fastify/docs/Reference/Decorators.md

Fast and low overhead web framework, for Node.js

<h1 align="center">Fastify</h1>

## Decorators

The decorators API customizes core Fastify objects, such as the server instance
and any request and reply objects used during the HTTP request lifecycle. It
can attach any type of property to core objects, e.g., functions, plain
objects, or native types.

This API is *synchronous*. Defining a decoration asynchronously could result in
the Fastify instance booting before the decoration completes. To register an
asynchronous decoration, use the `register` API with `fastify-plugin`. See the
[Plugins](./Plugins.md) documentation for more details.

Decorating core objects with this API allows the underlying JavaScript engine to
optimize the handling of server, request, and reply objects. This is
accomplished by defining the shape of all such object instances before they are
instantiated and used. As an example, the following is not recommended because
it will change the shape of objects during their lifecycle:

```js
// Bad example! Continue reading.

// Attach a user property to the incoming request before the request
// handler is invoked.
fastify.addHook('preHandler', function (req, reply, done) {
  req.user = 'Bob Dylan'
  done()
})

// Use the attached user property in the request handler.
fastify.get('/', function (req, reply) {
  reply.send(`Hello, ${req.user}`)
})
```

The above example mutates the request object after instantiation, causing the
JavaScript engine to deoptimize access. Using the decoration API avoids this
deoptimization:

```js
// Decorate request with a 'user' property
fastify.decorateRequest('user', '')

// Update our property
fastify.addHook('preHandler', (req, reply, done) => {
  req.user = 'Bob Dylan'
  done()
})
// And finally access it
fastify.get('/', (req, reply) => {
  reply.send(`Hello, ${req.user}!`)
})
```

Keep the initial shape of a decorated field close to its future dynamic value.
Initialize a decorator as `''` for strings and `null` for objects or functions.
This works only with value types; reference types will throw an error during
Fastify startup. See [decorateRequest](#decorate-request) and
[JavaScript engine fundamentals: Shapes
and Inline Caches](https://mathiasbynens.be/notes/shapes-ics)
for more information.

### Usage
<a id="usage"></a>

#### `decorate(name, value, [dependencies])`
<a id="decorate"></a>

This method customizes the Fastify [server](./Server.md) instance.

For example, to attach a new method to the server instance:

```js
fastify.decorate('utility', function () {
  // Something very useful
})
```

Non-function values can also be attached to the server instance:

```js
fastify.decorate('conf', {
  db: 'some.db',
  port: 3000
})
```

To access decorated properties, use the name provided to the decoration API:

```js
fastify.utility()

console.log(fastify.conf.db)
```

The decorated [Fastify server](./Server.md) is bound to `this` in
[route](./Routes.md) handlers:

```js
fastify.decorate('db', new DbConnection())

fastify.get('/', async function (request, reply) {
  // using return
  return { hello: await this.db.query('world') }

  // or
  // using reply.send()
  reply.send({ hello: await this.db.query('world') })
  await reply
})
```

The `dependencies` parameter is an optional list of decorators that the
decorator being defined relies upon. This list contains the names of other
decorators. In the following example, the "utility" decorator depends on the
"greet" and "hi" decorators:

```js
async function greetDecorator (fastify, opts) {
  fastify.decorate('greet', () => {
    return 'greet message'
  })
}

async function hiDecorator (fastify, opts) {
  fastify.decorate('hi', () => {
    return 'hi message'
  })
}

async function utilityDecorator (fastify, opts) {
  fastify.decorate('utility', () => {
    return `${fastify.greet()} | ${fastify.hi()}`
  })
}

fastify.register(fastifyPlugin(greetDecorator, { name: 'greet' }))
fastify.register(fastifyPlugin(hiDecorator, { name: 'hi' }))
fastify.register(fastifyPlugin(utilityDecorator, { dependencies: ['greet', 'hi'] }))

fastify.get('/', function (req, reply) {
  // Response: {"hello":"greet message | hi message"}
  reply.send({ hello: fastify.utility() })
})

fastify.listen({ port: 3000 }, (err, address) => {
  if (err) throw err
})
```

Using an arrow function breaks the binding of `this` to
the `FastifyInstance`.

If a dependency is not satisfied, the `decorate` method throws an exception.
The dependency check occurs before the server instance boots, not during
runtime.

#### `decorateReply(name, value, [dependencies])`
<a id="decorate-reply"></a>

This API adds new methods/properties to the core `Reply` object:

```js
fastify.decorateReply('utility', function () {
  // Something very useful
})
```

Using an arrow function will break the binding of `this` to the Fastify
`Reply` instance.

