Simple JWT decoder

info.md

Gets a JS object which contains the decoded body of the JWT. For now I am pasting this in to postman tests when I need to get JWT decoding

#Important Does not validate the token at all!

universal_jwt_decode.js

function jwt_decode(jwt){
    var parts = null;
    try{
        parts = decode_b64(jwt.split('.')[1]);
    } catch (e){
        console.log("error parsing JWT");
        throw (e)
    }
    console.log(parts)
    if (parts){
        return JSON.parse(parts);
    }
    return {};
}

function InvalidCharacterError(message) {
  this.message = message;
}

InvalidCharacterError.prototype = new Error();
InvalidCharacterError.prototype.name = 'InvalidCharacterError';

function atob (input) {
  var chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';
  var str = String(input).replace(/=+$/, '');
  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);
  }
  return output;
}
function b64DecodeUnicode(str) {
  return decodeURIComponent(atob(str).replace(/(.)/g, function (m, p) {
    var code = p.charCodeAt(0).toString(16).toUpperCase();
    if (code.length < 2) {
      code = '0' + code;
    }
    return '%' + code;
  }));
}

function decode_b64(str) {
  var output = str.replace(/-/g, "+").replace(/_/g, "/");
  switch (output.length % 4) {
    case 0:
      break;
    case 2:
      output += "==";
      break;
    case 3:
      output += "=";
      break;
    default:
      throw "Illegal base64url string!";
  }

  try{
    return b64DecodeUnicode(output);
  } catch (err) {
    return atob(output);
  }
};

Hmm, when I tried locally with one of our projects, I didn't need to implement atob myself--it seems to be provided in the JS environment Postman exposes. (It's commonly part of browsers too, so I suspected it might be.)

The following seems to work at least with our tokens, in Postman 6.7.1:

function jwt_decode(jwt) {
    var parts = jwt.split('.'); // header, payload, signature
    return JSON.parse(atob(parts[1]));
}