Server-Side Development with Fastify — Error Handler, Route List, and Parsing Content

Fastify is a small Node framework for developing back end web apps.

In this article, we’ll look at how to create back end apps with Fastify.

Set Error Handler

We can call setErrorHandler to ad an error handler.

For example, we can write:

const fastify = require('fastify')({
  logger: true
})

fastify.setErrorHandler(function (error, request, reply) {
  this.log.error(error)
  reply.status(409).send({ ok: false })
})

fastify.get('/', async (request, reply) => {
  return 'hello world'
})

const start = async () => {
  try {
    await fastify.listen(3000, '0.0.0.0')
  } catch (err) {
    fastify.log.error(err)
    await fastify.close()
    process.exit(1)
  }
}
start()

to call setErrorHandler with our handler.

We can get the status code with the error.statusCode property:

const fastify = require('fastify')({
  logger: true
})

fastify.setErrorHandler(function (error, request, reply) {
  const { statusCode } = error.statusCode
  if (statusCode >= 500) {
    this.log.error(error)
  } else if (statusCode >= 400) {
    this.log.info(error)
  } else {
    this.log.error(error)
  }
  reply.status(409).send({ ok: false })
})

fastify.get('/', async (request, reply) => {
  return 'hello world'
})

const start = async () => {
  try {
    await fastify.listen(3000, '0.0.0.0')
  } catch (err) {
    fastify.log.error(err)
    await fastify.close()
    process.exit(1)
  }
}
start()

We check the statusCode in our error handler.

Print a List of Routes

We can print the list of routes with the fastify.printRoutes method.

For instance, we can write:

const fastify = require('fastify')({
  logger: true
})

fastify.ready(() => {
  console.log(fastify.printRoutes())
})

fastify.get('/', async (request, reply) => {
  return 'hello world'
})

const start = async () => {
  try {
    await fastify.listen(3000, '0.0.0.0')
  } catch (err) {
    fastify.log.error(err)
    process.exit(1)
  }
}
start()

to print the listen of routes in the fastify.ready handler.

We get:

└── / (GET)

because we defined the / route with get .

Content-Type Parser

We can add our own content type parser with the fastify.addContentTypeParser method.

For example, we can write:

const fastify = require('fastify')({
  logger: true
})

fastify.addContentTypeParser('text/json', { asString: true }, fastify.getDefaultJsonParser('ignore', 'ignore'))

fastify.get('/', async (request, reply) => {
  return 'hello world'
})

const start = async () => {
  try {
    await fastify.listen(3000, '0.0.0.0')
  } catch (err) {
    fastify.log.error(err)
    process.exit(1)
  }
}
start()

We add a content type handler for the text/json MIME-type.

Error Handler

We can add an error handler with the fastify.errorHandler method.

For example, we can write:

const fastify = require('fastify')({
  logger: true
})

fastify.get('/', {
  errorHandler: (error, request, reply) => {
    if (error.code === 'SOMETHING_SPECIFIC') {
      reply.send({ custom: 'response' })
      return
    }
    fastify.errorHandler(error, request, response)
  }
}, (request, reply) => {
  return 'hello world'
})

const start = async () => {
  try {
    await fastify.listen(3000, '0.0.0.0')
  } catch (err) {
    fastify.log.error(err)
    process.exit(1)
  }
}
start()

to add our / route with an error handler.

We check the error.code property to check for the type of error raised.

Then we call reply.send to return the response when an error occurred.

Conclusion

We can set an error handler, content type parser, and print a list of routes with Fastify.