it-tools/src/tools/jwt-parser/jwt-parser.service.ts

import jwtDecode, { type JwtHeader, type JwtPayload } from 'jwt-decode';
import _ from 'lodash';
import { match } from 'ts-pattern';
import { ALGORITHM_DESCRIPTIONS, CLAIM_DESCRIPTIONS } from './jwt-parser.constants';

export { decodeJwt };

function decodeJwt({ jwt }: { jwt: string }) {
  const rawHeader = jwtDecode<JwtHeader>(jwt, { header: true });
  const rawPayload = jwtDecode<JwtPayload>(jwt);

  const header = _.map(rawHeader, (value, claim) => parseClaims({ claim, value }));
  const payload = _.map(rawPayload, (value, claim) => parseClaims({ claim, value }));

  return {
    header,
    payload,
  };
}

function parseClaims({ claim, value }: { claim: string; value: unknown }) {
  const claimDescription = CLAIM_DESCRIPTIONS[claim];
  const formattedValue = _.isPlainObject(value) ? JSON.stringify(value, null, 3) : _.toString(value);
  const friendlyValue = getFriendlyValue({ claim, value });

  return {
    value: formattedValue,
    friendlyValue,
    claim,
    claimDescription,
  };
}

function getFriendlyValue({ claim, value }: { claim: string; value: unknown }) {
  return match(claim)
    .with('exp', 'nbf', 'iat', () => dateFormatter(value))
    .with('alg', () => (_.isString(value) ? ALGORITHM_DESCRIPTIONS[value] : undefined))
    .otherwise(() => undefined);
}

const dateFormatter = (value: unknown) => {
  if (_.isNil(value)) return undefined;

  const date = new Date(Number(value) * 1000);
  return `${date.toLocaleDateString()} ${date.toLocaleTimeString()}`;
};