64BitsXBase64.js

Base64 = {
    // @param n     Number, 6 bits, e.g. 0b111111
    // @return      String, a char, e.g. "/"
    encode6Bits: function(n) {
        n = Math.min(Math.max(0, n), 63);
        if      (n >=  0 &&  n <= 25)   return String.fromCharCode("A".charCodeAt(0) + n);      // ["A", "Z"]
        else if (n >= 26 &&  n <= 51)   return String.fromCharCode("a".charCodeAt(0) + n - 26); // ["a", "z"]
        else if (n >= 52 &&  n <= 61)   return String.fromCharCode("0".charCodeAt(0) + n - 52); // ["0", "9"]
        else if (n == 62)               return "+";
        else /* if (n == 63) */         return "/";
    },
    // @param c     Number, char code, e.g. 47
    // @return      Number, 6 bits, e.g. 0b111111
    decode6Bits: function(c) { // c: charCode
        if      (c >= "A".charCodeAt(0) && c <= "Z".charCodeAt(0))  return c - "A".charCodeAt(0);      // [ 0, 25]
        else if (c >= "a".charCodeAt(0) && c <= "z".charCodeAt(0))  return c - "a".charCodeAt(0) + 26; // [26, 51]
        else if (c >= "0".charCodeAt(0) && c <= "9".charCodeAt(0))  return c - "0".charCodeAt(0) + 52; // [52, 61]
        else if (c == "+".charCodeAt(0))                            return 62;
        else /* if (c == "/".charCodeAt(0)) */                      return 63;
    },
    // @param bits64    BigInt, 64 bits, e.g. 0xFFFFFFFFFFFFFFFFn
    // @return          String, 1 ~ 11 chars, e.g. "//////////P"
    encode64Bits: function(bits64) {
        let base64 = "";
        while (bits64) {
            base64 += this.encode6Bits(Number(bits64 & 0b111111n)); // 0b111111: 6 bits
            bits64 >>= 6n;
        }
        return base64 || "A";
    },
    // @param base64    String, 1 ~ 11 chars, e.g. "//////////P"
    // @return          BigInt, 64 bits, e.g. 0xFFFFFFFFFFFFFFFFn
    decode64Bits: function(base64) {
        let length = Math.min(base64.length, 11); // 11: 64 bits ~= 11 chars * 6 bits
        let bits64 = 0n;
        for (let i = 0; i < length; ++i) {
            let bits06 = this.decode6Bits(base64.charCodeAt(i));
            bits64 |= BigInt(bits06) << BigInt(6 * i);
        }
        return bits64;
    }
};

BigInt.js

BigInt.max = function(a, b) {
    return a > b ? a : b;
};

BigInt.min = function(a, b) {
    return a < b ? a : b;
};

let bits64 = 0b1111111111111111111111111111111111111111111111111111111111111111n;
let base64 = Base64.encode64Bits(0b1111111111111111111111111111111111111111111111111111111111111111n);
console.log(bits64);
console.log(base64);
console.log(Base64.decode64Bits(base64));