# Ack

**Kind:** Function

**Source:** [`packages/websockets/decorators/ack.decorator.ts`](https://github.com/nestjs/nest/blob/master/packages/websockets/decorators/ack.decorator.ts#L26)

**Part of:** [Websockets](subsystem-packages-websockets)

WebSockets `ack` parameter decorator.
Extracts the `ack` callback function from the arguments of a ws event.

This decorator signals to the framework that the `ack` callback will be
handled manually within the method, preventing the framework from
automatically sending an acknowledgement based on the return value.

`Ack` is a WebSocket parameter decorator that injects the event acknowledgement callback into a handler method. Using this decorator tells the framework that the handler will send its own acknowledgement response instead of automatically acknowledging the event from the method’s return value.

## Signature

```ts
function Ack(): ParameterDecorator
```

**Returns:** `ParameterDecorator`

## Diagram

```mermaid
graph LR
  Client[WebSocket client] -->|emit event with ack callback| Gateway[Gateway event handler]
  Gateway -->|@Ack()| AckParam[ack callback parameter]
  AckParam -->|manual acknowledgement| Client
  Gateway -. return value is not auto-acknowledged .-> Framework[WebSocket framework]
```

## Usage

```ts
import { Ack, SubscribeMessage } from '@nestjs/websockets';

export class ChatGateway {
  @SubscribeMessage('send-message')
  handleMessage(
    @Ack() ack: (response: { ok: boolean; messageId?: string }) => void,
  ) {
    const messageId = crypto.randomUUID();

    // Perform application work before responding.
    ack({
      ok: true,
      messageId,
    });
  }
}
```

## AI Coding Instructions

- Use `@Ack()` only on WebSocket event handler parameters where the acknowledgement must be sent manually.
- Call the injected `ack` callback with the response payload expected by the connected client.
- Do not rely on the handler return value to acknowledge an event when `@Ack()` is present.
- Keep acknowledgement handling inside the handler or delegate it to a service, ensuring the callback is invoked after the relevant async work completes.
- Ensure client-side event emissions provide an acknowledgement callback when the server handler expects one.

## Used by

1 reference from 1 file. Each is a place in this repository where the symbol is actually used — go read one rather than trusting an example.

### Imported by (1)

- `AckGateway` — `integration/websockets/src/ack.gateway.ts`:8