From MDN: https://developer.mozilla.org/en-US/docs/Web/API/WindowBase64/Base64_encoding_and_decoding#Solution_.232_.E2.80.93_rewriting_atob()_and_btoa()_using_TypedArrays_and_UTF-8

base64-utf8.js

(function (global) {
    'use strict';
    
    /*\
    |*|
    |*|  Base64 / binary data / UTF-8 strings utilities
    |*|
    |*|  https://developer.mozilla.org/en-US/docs/Web/JavaScript/Base64_encoding_and_decoding
    |*|
    \*/

    /* Array of bytes to base64 string decoding */
    function b64ToUint6(nChr) {
        return nChr > 64 && nChr < 91 ?
            nChr - 65 : nChr > 96 && nChr < 123 ?
            nChr - 71 : nChr > 47 && nChr < 58 ?
            nChr + 4 : nChr === 43 ?
            62 : nChr === 47 ?
            63 :
            0;

    }

    function base64DecToArr(sBase64, nBlocksSize) {

        var
            sB64Enc = sBase64.replace(/[^A-Za-z0-9\+\/]/g, ""),
            nInLen = sB64Enc.length,
            nOutLen = nBlocksSize ? Math.ceil((nInLen * 3 + 1 >> 2) / nBlocksSize) * nBlocksSize : nInLen * 3 + 1 >> 2,
            taBytes = new Uint8Array(nOutLen);

        for (var nMod3, nMod4, nUint24 = 0, nOutIdx = 0, nInIdx = 0; nInIdx < nInLen; nInIdx++) {
            nMod4 = nInIdx & 3;
            nUint24 |= b64ToUint6(sB64Enc.charCodeAt(nInIdx)) << 18 - 6 * nMod4;
            if (nMod4 === 3 || nInLen - nInIdx === 1) {
                for (nMod3 = 0; nMod3 < 3 && nOutIdx < nOutLen; nMod3++, nOutIdx++) {
                    taBytes[nOutIdx] = nUint24 >>> (16 >>> nMod3 & 24) & 255;
                }
                nUint24 = 0;

            }
        }

        return taBytes;
    }

    /* Base64 string to array encoding */

    function uint6ToB64(nUint6) {

        return nUint6 < 26 ?
            nUint6 + 65 : nUint6 < 52 ?
            nUint6 + 71 : nUint6 < 62 ?
            nUint6 - 4 : nUint6 === 62 ?
            43 : nUint6 === 63 ?
            47 :
            65;

    }

    function base64EncArr(aBytes) {

        var nMod3 = 2,
            sB64Enc = "";

        for (var nLen = aBytes.length, nUint24 = 0, nIdx = 0; nIdx < nLen; nIdx++) {
            nMod3 = nIdx % 3;
            if (nIdx > 0 && (nIdx * 4 / 3) % 76 === 0) {
                sB64Enc += "\r\n";
            }
            nUint24 |= aBytes[nIdx] << (16 >>> nMod3 & 24);
            if (nMod3 === 2 || aBytes.length - nIdx === 1) {
                sB64Enc += String.fromCharCode(uint6ToB64(nUint24 >>> 18 & 63), uint6ToB64(nUint24 >>> 12 & 63), uint6ToB64(nUint24 >>> 6 & 63), uint6ToB64(nUint24 & 63));
                nUint24 = 0;
            }
        }

        return sB64Enc.substr(0, sB64Enc.length - 2 + nMod3) + (nMod3 === 2 ? '' : nMod3 === 1 ? '=' : '==');

    }

    /* UTF-8 array to DOMString and vice versa */

    function UTF8ArrToStr(aBytes) {

        var sView = "";

