Fastify
## Errors
### Error Handling In Node.js
#### Uncaught Errors
In Node.js, uncaught errors are likely to cause memory leaks, file descriptor leaks, and other major production issues. [Domains](https://nodejs.org/en/docs/guides/domain-postmortem/) were a failed attempt to fix this.
Given that it is not possible to process all uncaught errors sensibly, the best way to deal with them is to [crash](https://nodejs.org/api/process.html#process_warning_using_uncaughtexception_correctly).
#### Catching Errors In Promises
In Node.js, unhandled promise rejections (that is, without a `.catch()` handler) can also cause memory and file descriptor leaks. While `unhandledRejection` is deprecated in Node.js, unhandled rejections will not throw, and still potentially leak. You should use a module like [`make-promises-safe`](https://github.com/mcollina/make-promises-safe) to ensure unhandled rejections _always_ throw.
If you are using promises, you should attach a `.catch()` handler synchronously.
### Errors In Fastify
Fastify follows an all-or-nothing approach and aims to be lean and optimal as much as possible. The developer is responsible for making sure that the errors are handled properly.
#### Errors In Input Data
Most errors are a result of unexpected input data, so we recommend [validating your input data against a JSON schema](Validation-and-Serialization.md).
#### Catching Uncaught Errors In Fastify
Fastify tries to catch as many uncaught errors as it can without hindering performance. This includes:
1. synchronous routes, e.g. `app.get('/', () => { throw new Error('kaboom') })`
2. `async` routes, e.g. `app.get('/', async () => { throw new Error('kaboom') })`
The error in both cases will be caught safely and routed to Fastify's default error handler for a generic `500 Internal Server Error` response.
To customize this behavior you should use [`setErrorHandler`](Server.md#seterrorhandler).
### Errors In Fastify Lifecycle Hooks And A Custom Error Handler
From the [Hooks documentation](Hooks.md#manage-errors-from-a-hook):
> If you get an error during the execution of your hook, just pass it to `done()` and Fastify will automatically close the request and send the appropriate error code to the user.
If you have defined a custom error handler for using `setErrorHandler` the error will be routed there. otherwise, it will be routed to Fastify’s generic error handler.
Some things to consider in your custom error handler:
- you can `reply.send(data)`, which will behave as it would in [regular route handlers](Reply.md#senddata)
- objects are serialized, triggering the `preSerialization` lifecycle hook if you have one defined
- strings, buffers, and streams are sent to the client, with appropriate headers (no serialization)
- You can throw a new error in your custom error handler
- errors (new error or the received error parameter re-thrown) - will trigger the `onError` lifecycle hook and send the error to the user
- an error will not be triggered twice from a lifecycle hook - Fastify internally monitors the error invocation to avoid infinite loops for errors thrown in the reply phases of the lifecycle. (those after the route handler)
### Fastify Error Codes
#### FST_ERR_BAD_URL
The router received an invalid url.
#### FST_ERR_CTP_ALREADY_PRESENT
The parser for this content type was already registered.
#### FST_ERR_CTP_BODY_TOO_LARGE
The request body is larger than the provided limit.
#### FST_ERR_CTP_EMPTY_TYPE
The content type cannot be an empty string.
#### FST_ERR_CTP_INVALID_CONTENT_LENGTH
Request body size did not match Content-Length.
#### FST_ERR_CTP_INVALID_HANDLER
An invalid handler was passed for the content type.
#### FST_ERR_CTP_INVALID_MEDIA_TYPE
The received media type is not supported (i.e. there is no suitable `Content-Type` parser for it).
#### FST_ERR_CTP_INVALID_PARSE_TYPE
The provided parse type is not supported. Accepted values are `string` or `buffer`.
#### FST_ERR_CTP_INVALID_TYPE
The `Content-Type` should be a string.
#### FST_ERR_DEC_ALREADY_PRESENT
A decorator with the same name is already registered.
#### FST_ERR_DEC_MISSING_DEPENDENCY
The decorator cannot be registered due to a missing dependency.
#### FST_ERR_HOOK_INVALID_HANDLER
The hook callback must be a function.
#### FST_ERR_HOOK_INVALID_TYPE
The hook name must be a string.
#### FST_ERR_LOG_INVALID_DESTINATION
The logger accepts either a `'stream'` or a `'file'` as the destination.
#### FST_ERR_PROMISE_NOT_FULFILLED
A promise may not be fulfilled with 'undefined' when statusCode is not 204.
#### FST_ERR_REP_ALREADY_SENT
A response was already sent.
#### FST_ERR_REP_INVALID_PAYLOAD_TYPE
Reply payload can be either a `string` or a `Buffer`.
#### FST_ERR_SCH_ALREADY_PRESENT
A schema with the same `$id` already exists.
#### FST_ERR_SCH_MISSING_ID
The schema provided does not have `$id` property.
#### FST_ERR_SCH_SERIALIZATION_BUILD
The JSON schema provided for serialization of a route response is not valid.
#### FST_ERR_SCH_VALIDATION_BUILD
The JSON schema provided for validation to a route is not valid.
#### FST_ERR_SEND_INSIDE_ONERR
You cannot use `send` inside the `onError` hook.
#### FST_ERR_SEND_UNDEFINED_ERR
Undefined error has occurred.