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Errors

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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 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.

Catching Errors In Promises

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.

Catching Uncaught Errors In Fastify

Fastify tries to catch as many uncaught errors as it can without hindering performance. This includes:

synchronous routes, e.g. app.get('/', () => { throw new Error('kaboom') })
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.

Errors In Fastify Lifecycle Hooks And A Custom Error Handler

From the Hooks documentation:

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.

When a custom error handler has been defined through setErrorHandler, the custom error handler will receive the error passed to the done() callback (or through other supported automatic error handling mechanisms). If setErrorHandler has been used multiple times to define multiple handlers, the error will be routed to the most precedent handler defined within the error encapsulation context. Error handlers are fully encapsulated, so a setErrorHandler call within a plugin will limit the error handler to that plugin's context.

The root error handler is Fastify's generic error handler. This error handler will use the headers and status code in the Error object, if they exist. The headers and status code will not be automatically set if a custom error handler is provided.

Some things to consider in your custom error handler:

you can reply.send(data), which will behave as it would in regular route handlers
- 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 call the parent errorHandler.
- onError hook will be triggered once only for the first error being thrown.
- 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)

When utilizing Fastify's custom error handling through setErrorHandler, you should be aware of how errors are propagated between custom and default error handlers.

If a plugin's error handler re-throws an error, and the error is not an instance of Error (as seen in the /bad route in the following example), it will not propagate to the parent context error handler. Instead, it will be caught by the default error handler.

To ensure consistent error handling, it is recommended to throw instances of Error. For instance, in the following example, replacing throw 'foo' with throw new Error('foo') in the /bad route ensures that errors propagate through the custom error handling chain as intended. This practice helps avoid potential pitfalls when working with custom error handling in Fastify.

For example:

const Fastify = require('fastify')

// Instantiate the framework
const fastify = Fastify({
  logger: true
})

// Register parent error handler
fastify.setErrorHandler((error, request, reply) => {
  reply.status(500).send({ ok: false })
})

fastify.register((app, options, next) => {
  // Register child error handler
  fastify.setErrorHandler((error, request, reply) => {
    throw error
  })

  fastify.get('/bad', async () => {
    // Throws a non-Error type, 'bar'
    throw 'foo'
  })

  fastify.get('/good', async () => {
    // Throws an Error instance, 'bar'
    throw new Error('bar')
  })

  next()
})

// Run the server
fastify.listen({ port: 3000 }, function (err, address) {
  if (err) {
    fastify.log.error(err)
    process.exit(1)
  }
  // Server is listening at ${address}
})

Fastify Error Codes

You can access errorCodes for mapping:

// ESM
import { errorCodes } from 'fastify'

// CommonJs
const errorCodes = require('fastify').errorCodes

For example:

const Fastify = require('fastify')

// Instantiate the framework
const fastify = Fastify({
  logger: true
})

// Declare a route
fastify.get('/', function (request, reply) {
  reply.code('bad status code').send({ hello: 'world' })
})

fastify.setErrorHandler(function (error, request, reply) {
  if (error instanceof Fastify.errorCodes.FST_ERR_BAD_STATUS_CODE) {
    // Log error
    this.log.error(error)
    // Send error response
    reply.status(500).send({ ok: false })
  } else {
    // fastify will use parent error handler to handle this
    reply.send(error)
  }
})

// Run the server!
fastify.listen({ port: 3000 }, function (err, address) {
  if (err) {
    fastify.log.error(err)
    process.exit(1)
  }
  // Server is now listening on ${address}
})

Below is a table with all the error codes that Fastify uses.

