How to decode jwt token in javascript without using a library

Question

How can I use JavaScript to decode the payload of a JWT? There is no library. 

So the token just provides a payload object that my front-end app may consume.

Token example: xxxxxxxxx.XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

As a result, the payload is as follows:

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

Answer 1

Working unicode text JWT parser function:

function parseJwt (token) {
    var base64Url = token.split('.')[1];
    var base64 = base64Url.replace(/-/g, '+').replace(/_/g, '/');
    var jsonPayload = decodeURIComponent(window.atob(base64).split('').map(function(c) {
        return '%' + ('00' + c.charCodeAt(0).toString(16)).slice(-2);
    }).join(''));

    return JSON.parse(jsonPayload);
}

JWT uses base64url (RFC 4648 §5), so using only atob (which uses base64) isn't enough.

Node.js

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