Authentication Error Handling

Error Handling Overview

Error Handling is a building block you will reach for often in authentication. It keeps related logic together and makes your intent obvious to reviewers and future maintainers.

When you learn error handling properly, you avoid the guesswork that leads to bugs and rework. The example below shows the shape you will use in most real authentication projects.

import express from 'express';
import jwt from 'jsonwebtoken';

const app = express();
app.use(express.json());

function authenticate(req, res, next) {
  const token = req.headers.authorization?.replace('Bearer ', '');
  try {
    req.user = jwt.verify(token, process.env.JWT_SECRET);
    next();
  } catch {
    res.status(401).json({ error: 'Unauthorized' });
  }
}

Authentication middleware verifies the caller's identity before protected handlers run.

Error Handling Example

// verify the caller on every protected request
function authenticate(req, res, next) {
  const token = req.headers.authorization?.replace('Bearer ', '');
  req.user = verifyToken(token); // throws if invalid
  next();
}

• Start from a minimal Error Handling example and grow it only as needed.
• Keep configuration explicit so Error Handling behaves the same in every environment.
• Name things clearly so teammates understand your Error Handling at a glance.
• Add tests around Error Handling early to lock in expected behaviour.

Authentication Cheatsheet

Quick authentication reference related to error handling.

How Error Handling Works

Error Handling is part of proving who a user is and what they are allowed to do. Authentication verifies identity, while authorization decides access — and error handling plays a specific role in that flow.

Authentication middleware verifies the caller's identity before protected handlers run.

• Never store or log plain-text passwords or secrets.
• Prefer short-lived tokens with refresh over long-lived ones.
• Always send credentials over HTTPS.
• Fail closed: deny access when anything is uncertain.

Security Guidance for Error Handling

Security is the whole point of error handling. Small mistakes — weak hashing, tokens in localStorage, missing rate limits — are exactly what attackers look for, so follow proven defaults.

Common Mistakes

• Skipping error handling and edge cases when wiring up error handling.
• Leaving error handling untested, so regressions slip into production.
• Over-engineering error handling before you actually need the extra flexibility.
• Ignoring documentation, which makes error handling hard for the next developer to change.

Key Takeaways

• Error Handling is a core part of working effectively with authentication.
• Start small and keep error handling focused on a single responsibility.
• Apply consistent patterns so error handling scales across your project.
• Test and document error handling to keep it maintainable over time.