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Fastify Exception Handler ​

This guide explains how to integrate Lens's exception watcher with your Fastify application to monitor and report exceptions.

Configuration ​

When initializing Lens with your Fastify application, you can configure its exception watcher using the exceptionWatcherEnabled and registerErrorHandler options:

ts
await lens({
  app: fastify,
  exceptionWatcherEnabled: true, // Set to true to enable exception watching (default is true)
  registerErrorHandler: true, // Set to true to allow Lens to register its own error handler (default is true)
});

await fastify.listen({ port: 3000 });

console.log("Server listening on http://localhost:3000");

Fastify allows only one error handler at a time. If you need to use your own custom error handler, you should set registerErrorHandler to false. In this scenario, you can use the logException method returned by lens to manually log exceptions to Lens:

ts
const { logException } = await lens({
  app: fastify,
  exceptionWatcherEnabled: true,
  registerErrorHandler: false, // Set to false to use your custom error handler
});

fastify.setErrorHandler((err) => {
  logException(err);

  throw err;
});

Verification ​

To verify that the exception handler is working correctly, you can intentionally throw an exception in one of your Fastify routes. For example:

ts
fastify.get('/throw-error', () => {
  throw new Error("Something went wrong");
});

After triggering this route (e.g., by visiting http://localhost:3000/throw-error in your browser), navigate to the Lens /exceptions path in your browser. You should see the details of the captured exception there.