btoa, atob - NODE JS - ES6

Base64.js

class Base64 {
    constructor() {

    }

    // A helper that returns Base64 characters and their indices.
    static chars = {
        ascii: function () {
            return 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';
        },
        indices: function () {
            if (!this.cache) {
                this.cache = {};
                let ascii = Base64.chars.ascii();

                for (let c = 0; c < ascii.length; c++) {
                    let chr = ascii[c];
                    this.cache[chr] = c;
                }
            }
            return this.cache;
        }
    };

    /**
     * Binary to ASCII (encode data to Base64)
     * @param {String} data
     * @returns {String}
     */
    static btoa(data) {
        let ascii = Base64.chars.ascii(),
            len = data.length - 1,
            i = -1,
            b64 = '';

        while (i < len) {
            let code = data.charCodeAt(++i) << 16 | data.charCodeAt(++i) << 8 | data.charCodeAt(++i);
            b64 += ascii[(code >>> 18) & 63] + ascii[(code >>> 12) & 63] + ascii[(code >>> 6) & 63] + ascii[code & 63];
        }

        let pads = data.length % 3;
        if (pads > 0) {
            b64 = b64.slice(0, pads - 3);

            while (b64.length % 4 !== 0) {
                b64 += '=';
            }
        }

        return b64;
    };

    /**
     * ASCII to binary (decode Base64 to original data)
     * @param {String} b64
     * @returns {String}
     */
    static atob(b64) {
        let indices = Base64.chars.indices(),
            pos = b64.indexOf('='),
            padded = pos > -1,
            len = padded ? pos : b64.length,
            i = -1,
            data = '';

        while (i < len) {
            let code = indices[b64[++i]] << 18 | indices[b64[++i]] << 12 | indices[b64[++i]] << 6 | indices[b64[++i]];
            if (code !== 0) {
                data += String.fromCharCode((code >>> 16) & 255, (code >>> 8) & 255, code & 255);
            }
        }

        if (padded) {
            data = data.slice(0, pos - b64.length);
        }

        return data;
    };
}

console.log("btoa('QQ==') = " + Base64.atob('QXJndW1lbnQxO0FyZ3VtZW50MjtBcmd1bWVudDMNCnBsdXRvO+g7YXJyYWJiaWF0bw0K'));