[javascript] How to decode jwt token in javascript without using a library?

How can I decode the payload of JWT using JavaScript? Without a library. So the token just returns a payload object that can consumed by my front-end app.

Example token: xxxxxxxxx.XXXXXXXX.xxxxxxxx

And the result is the payload:

{exp: 10012016 name: john doe, scope:['admin']}

As "window" object is not present in nodejs environment, we could use the following lines of code :

let base64Url = token.split('.')[1]; // token you get
let base64 = base64Url.replace('-', '+').replace('_', '/');
let decodedData = JSON.parse(Buffer.from(base64, 'base64').toString('binary'));

It's working for me perfectly. Hope it helps.

Simple function with try - catch

const parseJwt = (token) => {
  try {
    return JSON.parse(atob(token.split('.')[1]));
  } catch (e) {
    return null;
  }
};

Thanks!

you can use pure javascript atob() function to decode token into a string:

atob(token.split('.')[1]);

or parse directly it into a json object:

JSON.parse(atob(token.split('.')[1]));

read about atob() and btoa() built-in javascript functions Base64 encoding and decoding - Web APIs | MDN.

You can use jwt-decode, so then you could write:

import jwt_decode from 'jwt-decode';

var token = 'eyJ0eXAiO.../// jwt token';

var decoded = jwt_decode(token);
console.log(decoded);
/*{exp: 10012016 name: john doe, scope:['admin']}*/

If you use Node.JS, You can use the native Buffer module by doing :

const token = 'eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWUsImp0aSI6ImU3YjQ0Mjc4LTZlZDYtNDJlZC05MTZmLWFjZDQzNzhkM2U0YSIsImlhdCI6MTU5NTg3NzUxOCwiZXhwIjoxNTk1ODgxMTE4fQ.WXyDlDMMSJAjOFF9oAU9JrRHg2wio-WolWAkAaY3kg4';
const base64Url = token.split('.')[1];
const decoded = Buffer.from(base64Url, 'base64').toString();
console.log(decoded)

And you're good to go :-)

Based on answers here and here:

const dashRE = /-/g;
const lodashRE = /_/g;

module.exports = function jwtDecode(tokenStr) {
  const base64Url = tokenStr.split('.')[1];
  if (base64Url === undefined) return null;
  const base64 = base64Url.replace(dashRE, '+').replace(lodashRE, '/');
  const jsonStr = Buffer.from(base64, 'base64').toString();
  return JSON.parse(jsonStr);
};

all features of jwt.io doesn't support all languages. In NodeJs you can use

var decoded = jwt.decode(token);

I use this function to get payload , header , exp(Expiration Time), iat (Issued At) based on this answer

function parseJwt(token) {
  try {
    // Get Token Header
    const base64HeaderUrl = token.split('.')[0];
    const base64Header = base64HeaderUrl.replace('-', '+').replace('_', '/');
    const headerData = JSON.parse(window.atob(base64Header));

    // Get Token payload and date's
    const base64Url = token.split('.')[1];
    const base64 = base64Url.replace('-', '+').replace('_', '/');
    const dataJWT = JSON.parse(window.atob(base64));
    dataJWT.header = headerData;

// TODO: add expiration at check ...


    return dataJWT;
  } catch (err) {
    return false;
  }
}

const jwtDecoded = parseJwt('YOUR_TOKEN') ;
if(jwtDecoded)
{
    console.log(jwtDecoded)
}

function parseJwt(token) {
  var base64Payload = token.split('.')[1];
  var payload = Buffer.from(base64Payload, 'base64');
  return JSON.parse(payload.toString());
}

let payload= parseJwt("eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c");
console.log("payload:- ", payload);

If using node, you might have to use buffer package:

npm install buffer
var Buffer = require('buffer/').Buffer

I found this code at jwt.io and it works well.

//this is used to parse base64
function url_base64_decode(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!';
  }
  var result = window.atob(output); //polifyll https://github.com/davidchambers/Base64.js
  try{
    return decodeURIComponent(escape(result));
  } catch (err) {
    return result;
  }
}

In some cases(certain development platforms),
the best answer(for now) faces a problem of invalid base64 length.
So, I needed a more stable way.

I hope it would help you.

In Node.js (TypeScript):

import { TextDecoder } from 'util';

function decode(jwt: string) {
    const { 0: encodedHeader, 1: encodedPayload, 2: signature, length } = jwt.split('.');

    if (length !== 3) {
        throw new TypeError('Invalid JWT');
    }

    const decode = (input: string): JSON => { return JSON.parse(new TextDecoder().decode(new Uint8Array(Buffer.from(input, 'base64')))); };

    return { header: decode(encodedHeader), payload: decode(encodedPayload), signature: signature };
}

With jose by panva on GitHub, you could use the minimal import { decode as base64Decode } from 'jose/util/base64url' and replace new Uint8Array(Buffer.from(input, 'base64')) with base64Decode(input). Code should then work in both browser and Node.js.

If you're using Typescript or vanilla JavaScript, here's a zero-dependency, ready to copy-paste in your project simple function (building on @Rajan Maharjan 's answer).

