feathersjs/feathers

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outline: deep
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# Hooks

## Generating a hook

A new hook can be generated via

```
npx feathers generate hook
```

## Hook name

The hook generator will first ask for a name. Based on the name it will create a kebab-cased filename in the `hooks/` folder that exports a camelCased hook function. For example a name of `my fancy Hook` will create a `src/my-fancy-hook.ts` file that exports a `myFancyHook` [hook function](../../api/hooks.md#hook-functions).

## Hook types

There are two hook types that can be generated.

<BlockQuote type="tip">

For more information see the [hooks API documentation](../../api/hooks.md).

</BlockQuote>

### Around hooks

[Around hooks](../../api/hooks.md#around) allow to control the entire `before`, `after` and `error` flow in a single function. An `around` hook is an `async` function that accepts two arguments:

- The [hook context](../../api/hooks.md#hook-context)
- An asynchronous `next` function. Somewhere in the body of the hook function, there is a call to `await next()`, which calls the `next` hooks OR the original function if all other hooks have run.

```ts
import type { HookContext, NextFunction } from '../declarations'

export const myFancyHook = async (context: HookContext, next: NextFunction) => {
  console.log(`Running hook ${name} on ${context.path}.${context.method}`)
  await next()
  // Do things after here
}
```

You can wrap the `await next()` in a `try/catch` block to also handle errors.

### Before, after, error

[Before, after or error hooks](../../api/hooks.md#before-after-and-error) are `async` functions that take the [hook context](#hook-context) as the parameter.

```ts
import type { HookContext } from '../declarations'

export const myFancyHook = async (context: HookContext) => {
  console.log(`Running hook ${name} on ${context.path}.${context.method}`)
}
```

## Context types

If the hook is for a specific service, you can pass the service as a generic to the [HookContext](./declarations.md#hook-context) type which will give you the correct types for [context.data](../../api/hooks.md#contextdata), [context.result](../../api/hooks.md#contextresult) and [context.params](../../api/hooks.md#contextparams):

```ts
import type { UserService } from '../services/users/users'
import type { HookContext } from '../declarations'

export const myFancyUserHook = async (context: HookContext<UserService>) => {
  console.log(`Running hook ${name} on ${context.path}.${context.method}`)
}
```

## Registering hooks

A generated hook can be registered as an [application hook](./app.md#application-hooks) or as a [service hook](./service.md#registering-hooks). Also see the [hook registration API documentation](../../api/hooks.md#registering-hooks).

## Profiling example

To log some basic profiling information like which method was called and how long it took to run you can create a new _around_ hook called `profiler` via

```
npx feathers generate hook
```

Then update `src/hooks/profiler.ts` as follows:

```ts
import type { HookContext, NextFunction } from '../declarations'
import { logger } from '../logger'

export const profiler = async (context: HookContext, next: NextFunction) => {
  const startTime = Date.now()

  await next()

  const runtime = Date.now() - startTime

  console.log(`Calling ${context.method} on service ${context.path} took ${runtime}ms`)
}
```

And add it in `src/app.ts` as an application hook after the `logError` hook as follows:

```ts{1,8}
import { profiler } from './hooks/profiler'

//...

// Register hooks that run on all service methods
app.hooks({
  around: {
    all: [ logError, profiler ]
  },
  before: {},
  after: {},
  error: {}
})
```