Issue verifying token via query param using FastifyJWT: TypeError - request.jwtVerify is not a function

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04-10-2024

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Decoding the "request.jwtVerify is not a function" Error with FastifyJWT

The Problem: You're attempting to verify a JWT token passed as a query parameter in your Fastify application using the FastifyJWT plugin, but you're met with the cryptic error "TypeError: request.jwtVerify is not a function".

In simpler terms: You're trying to use a built-in method to check if the token is valid, but FastifyJWT isn't providing it. This typically happens when you're not correctly configuring the plugin or accessing the token verification functionality in the wrong way.

Let's break it down:

Imagine you're building a protected API endpoint that requires a valid JWT token to access it. The token is passed in the URL as a query parameter. Your code might look something like this:

const fastify = require('fastify')({ logger: true });
const fastifyJWT = require('fastify-jwt');

fastify.register(fastifyJWT, {
  secret: 'your-secret-key',
});

fastify.get('/protected-route', async (request, reply) => {
  // Try to verify the token from the query parameter
  try {
    const decoded = await request.jwtVerify(); 
    reply.send({ message: 'Token verified!' });
  } catch (err) {
    reply.code(401).send({ message: 'Unauthorized' });
  }
});

fastify.listen(3000, (err, address) => {
  if (err) {
    fastify.log.error(err);
    process.exit(1);
  }
  fastify.log.info(`Server listening at ${address}`);
});

However, this code will throw the "TypeError: request.jwtVerify is not a function" error.

Why does this happen?

The request.jwtVerify function is not automatically added to your Fastify request object. FastifyJWT provides this functionality in a different way:

1. It modifies the request.headers object. FastifyJWT parses the token (either from a cookie, authorization header, or query parameter) and injects the decoded token into the request.headers object under the key 'Authorization'.
2. You access the token via the request.headers.Authorization property. This property will hold the decoded token if the verification is successful.

Here's the corrected code:

const fastify = require('fastify')({ logger: true });
const fastifyJWT = require('fastify-jwt');

fastify.register(fastifyJWT, {
  secret: 'your-secret-key',
});

fastify.get('/protected-route', {
  preHandler: fastify.authenticate, // Add authentication pre-handler
}, async (request, reply) => {
  // Access the decoded token from the request headers
  const decoded = request.headers.Authorization; 
  if (decoded) {
    reply.send({ message: 'Token verified!', decoded });
  } else {
    reply.code(401).send({ message: 'Unauthorized' });
  }
});

fastify.listen(3000, (err, address) => {
  if (err) {
    fastify.log.error(err);
    process.exit(1);
  }
  fastify.log.info(`Server listening at ${address}`);
});

Important Notes:

• fastify.authenticate Pre-Handler: The fastify.authenticate pre-handler is crucial. It triggers the JWT verification process, making the decoded token available in the request headers.
• Token Location: In this example, we're assuming the token is passed in the query parameter. If your token is located elsewhere (like in a cookie or authorization header), adjust your code accordingly.
• Error Handling: Make sure to handle potential errors during token verification gracefully.

By understanding the nuances of how FastifyJWT works, you can effectively leverage this powerful plugin to secure your Fastify APIs and avoid common pitfalls like the "request.jwtVerify is not a function" error.

Further Reading:

• FastifyJWT Documentation
• JWT (JSON Web Token) Specification

This article offers a practical solution to a common issue with FastifyJWT. It emphasizes understanding the plugin's mechanisms, providing you with a deeper insight into how to utilize its features effectively.