Simple JWT deocder

decode-jwt.js

/**
 * @typedef {object} Header - JOSE Header https://datatracker.ietf.org/doc/html/rfc7519#section-5
 * @property {string} typ - Type https://datatracker.ietf.org/doc/html/rfc7519#section-5.1
 * @property {string} alg - Algorithm https://datatracker.ietf.org/doc/html/rfc7515#section-4.1.1
 */

/**
 * @param {string} char - a string containing a single character
 */
const toURIComponent = (char) => '%' + ('00' + char.charCodeAt(0).toString(16)).slice(-2);

/**
 * @template T
 * @param {string} part - The encoded header or payload part of the JWT to decode
 * @returns {T} - object containing the payload of the JWT
 */
const decodeJWTPart = (part) =>
  JSON.parse(decodeURIComponent(Array.from(window.atob(part)).map(toURIComponent).join('')));

/**
 * @template T - The payload type
 * @param {string} jwt - The JWT to decode
 * @returns {{ header: Header, payload: T, signature: string }}
 */
export function decodeJWT(jwt) {
  const [encodedHeader, encodedPayload, signature] = jwt.split('.');
  return {
    header: decodeJWTPart(encodedHeader),
    payload: decodeJWTPart(encodedPayload),
    signature,
  };
}