base64_decode_encode.js

/*
// This constant can also be computed with the following algorithm:
const base64abc = [],
  A = "A".charCodeAt(0),
  a = "a".charCodeAt(0),
  n = "0".charCodeAt(0);
for (let i = 0; i < 26; ++i) {
  base64abc.push(String.fromCharCode(A + i));
}
for (let i = 0; i < 26; ++i) {
  base64abc.push(String.fromCharCode(a + i));
}
for (let i = 0; i < 10; ++i) {
  base64abc.push(String.fromCharCode(n + i));
}
base64abc.push("+");
base64abc.push("/");
 */
const base64abc = [
  "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", "+", "/",
];

/*
// This constant can also be computed with the following algorithm:
const l = 256, base64codes = new Uint8Array(l);
for (let i = 0; i < l; ++i) {
  base64codes[i] = 255; // invalid character
}
base64abc.forEach((char, index) => {
  base64codes[char.charCodeAt(0)] = index;
});
base64codes["=".charCodeAt(0)] = 0; // ignored anyway, so we just need to prevent an error
 */
const base64codes = [
  255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
  255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
  255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 62, 255, 255, 255, 63,
  52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 255, 255, 255, 0, 255, 255,
  255, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
  15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 255, 255, 255, 255, 255,
  255, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
  41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51,
];

/**
 * Obtain the value corresponding to a base64 char code.
 * /!\ Can throw if the char code given is invalid.
 * @param {number} charCode
 * @returns {number}
 */
function getBase64Code(charCode) {
  if (charCode >= base64codes.length) {
    throw new Error("Unable to parse base64 string.");
  }
  const code = base64codes[charCode];
  if (code === 255) {
    throw new Error("Unable to parse base64 string.");
  }
  return code;
}

/**
 * Convert an array of bytes into a base64 string.
 * @param {Array.<number>|Uint8Array} bytes
 * @returns {string}
 */
function bytesToBase64(bytes) {
  let result = "";
  let i;
  const length = bytes.length;
  for (i = 2; i < length; i += 3) {
    result += base64abc[bytes[i - 2] >> 2];
    result += base64abc[((bytes[i - 2] & 0x03) << 4) | (bytes[i - 1] >> 4)];
    result += base64abc[((bytes[i - 1] & 0x0F) << 2) | (bytes[i] >> 6)];
    result += base64abc[bytes[i] & 0x3F];
  }
  if (i === length + 1) { // 1 octet yet to write
    result += base64abc[bytes[i - 2] >> 2];
    result += base64abc[(bytes[i - 2] & 0x03) << 4];
    result += "==";
  }
  if (i === length) { // 2 octets yet to write
    result += base64abc[bytes[i - 2] >> 2];
    result += base64abc[((bytes[i - 2] & 0x03) << 4) | (bytes[i - 1] >> 4)];
    result += base64abc[(bytes[i - 1] & 0x0F) << 2];
    result += "=";
  }
  return result;
}

/**
 * Convert a base64 string into the corresponding Uint8Array containing its
 * corresponding binary data.
 * /!\ Can throw if an invalid base64 string was given.
 * @param {Array.<number>|Uint8Array} bytes
 * @returns {string}
 */
function base64ToBytes(str) {
  const paddingNeeded = str.length % 4;
  let paddedStr = str;
  if (paddingNeeded !== 0) {
    paddedStr += paddingNeeded === 3 ? "=" :
                 paddingNeeded === 2 ? "==" :
                                       "==="; // invalid, but we will catch it
  }
  const index = paddedStr.indexOf("=");
  if (index !== -1 && index < paddedStr.length - 2) {
    throw new Error("Unable to parse base64 string.");
  }
  const missingOctets = paddedStr.endsWith("==") ? 2 :
                        paddedStr.endsWith("=") ? 1 : 0;
  const n = paddedStr.length;
  const result = new Uint8Array((n / 4) * 3);
  let buffer;
  for (let i = 0, j = 0; i < n; i += 4, j += 3) {
    buffer = getBase64Code(paddedStr.charCodeAt(i)) << 18 |
             getBase64Code(paddedStr.charCodeAt(i + 1)) << 12 |
             getBase64Code(paddedStr.charCodeAt(i + 2)) << 6 |
             getBase64Code(paddedStr.charCodeAt(i + 3));
    result[j] = buffer >> 16;
    result[j + 1] = (buffer >> 8) & 0xFF;
    result[j + 2] = buffer & 0xFF;
  }
  return result.subarray(0, result.length - missingOctets);
}