgoJCQkJPC9jb25kaXRpb25zPgoJCQk=";
var arrayBuffer = _base64ToArrayBuffer(content_base64);
var content = new TextDecoder("gbk").decode(arrayBuffer);
console.log(content);

function _base64ToArrayBuffer(base64) {
    var binary_string =  window.atob(base64);
    var len = binary_string.length;
    var bytes = new Uint8Array( len );
    for (var i = 0; i < len; i++)        {
        bytes[i] = binary_string.charCodeAt(i);
    }
    return bytes.buffer;
}

function _arrayBufferToBase64( buffer ) {
    var binary = '';
    var bytes = new Uint8Array( buffer );
    var len = bytes.byteLength;
    for (var i = 0; i < len; i++) {
        binary += String.fromCharCode( bytes[ i ] );
    }
    return window.btoa( binary );
}

怎么把字符串再轉回base64(gbk編碼格式)???