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

Note: this does not validate the signature, it just extracts the JSON payload from the token, which could have been tampered with.

Browser

Working unicode text JWT parser function:

function parseJwt (token) {
    var base64Url = token.split('.')[1];
    var base64 = base64Url.replace(/-/g, '+').replace(/_/g, "https://stackoverflow.com/");
    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());
}