var dom = {};
    dom.base64encode = function (data) {
      var xml_dom = new ActiveXObject("Microsoft.XMLDOM");
      xml_dom.loadXML('<!--?xml version="1.0" ?--> <root>');
      xml_dom.documentElement.setAttribute("xmlns:dt", "urn:schemas-microsoft-com:datatypes");
      var node1 = xml_dom.createElement("file1");
      node1.dataType = "bin.base64";
      node1.nodeTypedValue=data;
      xml_dom.documentElement.appendChild(node1);
      return node1.text;
    }
    var fso = new ActiveXObject('Scripting.FileSystemObject');
    var stream = new ActiveXObject('ADODB.Stream');
    stream.Type=1;//1=binary,2=text
    stream.Open();
    stream.LoadFromFile(fso.GetAbsolutePathName("/images/rails.png"));
    var bin = stream.Read(-1);//-1:read all , -2 : read line
    var ret = dom.base64encode(bin);
     
</root>

上述代码需要把浏览器的安全级别放到最低，可能还要做其他设置才行。不过，后来我发现更好的读入数据流的方法，毕竟Scripting.FileSystemObject太不可靠，我们需要一个更常用的宿主对象。IE的AJAX实现无疑是最好的选择。

var http = new ActiveXObject("Microsoft.XmlHttp");
    http.open("GET", "images/default/logo.gif", false);
    http.send();
    var xml = document.createElement("xml");
    xml.loadXML("<root>");
    var root = xml.documentElement;
    root.dataType = "bin.base64";
    root.nodeTypedValue = http.responseBody;
    alert(root.text);
</root>

在能支持HTML5 的新锐游览器，它们拥有一个叫toDataURL的API可以实现这种二进制转换为base64的功能。

如果是字符串转换编码则简单多了，firefox拥有两个叫atob与btoa的方法，详见这里。

最后附上日本一JS高手的兼容方案：

/*
 * $Id: base64.js,v 1.1 2009/03/01 22:38:45 dankogai Exp dankogai $
 *
 * History:
 *   dankogai's original: character-based
 *   drry's fix: split string to array then join
 *   new version: regexp-based
 */
 
(function(){
 
var b64chars 
    = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';
var b64tab = function(bin){
    var t = {};
    for (var i = 0, l = bin.length; i < l; i++) t[bin.charAt(i)] = i;
    return t;
}(b64chars);
 
var sub_toBase64 = function(m){
    var n = (m.charCodeAt(0) << 16)
          | (m.charCodeAt(1) <<  8)
          | (m.charCodeAt(2)      );
    return b64chars.charAt( n >>> 18)
         + b64chars.charAt((n >>> 12) & 63)
         + b64chars.charAt((n >>>  6) & 63)
         + b64chars.charAt( n         & 63);
};
 
var toBase64 = function(bin){
    if (bin.match(/[^\x00-\xFF]/)) throw 'unsupported character found' ;
    var padlen = 0;
    while(bin.length % 3) {
        bin += '\0';
        padlen++;
    };
    var b64 = bin.replace(/[\x00-\xFF]{3}/g, sub_toBase64);
    if (!padlen) return b64;
    b64 = b64.substr(0, b64.length - padlen);
    while(padlen--) b64 += '=';
    return b64;
};
 
var btoa = window.btoa || toBase64;
 
var sub_fromBase64 = function(m){
        var n = (b64tab[ m.charAt(0) ] << 18)
            |   (b64tab[ m.charAt(1) ] << 12)
            |   (b64tab[ m.charAt(2) ] <<  6)
            |   (b64tab[ m.charAt(3) ]);
    return String.fromCharCode(  n >> 16 )
        +  String.fromCharCode( (n >>  8) & 0xff )
        +  String.fromCharCode(  n        & 0xff );
};
 
var fromBase64 = function(b64){
    b64 = b64.replace(/[^A-Za-z0-9\+\/]/g, '');
    var padlen = 0;
    while(b64.length % 4){
        b64 += 'A';
        padlen++;
    }
    var bin = b64.replace(/[A-Za-z0-9\+\/]{4}/g, sub_fromBase64);
    bin.length -= [0,0,2,1][padlen];
    return bin;
};
 
var atob = window.atob || fromBase64;
 
var re_char_nonascii = /[^\x00-\xFF]/g;
 
var sub_char_nonascii = function(m){
    var n = m.charCodeAt(0);
    return n < 0x800 ? String.fromCharCode(0xc0 | (n >>>  6))
                     + String.fromCharCode(0x80 | (n & 0x3f))
        :              String.fromCharCode(0xe0 | ((n >>> 12) & 0x0f))
                     + String.fromCharCode(0x80 | ((n >>>  6) & 0x3f))
                     + String.fromCharCode(0x80 |  (n         & 0x3f))
        ;
};
 
var utob = function(uni){
    return uni.replace(re_char_nonascii, sub_char_nonascii);
};
 
var re_bytes_nonascii
    = /[\xC0-\xDF][\x80-\xBF]|[\xE0-\xEF][\x80-\xBF]{2}|[\xF0-\xF7][\x80-\xBF]{3}/g;
 
var sub_bytes_nonascii = function(m){
    var c0 = m.charCodeAt(0);
    var c1 = m.charCodeAt(1);
    if(c0 < 0xe0){
        return String.fromCharCode(((c0 & 0x1f) << 6) | (c1 & 0x3f));
    }else{
        var c2 = m.charCodeAt(2);
        return String.fromCharCode(
            ((c0 & 0x0f) << 12) | ((c1 & 0x3f) <<  6) | (c2 & 0x3f)
        );
    }
};
     
var btou = function(bin){
    return bin.replace(re_bytes_nonascii, sub_bytes_nonascii);
};
 
if (!this['Base64']) Base64 = {
    fromBase64:fromBase64,
    toBase64:toBase64,
    atob:atob,
    btoa:btoa,
    utob:utob,
    btou:btou,
    encode:function(u){ return btoa(utob(u)) },
    encodeURI:function(u){
        return btoa(utob(u)).replace(/[+\/]/g, function(m0){
            return m0 == '+' ? '-' : '_';
        }).replace(/=+$/, '');
    },
    decode:function(a){ 
        return btou(atob(a.replace(/[-_]/g, function(m0){
            return m0 == '-' ? '+' : '/';
        })));
    }
};
 
})();