作为Stefan Steiger回答的补充:(作为评论看起来不太好)

扩展字符串原型:

String.prototype.b64encode = function() { 
    return btoa(unescape(encodeURIComponent(this))); 
};
String.prototype.b64decode = function() { 
    return decodeURIComponent(escape(atob(this))); 
};

用法:

var str = "äöüÄÖÜçéèñ";
var encoded = str.b64encode();
console.log( encoded.b64decode() );

注意:

正如评论中所述，不建议使用unescape，因为它将来可能会被删除:

警告:尽管unescape()并没有被严格弃用(就像“从Web标准中删除”一样)，但它在ECMA-262标准的附件B中有定义，其介绍说明: 本附录中规定的所有语言特性和行为都有一个或多个不受欢迎的特性，如果没有遗留用法，将从本规范中删除。 注意:不要使用unescape来解码uri，而是使用decodeURI或decodeURIComponent来代替。

另一个不使用unescape的浏览器解决方案:

function ToBinary(str)
{
    let result="";

    str=encodeURIComponent(str);

    for(let i=0;i<str.length;i++)
        if(str[i]=="%")
        {
            result+=String.fromCharCode(parseInt(str.substring(i+1,i+3),16));
            i+=2;
        }
        else
            result+=str[i];

    return result;
}

btoa(ToBinary("тест"));//0YLQtdGB0YI=

如果你有UTF8，使用这个(实际上与SVG源代码一起工作)，像这样:

btoa(unescape(encodeURIComponent(str)))

例子:

 var imgsrc = 'data:image/svg+xml;base64,' + btoa(unescape(encodeURIComponent(markup)));
 var img = new Image(1, 1); // width, height values are optional params 
 img.src = imgsrc;

如果你需要解码base64，使用这个:

var str2 = decodeURIComponent(escape(window.atob(b64)));
console.log(str2);

例子:

var str = "äöüÄÖÜçéèñ";
var b64 = window.btoa(unescape(encodeURIComponent(str)))
console.log(b64);

var str2 = decodeURIComponent(escape(window.atob(b64)));
console.log(str2);

注意:如果你需要让它在mobile-safari中工作，你可能需要从base64数据中剥离所有的空白…

function b64_to_utf8( str ) {
    str = str.replace(/\s/g, '');    
    return decodeURIComponent(escape(window.atob( str )));
}

2017年更新

这个问题又困扰我了。 简单的事实是，atob并不真正处理utf8 -string -它只处理ASCII。 另外，我不会使用像js-base64这样的臃肿软件。 但webtoolkit确实有一个小的、漂亮的、非常可维护的实现:

/**
*
*  Base64 encode / decode
*  http://www.webtoolkit.info
*
**/
var Base64 = {

    // private property
    _keyStr: "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/="

    // public method for encoding
    , encode: function (input)
    {
        var output = "";
        var chr1, chr2, chr3, enc1, enc2, enc3, enc4;
        var i = 0;

        input = Base64._utf8_encode(input);

        while (i < input.length)
        {
            chr1 = input.charCodeAt(i++);
            chr2 = input.charCodeAt(i++);
            chr3 = input.charCodeAt(i++);

            enc1 = chr1 >> 2;
            enc2 = ((chr1 & 3) << 4) | (chr2 >> 4);
            enc3 = ((chr2 & 15) << 2) | (chr3 >> 6);
            enc4 = chr3 & 63;

            if (isNaN(chr2))
            {
                enc3 = enc4 = 64;
            }
            else if (isNaN(chr3))
            {
                enc4 = 64;
            }

            output = output +
                this._keyStr.charAt(enc1) + this._keyStr.charAt(enc2) +
                this._keyStr.charAt(enc3) + this._keyStr.charAt(enc4);
        } // Whend 

        return output;
    } // End Function encode 


    // public method for decoding
    ,decode: function (input)
    {
        var output = "";
        var chr1, chr2, chr3;
        var enc1, enc2, enc3, enc4;
        var i = 0;

        input = input.replace(/[^A-Za-z0-9\+\/\=]/g, "");
        while (i < input.length)
        {
            enc1 = this._keyStr.indexOf(input.charAt(i++));
            enc2 = this._keyStr.indexOf(input.charAt(i++));
            enc3 = this._keyStr.indexOf(input.charAt(i++));
            enc4 = this._keyStr.indexOf(input.charAt(i++));

            chr1 = (enc1 << 2) | (enc2 >> 4);
            chr2 = ((enc2 & 15) << 4) | (enc3 >> 2);
            chr3 = ((enc3 & 3) << 6) | enc4;

            output = output + String.fromCharCode(chr1);

            if (enc3 != 64)
            {
                output = output + String.fromCharCode(chr2);
            }

            if (enc4 != 64)
            {
                output = output + String.fromCharCode(chr3);
            }

