common-kit/lib/base64/index.js

JavaScript Common Kit

const utf8 = require('../utf8/index.js')

const chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/='

function InvalidCharacterError(message) {
  this.message = message
}
InvalidCharacterError.prototype = new Error()
InvalidCharacterError.prototype.name = 'InvalidCharacterError'

// encoder
// [https://gist.github.com/999166] by [https://github.com/nignag]
function btoa(input) {
  var str = String(input)
  str = utf8.utf16to8(str) // 支持中文
  for (
    // initialize result and counter
    var block, charCode, idx = 0, map = chars, output = '';
    // if the next str index does not exist:
    //   change the mapping table to "="
    //   check if d has no fractional digits
    str.charAt(idx | 0) || ((map = '='), idx % 1);
    // "8 - idx % 1 * 8" generates the sequence 2, 4, 6, 8
    output += map.charAt(63 & (block >> (8 - (idx % 1) * 8)))
  ) {
    charCode = str.charCodeAt((idx += 3 / 4))
    if (charCode > 0xff) {
      throw new InvalidCharacterError(
        "'btoa' failed: The string to be encoded contains characters outside of the Latin1 range."
      )
    }
    block = (block << 8) | charCode
  }
  return output
}

// decoder
// [https://gist.github.com/1020396] by [https://github.com/atk]
function atob(input) {
  var str = String(input).replace(/[=]+$/, '') // #31: ExtendScript bad parse of /=
  if (str.length % 4 == 1) {
    throw new InvalidCharacterError("'atob' failed: The string to be decoded is not correctly encoded.")
  }
  for (
    // initialize result and counters
    var bc = 0, bs, buffer, idx = 0, output = '';
    // get next character
    (buffer = str.charAt(idx++));
    // character found in table? initialize bit storage and add its ascii value;
    ~buffer &&
    ((bs = bc % 4 ? bs * 64 + buffer : buffer),
    // and if not first of each 4 characters,
    // convert the first 8 bits to one ascii character
    bc++ % 4)
      ? (output += String.fromCharCode(255 & (bs >> ((-2 * bc) & 6))))
      : 0
  ) {
    // try to find character in table (0-63, not found => -1)
    buffer = chars.indexOf(buffer)
  }
  output = utf8.utf8to16(output) // 支持中文
  return output
}

module.exports = {
  encode: btoa,
  decode: atob
}