I’ve written a simple class based on the existing encode/decode functions in the internet for base64 encoding/decoding.

/**
 * Copyright (c) 2022 Vladimir Sviridov.
 * Distributed under the MIT License (license terms are at http://opensource.org/licenses/MIT).
 * 
 * Base64 encrypting module.
 */
define([], function () {

    // Base 64 table
    var table = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 
                 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 
                 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 
                 'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f', 
                 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 
                 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 
                 'w', 'x', 'y', 'z', '0', '1', '2', '3', 
                 '4', '5', '6', '7', '8', '9', '+', '/'];

    function u16tou8(str) {
        var res = [],
        len = str.length;
        for (var i = 0; i < len; i++) {
            var code = str.charCodeAt(i);
            if (code > 0x0000 && code <= 0x007F) {
                // single character,does not need to check 0x0000, because it's empty character
                // U+00000000 - U+0000007F  0xxxxxxx
                res.push(str.charAt(i));
            } else if (code >= 0x0080 && code <= 0x07FF) {
                // double characters
                // U+00000080 - U+000007FF  110xxxxx 10xxxxxx
                // 110xxxxx
                var byte1 = 0xC0 | ((code >> 6) & 0x1F);
                // 10xxxxxx
                var byte2 = 0x80 | (code & 0x3F);
                res.push(String.fromCharCode(byte1), String.fromCharCode(byte2));
            } else if (code >= 0x0800 && code <= 0xFFFF) {
                // three characters
                // U+00000800 - U+0000FFFF  1110xxxx 10xxxxxx 10xxxxxx
                // 1110xxxx
                var byte1 = 0xE0 | ((code >> 12) & 0x0F);
                // 10xxxxxx
                var byte2 = 0x80 | ((code >> 6) & 0x3F);
                // 10xxxxxx
                var byte3 = 0x80 | (code & 0x3F);
                res.push(String.fromCharCode(byte1), String.fromCharCode(byte2), String.fromCharCode(byte3));
            } else if (code >= 0x00010000 && code <= 0x001FFFFF) {
                // four characters
                // U+00010000 - U+001FFFFF  11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
            } else if (code >= 0x00200000 && code <= 0x03FFFFFF) {
                // five characters
                // U+00200000 - U+03FFFFFF  111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
            } else/** if (code >= 0x04000000 && code <= 0x7FFFFFFF)*/
            {
                // six characters
                // U+04000000 - U+7FFFFFFF  1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
            }
        }
        return res.join('');
    }

    function u8tou16(str) {
        var res = [],
        len = str.length;
        var i = 0;
        for (var i = 0; i < len; i++) {
            var code = str.charCodeAt(i);
            if (((code >> 7) & 0xFF) == 0x0) {
                // single character
                // 0xxxxxxx
                res.push(str.charAt(i));
            } else if (((code >> 5) & 0xFF) == 0x6) {
                // double characters
                // 110xxxxx 10xxxxxx
                var code2 = str.charCodeAt(++i);
                var byte1 = (code & 0x1F) << 6;
                var byte2 = code2 & 0x3F;
                var utf16 = byte1 | byte2;
                res.push(Sting.fromCharCode(utf16));
            } else if (((code >> 4) & 0xFF) == 0xE) {
                // three characters
                // 1110xxxx 10xxxxxx 10xxxxxx
                var code2 = str.charCodeAt(++i);
                var code3 = str.charCodeAt(++i);
                var byte1 = (code << 4) | ((code2 >> 2) & 0x0F);
                var byte2 = ((code2 & 0x03) << 6) | (code3 & 0x3F);
                var utf16 = ((byte1 & 0x00FF) << 8) | byte2;
                res.push(String.fromCharCode(utf16));
            } else if (((code >> 3) & 0xFF) == 0x1E) {
                // four characters
                // 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
            } else if (((code >> 2) & 0xFF) == 0x3E) {
                // five characters
                // 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
            } else/** if (((code >> 1) & 0xFF) == 0x7E)*/
            {
                // six characters
                // 1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
            }
        }
        return res.join('');
    }

    function encode(str) {
        if (!str) {
            return '';
        }
        var utf8 = u16tou8(str);
        // convert to utf8
        var i = 0;
        var len = utf8.length;
        var res = [];
        while (i < len) {
            var c1 = utf8.charCodeAt(i++) & 0xFF;
            res.push(table[c1 >> 2]);
            if (i == len) {
                res.push(table[(c1 & 0x3) << 4]);
                res.push('==');
                break;
            }
            var c2 = utf8.charCodeAt(i++);
            if (i == len) {
                res.push(table[((c1 & 0x3) << 4) | ((c2 >> 4) & 0x0F)]);
                res.push(table[(c2 & 0x0F) << 2]);
                res.push('=');
                break;
            }
            var c3 = utf8.charCodeAt(i++);
            res.push(table[((c1 & 0x3) << 4) | ((c2 >> 4) & 0x0F)]);
            res.push(table[((c2 & 0x0F) << 2) | ((c3 & 0xC0) >> 6)]);
            res.push(table[c3 & 0x3F]);
        }
        return res.join('');
    }

    function decode(str) {
        if (!str) {
            return '';
        }

        var len = str.length;
        var i = 0;
        var res = [];

        while (i < len) {
            code1 = table.indexOf(str.charAt(i++));
            code2 = table.indexOf(str.charAt(i++));
            code3 = table.indexOf(str.charAt(i++));
            code4 = table.indexOf(str.charAt(i++));

            c1 = (code1 << 2) | (code2 >> 4);
            c2 = ((code2 & 0xF) << 4) | (code3 >> 2);
            c3 = ((code3 & 0x3) << 6) | code4;

            res.push(String.fromCharCode(c1));

            if (code3 != 64) {
                res.push(String.fromCharCode(c2));
            }
            if (code4 != 64) {
                res.push(String.fromCharCode(c3));
            }
        }
        return u8tou16(res.join(''));
    }

    return {
        encode: encode,
        decode: decode,
    };
});

But JavaScript has embedded base64 functionality:

function utf8_to_b64(str) {
    return window.btoa(unescape(encodeURIComponent(str)));
}

function b64_to_utf8(str) {
    return decodeURIComponent(escape(window.atob(str)));
}