This answer is particularly good, not only because it does not depend on any npm module, but also because it does not depend an any node.js built-in module (like Buffer) that some other solutions here are using and of course would fail in the browser (unless polyfilled, but there's no reason to do that in the first place). Additionally JSON.parse can fail at runtime and this version (especially in Typescript) will force handling of that. The JSDoc annotations will make future maintainers of your code thankful. :)

/**
 * Returns a JS object representation of a Javascript Web Token from it's common encoded
 * string form.
 *
 * @export
 * @template T the expected shape of the parsed token
 * @param {string} token a Javascript Web Token in base64 encoded, `.` separated form
 * @returns {(T | undefined)} an object-representation of the token
 * or undefined if parsing failed
 */
export function getParsedJwt<T extends object = { [k: string]: string | number }>(
  token: string,
): T | undefined {
  try {
    return JSON.parse(atob(token.split('.')[1]))
  } catch {
    return undefined
  }
}

For completion, here's the vanilla javascript version too:

/**
 * Returns a JS object representation of a Javascript Web Token from it's common encoded
 * string form.
 *
 * @export
 * @template T the expected shape of the parsed token
 * @param {string} token a Javascript Web Token in base64 encoded, `.` separated form
 * @returns {(object | undefined)} an object-representation of the token
 * or undefined if parsing failed
 */
export function getParsedJwt(token) {
  try {
    return JSON.parse(atob(token.split('.')[1]))
  } catch (e) {
    return undefined
  }
}

Answer based from GitHub - auth0/jwt-decode. Altered the input/output to include string splitting and return object { header, payload, signature } so you can just pass the whole token.

var jwtDecode = function (jwt) {

        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(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);
            }
        }

        var jwtArray = jwt.split('.');

        return {
            header: decode(jwtArray[0]),
            payload: decode(jwtArray[1]),
            signature: decode(jwtArray[2])
        };

    };

Simple NodeJS Solution for Decoding a JSON Web Token (JWT)

function decodeTokenComponent(value) {
    const buff = new Buffer(value, 'base64')
    const text = buff.toString('ascii')
    return JSON.parse(text)
}

const token = 'xxxxxxxxx.XXXXXXXX.xxxxxxxx'
const [headerEncoded, payloadEncoded, signature] = token.split('.')
const [header, payload] = [headerEncoded, payloadEncoded].map(decodeTokenComponent)

console.log(`header: ${header}`)
console.log(`payload: ${payload}`)
console.log(`signature: ${signature}`)

@Peheje will work, but you will have problem with unicode. To fix it I use the code on https://stackoverflow.com/a/30106551/5277071;

let b64DecodeUnicode = str =>_x000D_
  decodeURIComponent(_x000D_
    Array.prototype.map.call(atob(str), c =>_x000D_
      '%' + ('00' + c.charCodeAt(0).toString(16)).slice(-2)_x000D_
    ).join(''))_x000D_
_x000D_
let parseJwt = token =>_x000D_
  JSON.parse(_x000D_
    b64DecodeUnicode(_x000D_
      token.split('.')[1].replace('-', '+').replace('_', '/')_x000D_
    )_x000D_
  )_x000D_
_x000D_
_x000D_
let form = document.getElementById("form")_x000D_
form.addEventListener("submit", (e) => {_x000D_
   form.out.value = JSON.stringify(_x000D_
      parseJwt(form.jwt.value)_x000D_
   )_x000D_
   e.preventDefault();_x000D_
})

textarea{width:300px; height:60px; display:block}

<form id="form" action="parse">_x000D_
  <textarea name="jwt">eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkrDtGhuIETDs8OoIiwiYWRtaW4iOnRydWV9.469tBeJmYLERjlKi9u6gylb-2NsjHLC_6kZNdtoOGsA</textarea>_x000D_
  <textarea name="out"></textarea>_x000D_
  <input type="submit" value="parse" />_x000D_
</form>

Here is a more feature-rich solution I just made after studying this question:

const parseJwt = (token) => {
    try {
        if (!token) {
            throw new Error('parseJwt# Token is required.');
        }

        const base64Payload = token.split('.')[1];
        let payload = new Uint8Array();

        try {
            payload = Buffer.from(base64Payload, 'base64');
        } catch (err) {
            throw new Error(`parseJwt# Malformed token: ${err}`);
        }

        return {
            decodedToken: JSON.parse(payload),
        };
    } catch (err) {
        console.log(`Bonus logging: ${err}`);

        return {
            error: 'Unable to decode token.',
        };
    }
};

Here's some usage samples:

const unhappy_path1 = parseJwt('sk4u7vgbis4ewku7gvtybrose4ui7gvtmalformedtoken');
console.log('unhappy_path1', unhappy_path1);

const unhappy_path2 = parseJwt('sk4u7vgbis4ewku7gvtybrose4ui7gvt.malformedtoken');
console.log('unhappy_path2', unhappy_path2);

const unhappy_path3 = parseJwt();
console.log('unhappy_path3', unhappy_path3);

const { error, decodedToken } = parseJwt('eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c');
if (!decodedToken.exp) {
    console.log('almost_happy_path: token has illegal claims (missing expires_at timestamp)', decodedToken);
    // note: exp, iat, iss, jti, nbf, prv, sub
}

I wasn't able to make that runnable in StackOverflow code snippet tool, but here's approximately what you would see if you ran that code:

I made the parseJwt function always return an object (to some degree for static-typing reasons).

This allows you to utilize syntax such as:

const { decodedToken, error } = parseJwt(token);

Then you can test at run-time for specific types of errors and avoid any naming collision.

If anyone can think of any low effort, high value changes to this code, feel free to edit my answer for the benefit of next(person).