Code	Description	How to solve	Discussion
FST_ERR_NOT_FOUND	404 Not Found	-	#1168
FST_ERR_OPTIONS_NOT_OBJ	Fastify options wrongly specified.	Fastify options should be an object.	#4554
FST_ERR_QSP_NOT_FN	QueryStringParser wrongly specified.	QueryStringParser option should be a function.	#4554
FST_ERR_SCHEMA_CONTROLLER_BUCKET_OPT_NOT_FN	SchemaController.bucket wrongly specified.	SchemaController.bucket option should be a function.	#4554
FST_ERR_SCHEMA_ERROR_FORMATTER_NOT_FN	SchemaErrorFormatter option wrongly specified.	SchemaErrorFormatter option should be a non async function.	#4554
FST_ERR_AJV_CUSTOM_OPTIONS_OPT_NOT_OBJ	ajv.customOptions wrongly specified.	ajv.customOptions option should be an object.	#4554
FST_ERR_AJV_CUSTOM_OPTIONS_OPT_NOT_ARR	ajv.plugins option wrongly specified.	ajv.plugins option should be an array.	#4554
FST_ERR_VERSION_CONSTRAINT_NOT_STR	Version constraint wrongly specified.	Version constraint should be a string.	#4554
FST_ERR_CTP_ALREADY_PRESENT	The parser for this content type was already registered.	Use a different content type or delete the already registered parser.	#1168
FST_ERR_CTP_INVALID_TYPE	`Content-Type` wrongly specified	The `Content-Type` should be a string.	#1168
FST_ERR_CTP_EMPTY_TYPE	`Content-Type` is an empty string.	`Content-Type` cannot be an empty string.	#1168
FST_ERR_CTP_INVALID_HANDLER	Invalid handler for the content type.	Use a different handler.	#1168
FST_ERR_CTP_INVALID_PARSE_TYPE	The provided parse type is not supported.	Accepted values are `string` or `buffer`.	#1168
FST_ERR_CTP_BODY_TOO_LARGE	The request body is larger than the provided limit.	Increase the limit in the Fastify server instance setting: bodyLimit	#1168
FST_ERR_CTP_INVALID_MEDIA_TYPE	The received media type is not supported (i.e. there is no suitable `Content-Type` parser for it).	Use a different content type.	#1168
FST_ERR_CTP_INVALID_CONTENT_LENGTH	Request body size did not match `Content-Length`.	Check the request body size and the `Content-Length` header.	#1168
FST_ERR_CTP_EMPTY_JSON_BODY	Body cannot be empty when content-type is set to `application/json`.	Check the request body.	#1253
FST_ERR_CTP_INSTANCE_ALREADY_STARTED	Fastify is already started.	-	#4554
FST_ERR_INSTANCE_ALREADY_LISTENING	Fastify instance is already listening.	-	#4554
FST_ERR_DEC_ALREADY_PRESENT	A decorator with the same name is already registered.	Use a different decorator name.	#1168
FST_ERR_DEC_DEPENDENCY_INVALID_TYPE	The dependencies of decorator must be of type `Array`.	Use an array for the dependencies.	#3090
FST_ERR_DEC_MISSING_DEPENDENCY	The decorator cannot be registered due to a missing dependency.	Register the missing dependency.	#1168
FST_ERR_DEC_AFTER_START	The decorator cannot be added after start.	Add the decorator before starting the server.	#2128
FST_ERR_HOOK_INVALID_TYPE	The hook name must be a string.	Use a string for the hook name.	#1168
FST_ERR_HOOK_INVALID_HANDLER	The hook callback must be a function.	Use a function for the hook callback.	#1168
FST_ERR_HOOK_INVALID_ASYNC_HANDLER	Async function has too many arguments. Async hooks should not use the `done` argument.	Remove the `done` argument from the async hook.	#4367
FST_ERR_HOOK_NOT_SUPPORTED	The hook is not supported.	Use a supported hook.	#4554
FST_ERR_MISSING_MIDDLEWARE	You must register a plugin for handling middlewares, visit `Middleware` for more info.	Register a plugin for handling middlewares.	#2014
FST_ERR_HOOK_TIMEOUT	A callback for a hook timed out.	Increase the timeout for the hook.	#3106
FST_ERR_LOG_INVALID_DESTINATION	The logger does not accept the specified destination.	Use a `'stream'` or a `'file'` as the destination.	#1168
FST_ERR_LOG_INVALID_LOGGER	The logger should have all these methods: `'info'`, `'error'`, `'debug'`, `'fatal'`, `'warn'`, `'trace'`, `'child'`.	Use a logger with all the required methods.	#4520
FST_ERR_REP_INVALID_PAYLOAD_TYPE	Reply payload can be either a `string` or a `Buffer`.	Use a `string` or a `Buffer` for the payload.	#1168
FST_ERR_REP_RESPONSE_BODY_CONSUMED	Using `Response` as reply payload
but the body is being consumed.	Make sure you don't consume the `Response.body`	#5286
FST_ERR_REP_ALREADY_SENT	A response was already sent.	-	#1336
FST_ERR_REP_SENT_VALUE	The only possible value for `reply.sent` is `true`.	-	#1336
FST_ERR_SEND_INSIDE_ONERR	You cannot use `send` inside the `onError` hook.	-	#1348
FST_ERR_SEND_UNDEFINED_ERR	Undefined error has occurred.	-	#2074
FST_ERR_BAD_STATUS_CODE	The status code is not valid.	Use a valid status code.	#2082
FST_ERR_BAD_TRAILER_NAME	Called `reply.trailer` with an invalid header name.	Use a valid header name.	#3794
FST_ERR_BAD_TRAILER_VALUE	Called `reply.trailer` with an invalid type. Expected a function.	Use a function.	#3794