Using `decorateReply` will throw and error if used with a reference type:

```js
// Don't do this
fastify.decorateReply('foo', { bar: 'fizz'})
```
In this example, the object reference would be shared with all requests, and
**any mutation will impact all requests, potentially creating security
vulnerabilities or memory leaks**. Fastify blocks this.

To achieve proper encapsulation across requests configure a new value for each
incoming request in the [`'onRequest'` hook](./Hooks.md#onrequest).

```js
const fp = require('fastify-plugin')

async function myPlugin (app) {
  app.decorateReply('foo')
  app.addHook('onRequest', async (req, reply) => {
    reply.foo = { bar: 42 }
  })
}

module.exports = fp(myPlugin)
```

See [`decorate`](#decorate) for information about the `dependencies` parameter.

#### `decorateRequest(name, value, [dependencies])`
<a id="decorate-request"></a>

As with [`decorateReply`](#decorate-reply), this API adds new methods/properties
to the core `Request` object:

```js
fastify.decorateRequest('utility', function () {
  // something very useful
})
```

Using an arrow function will break the binding of `this` to the Fastify
`Request` instance.

Using `decorateRequest` will emit an error if used with a reference type:

```js
// Don't do this
fastify.decorateRequest('foo', { bar: 'fizz'})
```
In this example, the object reference would be shared with all requests, and
**any mutation will impact all requests, potentially creating security
vulnerabilities or memory leaks**. Fastify blocks this.

To achieve proper encapsulation across requests configure a new value for each
incoming request in the [`'onRequest'` hook](./Hooks.md#onrequest).

Example:

```js
const fp = require('fastify-plugin')

async function myPlugin (app) {
  app.decorateRequest('foo')
  app.addHook('onRequest', async (req, reply) => {
    req.foo = { bar: 42 }
  })
}

module.exports = fp(myPlugin)
```

The hook solution is more flexible and allows for more complex initialization
because more logic can be added to the `onRequest` hook.

Another approach is to use the getter/setter pattern, but it requires 2 decorators:

```js
fastify.decorateRequest('my_decorator_holder') // define the holder
fastify.decorateRequest('user', {
  getter () {
    this.my_decorator_holder ??= {} // initialize the holder
    return this.my_decorator_holder
  }
})

fastify.get('/', async function (req, reply) {
  req.user.access = 'granted'
  // other code
})
```

This ensures that the `user` property is always unique for each request.

See [`decorate`](#decorate) for information about the `dependencies` parameter.

#### `hasDecorator(name)`
<a id="has-decorator"></a>

Used to check for the existence of a server instance decoration:

```js
fastify.hasDecorator('utility')
```

#### hasRequestDecorator
<a id="has-request-decorator"></a>

Used to check for the existence of a Request decoration:

```js
fastify.hasRequestDecorator('utility')
```

#### hasReplyDecorator
<a id="has-reply-decorator"></a>

Used to check for the existence of a Reply decoration:

```js
fastify.hasReplyDecorator('utility')
```

### Decorators and Encapsulation
<a id="decorators-encapsulation"></a>

Defining a decorator (using `decorate`, `decorateRequest`, or `decorateReply`)
with the same name more than once in the same **encapsulated** context will
throw an exception. For example, the following will throw:

```js
const server = require('fastify')()

server.decorateReply('view', function (template, args) {
  // Amazing view rendering engine
})

server.get('/', (req, reply) => {
  reply.view('/index.html', { hello: 'world' })
})

// Somewhere else in our codebase, we define another
// view decorator. This throws.
server.decorateReply('view', function (template, args) {
  // Another rendering engine
})

server.listen({ port: 3000 })
```


But this will not:

```js
const server = require('fastify')()

server.decorateReply('view', function (template, args) {
  // Amazing view rendering engine.
})

server.register(async function (server, opts) {
  // We add a view decorator to the current encapsulated
  // plugin. This will not throw as outside of this encapsulated
  // plugin view is the old one, while inside it is the new one.
  server.decorateReply('view', function (template, args) {
    // Another rendering engine
  })

  server.get('/', (req, reply) => {
    reply.view('/index.page', { hello: 'world' })
  })
}, { prefix: '/bar' })

server.listen({ port: 3000 })
```

### Getters and Setters
<a id="getters-setters"></a>

Decorators accept special "getter/setter" objects with `getter` and optional
`setter` functions. This allows defining properties via decorators,
for example:

```js
fastify.decorate('foo', {
  getter () {
    return 'a getter'
  }
})
```

Will define the `foo` property on the Fastify instance:

```js
console.log(fastify.foo) // 'a getter'
```