在基于Gecko/ webkit的浏览器(Firefox、Chrome和Safari)和Opera中，可以使用btoa()和atob()。

原来的答案:如何在JavaScript中将字符串编码为Base64 ?

短而快速的Base64 JavaScript解码函数没有Failsafe:

function decode_base64 (s)
{
    var e = {}, i, k, v = [], r = '', w = String.fromCharCode;
    var n = [[65, 91], [97, 123], [48, 58], [43, 44], [47, 48]];

    for (z in n)
    {
        for (i = n[z][0]; i < n[z][1]; i++)
        {
            v.push(w(i));
        }
    }
    for (i = 0; i < 64; i++)
    {
        e[v[i]] = i;
    }

    for (i = 0; i < s.length; i+=72)
    {
        var b = 0, c, x, l = 0, o = s.substring(i, i+72);
        for (x = 0; x < o.length; x++)
        {
            c = e[o.charAt(x)];
            b = (b << 6) + c;
            l += 6;
            while (l >= 8)
            {
                r += w((b >>> (l -= 8)) % 256);
            }
         }
    }
    return r;
}

PHP .js项目有很多PHP函数的JavaScript实现。包括Base64_encode和base64_decode。

前端:上面的解决方案很好，但后端很快……

NodeJS -不弃用

使用Buffer.from。

> inBase64 = Buffer.from('plain').toString('base64')
'cGxhaW4='

> // DEPRECATED //
> new Buffer(inBase64, 'base64').toString()
'plain'
> (node:1188987) [DEP0005] DeprecationWarning: Buffer() is deprecated due to security and usability issues. Please use the Buffer.alloc(), Buffer.allocUnsafe(), or Buffer.from() methods instead.
(Use `node --trace-deprecation ...` to show where the warning was created)

// Works //
> Buffer.from(inBase64, 'base64').toString()
'plain'

函数b64_to_utf8(STR) { 返回decodeURIComponent(逃避(窗口。Atob (STR))); ｝

https://developer.mozilla.org/en-US/docs/Web/API/WindowBase64/Base64_encoding_and_decoding#The_.22Unicode_Problem.22

对于没有atob方法的JavaScripts框架，如果你不想导入外部库，这是一个简短的函数。

它将获得一个包含Base64编码值的字符串，并将返回一个解码后的字节数组(其中字节数组表示为数字数组，其中每个数字都是0到255之间的整数)。

function fromBase64String(str) {
    var alpha = 
    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    var value = [];
    var index = 0;
    var destIndex  = 0;
    var padding = false;
    while (true) {

        var first  = getNextChr(str, index, padding, alpha);
        var second = getNextChr(str, first .nextIndex, first .padding, alpha);
        var third  = getNextChr(str, second.nextIndex, second.padding, alpha);
        var fourth = getNextChr(str, third .nextIndex, third .padding, alpha);

        index = fourth.nextIndex;
        padding = fourth.padding;

        // ffffffss sssstttt ttffffff
        var base64_first  = first.code  == null ? 0 : first.code;
        var base64_second = second.code == null ? 0 : second.code;
        var base64_third  = third.code  == null ? 0 : third.code;
        var base64_fourth = fourth.code == null ? 0 : fourth.code;

        var a = (( base64_first << 2) & 0xFC ) | ((base64_second>>4) & 0x03);
        var b = (( base64_second<< 4) & 0xF0 ) | ((base64_third >>2) & 0x0F);
        var c = (( base64_third << 6) & 0xC0 ) | ((base64_fourth>>0) & 0x3F);

        value [destIndex++] = a;
        if (!third.padding) {
            value [destIndex++] = b;
        } else {
            break;
        }
        if (!fourth.padding) {
            value [destIndex++] = c;
        } else {
            break;
        }
        if (index >= str.length) {
            break;
        }
    }
    return value;
}

function getNextChr(str, index, equalSignReceived, alpha) {
    var chr = null;
    var code = 0;
    var padding = equalSignReceived;
    while (index < str.length) {
        chr = str.charAt(index);
        if (chr == " " || chr == "\r" || chr == "\n" || chr == "\t") {
            index++;
            continue;
        }
        if (chr == "=") {
            padding = true;
        } else {
            if (equalSignReceived) {
                throw new Error("Invalid Base64 Endcoding character \"" 
                    + chr + "\" with code " + str.charCodeAt(index) 
                    + " on position " + index 
                    + " received afer an equal sign (=) padding "
                    + "character has already been received. "
                    + "The equal sign padding character is the only "
                    + "possible padding character at the end.");
            }
            code = alpha.indexOf(chr);
            if (code == -1) {
                throw new Error("Invalid Base64 Encoding character \"" 
                    + chr + "\" with code " + str.charCodeAt(index) 
                    + " on position " + index + ".");
            }
        }
        break;
    }
    return { character: chr, code: code, padding: padding, nextIndex: ++index};
}

使用的资源:RFC-4648第4节