JWT to JSON converter that can be used as a Slack Command at https://putsreq.com

jwt_to_json_slack.js

// Parse JWT and respond with a JSON
// Used for slack command
//
// Created by Chris Ziogas on 4/9/2016
//
// For using it copy the following code in https://putsreq.com
// and then register a new Slack command

// polyfill for window.btoa and window.atob
!function(){function t(t){this.message=t}var r="undefined"!=typeof exports?exports:this,e="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";t.prototype=new Error,t.prototype.name="InvalidCharacterError",r.btoa||(r.btoa=function(r){for(var o,n,a=String(r),i=0,c=e,d="";a.charAt(0|i)||(c="=",i%1);d+=c.charAt(63&o>>8-i%1*8)){if(n=a.charCodeAt(i+=.75),n>255)throw new t("'btoa' failed: The string to be encoded contains characters outside of the Latin1 range.");o=o<<8|n}return d}),r.atob||(r.atob=function(r){var o=String(r).replace(/=+$/,"");if(o.length%4==1)throw new t("'atob' failed: The string to be decoded is not correctly encoded.");for(var n,a,i=0,c=0,d="";a=o.charAt(c++);~a&&(n=i%4?64*n+a:a,i++%4)?d+=String.fromCharCode(255&n>>(-2*i&6)):0)a=e.indexOf(a);return d})}();

function base64urlDecode(str) {
  str += Array(5 - (str.length % 4 || 4)).join('=');
  str = str.replace(/\-/g, '+').replace(/_/g, '/');
  return decodeURIComponent(escape(atob(str)));
}

// parse JWT from Slack command
var segments = request.params.text ? request.params.text.split('.') : [];

// All segment should be base64
var headerSeg = segments[0];
var payloadSeg = segments[1];
var signatureSeg = segments[2];

// base64 decode and parse JSON
var header = JSON.parse(base64urlDecode(headerSeg)) || undefined;
var payload = JSON.parse(base64urlDecode(payloadSeg)) || undefined;

var res = {
    "header": header,
    "payload": payload,
    "signature": signatureSeg
};

// respond with a JSON
response.headers["Content-Type"] = "application/json";

response.body = {
    // share with rest in channel
    "response_type": "in_channel",
    "text": JSON.stringify(res, null, 2),
    "attachments": [
        {
            "title": "Check JWT at jwt.io",
            "title_link": "https://jwt.io/?value=" + request.params.text,
            "color": "#764FA5"
        }
    ]
}