FST_ERR_FAILED_ERROR_SERIALIZATION	Failed to serialize an error.	-	#4601
FST_ERR_MISSING_SERIALIZATION_FN	Missing serialization function.	Add a serialization function.	#3970
FST_ERR_MISSING_CONTENTTYPE_SERIALIZATION_FN	Missing `Content-Type` serialization function.	Add a serialization function.	#4264
FST_ERR_REQ_INVALID_VALIDATION_INVOCATION	Invalid validation invocation. Missing validation function for HTTP part nor schema provided.	Add a validation function.	#3970
FST_ERR_SCH_MISSING_ID	The schema provided does not have `$id` property.	Add a `$id` property.	#1168
FST_ERR_SCH_ALREADY_PRESENT	A schema with the same `$id` already exists.	Use a different `$id`.	#1168
FST_ERR_SCH_CONTENT_MISSING_SCHEMA	A schema is missing for the corresponding content type.	Add a schema.	#4264
FST_ERR_SCH_DUPLICATE	Schema with the same attribute already present!	Use a different attribute.	#1954
FST_ERR_SCH_VALIDATION_BUILD	The JSON schema provided for validation to a route is not valid.	Fix the JSON schema.	#2023
FST_ERR_SCH_SERIALIZATION_BUILD	The JSON schema provided for serialization of a route response is not valid.	Fix the JSON schema.	#2023
FST_ERR_SCH_RESPONSE_SCHEMA_NOT_NESTED_2XX	Response schemas should be nested under a valid status code (2XX).	Use a valid status code.	#4554
FST_ERR_HTTP2_INVALID_VERSION	HTTP2 is available only from node >= 8.8.1.	Use a higher version of node.	#1346
FST_ERR_INIT_OPTS_INVALID	Invalid initialization options.	Use valid initialization options.	#1471
FST_ERR_FORCE_CLOSE_CONNECTIONS_IDLE_NOT_AVAILABLE	Cannot set forceCloseConnections to `idle` as your HTTP server does not support `closeIdleConnections` method.	Use a different value for `forceCloseConnections`.	#3925
FST_ERR_DUPLICATED_ROUTE	The HTTP method already has a registered controller for that URL.	Use a different URL or register the controller for another HTTP method.	#2954
FST_ERR_BAD_URL	The router received an invalid url.	Use a valid URL.	#2106
FST_ERR_ASYNC_CONSTRAINT	The router received an error when using asynchronous constraints.	-	#4323
FST_ERR_DEFAULT_ROUTE_INVALID_TYPE	The `defaultRoute` type should be a function.	Use a function for the `defaultRoute`.	#2733
FST_ERR_INVALID_URL	URL must be a string.	Use a string for the URL.	#3653
FST_ERR_ROUTE_OPTIONS_NOT_OBJ	Options for the route must be an object.	Use an object for the route options.	#4554
FST_ERR_ROUTE_DUPLICATED_HANDLER	Duplicate handler for the route is not allowed.	Use a different handler.	#4554
FST_ERR_ROUTE_HANDLER_NOT_FN	Handler for the route must be a function.	Use a function for the handler.	#4554
FST_ERR_ROUTE_MISSING_HANDLER	Missing handler function for the route.	Add a handler function.	#4554
FST_ERR_ROUTE_METHOD_INVALID	Method is not a valid value.	Use a valid value for the method.	#4750
FST_ERR_ROUTE_METHOD_NOT_SUPPORTED	Method is not supported for the route.	Use a supported method.	#4554
FST_ERR_ROUTE_BODY_VALIDATION_SCHEMA_NOT_SUPPORTED	Body validation schema route is not supported.	Use a different different method for the route.	#4554
FST_ERR_ROUTE_BODY_LIMIT_OPTION_NOT_INT	`bodyLimit` option must be an integer.	Use an integer for the `bodyLimit` option.	#4554
FST_ERR_ROUTE_REWRITE_NOT_STR	`rewriteUrl` needs to be of type `string`.	Use a string for the `rewriteUrl`.	#4554
FST_ERR_REOPENED_CLOSE_SERVER	Fastify has already been closed and cannot be reopened.	-	#2415
FST_ERR_REOPENED_SERVER	Fastify is already listening.	-	#2415
FST_ERR_PLUGIN_VERSION_MISMATCH	Installed Fastify plugin mismatched expected version.	Use a compatible version of the plugin.	#2549
FST_ERR_PLUGIN_CALLBACK_NOT_FN	Callback for a hook is not a function.	Use a function for the callback.	#3106
FST_ERR_PLUGIN_NOT_VALID	Plugin must be a function or a promise.	Use a function or a promise for the plugin.	#3106
FST_ERR_ROOT_PLG_BOOTED	Root plugin has already booted.	-	#3106
FST_ERR_PARENT_PLUGIN_BOOTED	Impossible to load plugin because the parent (mapped directly from `avvio`)	-	#3106
FST_ERR_PLUGIN_TIMEOUT	Plugin did not start in time.	Increase the timeout for the plugin.	#3106
FST_ERR_PLUGIN_NOT_PRESENT_IN_INSTANCE	The decorator is not present in the instance.	-	#4554
FST_ERR_VALIDATION	The Request failed the payload validation.	Check the request payload.	#4824
FST_ERR_LISTEN_OPTIONS_INVALID	Invalid listen options.	Check the listen options.	#4886
FST_ERR_ERROR_HANDLER_NOT_FN	Error Handler must be a function	Provide a function to `setErrorHandler`.	#5317

Errors​

Error Handling In Node.js​

Uncaught Errors​

Catching Errors In Promises​

Errors In Fastify​

Errors In Input Data​

Catching Uncaught Errors In Fastify​

Errors In Fastify Lifecycle Hooks And A Custom Error Handler​

Fastify Error Codes​