        } // Whend 

        output = Base64._utf8_decode(output);

        return output;
    } // End Function decode 


    // private method for UTF-8 encoding
    ,_utf8_encode: function (string)
    {
        var utftext = "";
        string = string.replace(/\r\n/g, "\n");

        for (var n = 0; n < string.length; n++)
        {
            var c = string.charCodeAt(n);

            if (c < 128)
            {
                utftext += String.fromCharCode(c);
            }
            else if ((c > 127) && (c < 2048))
            {
                utftext += String.fromCharCode((c >> 6) | 192);
                utftext += String.fromCharCode((c & 63) | 128);
            }
            else
            {
                utftext += String.fromCharCode((c >> 12) | 224);
                utftext += String.fromCharCode(((c >> 6) & 63) | 128);
                utftext += String.fromCharCode((c & 63) | 128);
            }

        } // Next n 

        return utftext;
    } // End Function _utf8_encode 

    // private method for UTF-8 decoding
    ,_utf8_decode: function (utftext)
    {
        var string = "";
        var i = 0;
        var c, c1, c2, c3;
        c = c1 = c2 = 0;

        while (i < utftext.length)
        {
            c = utftext.charCodeAt(i);

            if (c < 128)
            {
                string += String.fromCharCode(c);
                i++;
            }
            else if ((c > 191) && (c < 224))
            {
                c2 = utftext.charCodeAt(i + 1);
                string += String.fromCharCode(((c & 31) << 6) | (c2 & 63));
                i += 2;
            }
            else
            {
                c2 = utftext.charCodeAt(i + 1);
                c3 = utftext.charCodeAt(i + 2);
                string += String.fromCharCode(((c & 15) << 12) | ((c2 & 63) << 6) | (c3 & 63));
                i += 3;
            }

        } // Whend 

        return string;
    } // End Function _utf8_decode 

}

https://www.fileformat.info/info/unicode/utf8.htm

For any character equal to or below 127 (hex 0x7F), the UTF-8 representation is one byte. It is just the lowest 7 bits of the full unicode value. This is also the same as the ASCII value. For characters equal to or below 2047 (hex 0x07FF), the UTF-8 representation is spread across two bytes. The first byte will have the two high bits set and the third bit clear (i.e. 0xC2 to 0xDF). The second byte will have the top bit set and the second bit clear (i.e. 0x80 to 0xBF). For all characters equal to or greater than 2048 but less than 65535 (0xFFFF), the UTF-8 representation is spread across three bytes.

使用btoa和unescape和encodeURIComponent不适合我。用XML/HTML实体替换所有特殊字符，然后转换为base64表示是解决这个问题的唯一方法。一些代码:

base64 = btoa(str.replace(/[\u00A0-\u2666]/g, function(c) {
    return '&#' + c.charCodeAt(0) + ';';
}));

使用库代替

我们没必要白费力气。使用库来节省时间和麻烦。

js-base64

https://github.com/dankogai/js-base64很好，我确认它很好地支持unicode。

Base64.encode('dankogai');  // ZGFua29nYWk=
Base64.encode('小飼弾');    // 5bCP6aO85by+
Base64.encodeURI('小飼弾'); // 5bCP6aO85by-

Base64.decode('ZGFua29nYWk=');  // dankogai
Base64.decode('5bCP6aO85by+');  // 小飼弾
// note .decodeURI() is unnecessary since it accepts both flavors
Base64.decode('5bCP6aO85by-');  // 小飼弾

将字符串转换为utf-8的解决方案，这比许多其他答案建议的utf-16或URLEncoded版本略短。它也与其他语言(如python和PHP)解码字符串的方式更加兼容:

编码

function btoa_utf8(value) {
    return btoa(
        String.fromCharCode(
            ...new TextEncoder('utf-8')
                   .encode(hash_value)
        )
    );
}

解码

function atob_utf8(value) {
    const value_latin1 = atob(value);
    return new TextDecoder('utf-8').decode(
        Uint8Array.from(
            { length: value_latin1.length },
            (element, index) => value_latin1.charCodeAt(index)
        )
    )
}

如果您愿意，您可以使用不同的字符编码替换其中的'utf-8'字符串。

注意:这取决于TextEncoder类。目前大多数浏览器都支持这一功能，但如果您需要针对较老的浏览器，请检查它是否可用。