        for (var nPart, nLen = aBytes.length, nIdx = 0; nIdx < nLen; nIdx++) {
            nPart = aBytes[nIdx];
            sView += String.fromCharCode(
                nPart > 251 && nPart < 254 && nIdx + 5 < nLen ? /* six bytes */
                /* (nPart - 252 << 30) may be not so safe in ECMAScript! So...: */
                (nPart - 252) * 1073741824 + (aBytes[++nIdx] - 128 << 24) + (aBytes[++nIdx] - 128 << 18) + (aBytes[++nIdx] - 128 << 12) + (aBytes[++nIdx] - 128 << 6) + aBytes[++nIdx] - 128 : nPart > 247 && nPart < 252 && nIdx + 4 < nLen ? /* five bytes */
                (nPart - 248 << 24) + (aBytes[++nIdx] - 128 << 18) + (aBytes[++nIdx] - 128 << 12) + (aBytes[++nIdx] - 128 << 6) + aBytes[++nIdx] - 128 : nPart > 239 && nPart < 248 && nIdx + 3 < nLen ? /* four bytes */
                (nPart - 240 << 18) + (aBytes[++nIdx] - 128 << 12) + (aBytes[++nIdx] - 128 << 6) + aBytes[++nIdx] - 128 : nPart > 223 && nPart < 240 && nIdx + 2 < nLen ? /* three bytes */
                (nPart - 224 << 12) + (aBytes[++nIdx] - 128 << 6) + aBytes[++nIdx] - 128 : nPart > 191 && nPart < 224 && nIdx + 1 < nLen ? /* two bytes */
                (nPart - 192 << 6) + aBytes[++nIdx] - 128 : /* nPart < 127 ? */ /* one byte */
                nPart
            );
        }

        return sView;

    }

    function strToUTF8Arr(sDOMStr) {

        var aBytes, nChr, nStrLen = sDOMStr.length,
            nArrLen = 0;

        /* mapping... */

        for (var nMapIdx = 0; nMapIdx < nStrLen; nMapIdx++) {
            nChr = sDOMStr.charCodeAt(nMapIdx);
            nArrLen += nChr < 0x80 ? 1 : nChr < 0x800 ? 2 : nChr < 0x10000 ? 3 : nChr < 0x200000 ? 4 : nChr < 0x4000000 ? 5 : 6;
        }

        aBytes = new Uint8Array(nArrLen);

        /* transcription... */

        for (var nIdx = 0, nChrIdx = 0; nIdx < nArrLen; nChrIdx++) {
            nChr = sDOMStr.charCodeAt(nChrIdx);
            if (nChr < 128) {
                /* one byte */
                aBytes[nIdx++] = nChr;
            } else if (nChr < 0x800) {
                /* two bytes */
                aBytes[nIdx++] = 192 + (nChr >>> 6);
                aBytes[nIdx++] = 128 + (nChr & 63);
            } else if (nChr < 0x10000) {
                /* three bytes */
                aBytes[nIdx++] = 224 + (nChr >>> 12);
                aBytes[nIdx++] = 128 + (nChr >>> 6 & 63);
                aBytes[nIdx++] = 128 + (nChr & 63);
            } else if (nChr < 0x200000) {
                /* four bytes */
                aBytes[nIdx++] = 240 + (nChr >>> 18);
                aBytes[nIdx++] = 128 + (nChr >>> 12 & 63);
                aBytes[nIdx++] = 128 + (nChr >>> 6 & 63);
                aBytes[nIdx++] = 128 + (nChr & 63);
            } else if (nChr < 0x4000000) {
                /* five bytes */
                aBytes[nIdx++] = 248 + (nChr >>> 24);
                aBytes[nIdx++] = 128 + (nChr >>> 18 & 63);
                aBytes[nIdx++] = 128 + (nChr >>> 12 & 63);
                aBytes[nIdx++] = 128 + (nChr >>> 6 & 63);
                aBytes[nIdx++] = 128 + (nChr & 63);
            } else /* if (nChr <= 0x7fffffff) */ {
                /* six bytes */
                aBytes[nIdx++] = 252 + (nChr >>> 30);
                aBytes[nIdx++] = 128 + (nChr >>> 24 & 63);
                aBytes[nIdx++] = 128 + (nChr >>> 18 & 63);
                aBytes[nIdx++] = 128 + (nChr >>> 12 & 63);
                aBytes[nIdx++] = 128 + (nChr >>> 6 & 63);
                aBytes[nIdx++] = 128 + (nChr & 63);
            }
        }

        return aBytes
    }

    global.base64 = {
        encode: function (str) {
          return base64EncArr(strToUTF8Arr(str))
        },
        decode: function(str) {
            return UTF8ArrToStr(base64DecToArr(str))
        }
    }
}(window))