# BaseWsInstance

**Kind:** Interface

**Source:** [`packages/websockets/adapters/ws-adapter.ts`](https://github.com/nestjs/nest/blob/master/packages/websockets/adapters/ws-adapter.ts#L8)

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

`BaseWsInstance` defines the minimal WebSocket-like contract required by the WS adapter. Implementations must support event subscription through `on` and expose a `close` function so the adapter can listen for lifecycle events and terminate connections consistently.

## Properties

| Property | Type |
|---|---|
| `on` | `(event: string, callback: Function) => void` |
| `close` | `Function` |

## Diagram

```mermaid
graph LR
  Adapter[WS Adapter] --> Instance[BaseWsInstance]
  Instance --> On["on(event, callback)"]
  Instance --> Close["close()"]
  On --> Events[Connection and message events]
  Close --> Cleanup[Connection cleanup]
```

## Usage

```ts
import type { BaseWsInstance } from './ws-adapter';

function registerConnection(socket: BaseWsInstance) {
  socket.on('message', (data: unknown) => {
    console.log('Received message:', data);
  });

  socket.on('close', () => {
    console.log('Connection closed');
  });

  // Close the connection when it is no longer needed.
  socket.close();
}
```

## AI Coding Instructions

- Treat `BaseWsInstance` as an adapter boundary; only rely on `on` and `close` when writing transport-agnostic WebSocket code.
- Register event listeners with the expected event names supported by the underlying WebSocket implementation.
- Ensure `close` is called during shutdown, error handling, or resource cleanup paths.
- Avoid depending on implementation-specific socket APIs unless the adapter interface is explicitly extended.

## Relationships

- IMPORTS → `INestApplicationContext`
- IMPORTS → `WebSocketAdapter`
- IMPORTS → `WsMessageHandler`
- IMPORTS → `isFunction`
- IMPORTS → `NestApplication`