Calculate HMAC for JWT Signature (TypeScript/NodeJS)

jwt-sig-hmac.ts

import crypto from 'crypto';

export class JwtTokenVerifier {
  verifyToken(token: string) {
    const [headerEncoded, payloadEncoded, signature] = token.split('.');
    const header = this.decodeBase64ToAscii(headerEncoded);
    const payload = this.decodeBase64ToAscii(payloadEncoded);
    
    // Other validation checks here
    
    // Validate the token signature, ref ==> https://jwt.io/
    if (this.hmachWithSHA256(`${headerEncoded}.${payloadEncoded}`) !== signature){
      throw new Error('Token is invalid.')
    }
    
    return True;
  }

  private hmacWithSha256(input: string) {
    const hash = crypto.createHmac('sha256', "${secretKey}");
    hash.update(input, 'utf8');
    return hash.digest('base64url');
  }

  private decodeBase64ToAscii(encoded: string) {
    const buff = Buffer.from(encoded, 'base64');
    const text = buff.toString('ascii');

    return JSON.parse(text);
  }
}