> ## Documentation Index
> Fetch the complete documentation index at: https://ship-jskiller1404-add-ommiting-private-fields-feature.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Events

> Events API reference

### `eventBus.on`

```typescript theme={null}
on: (
  eventName: string,
  handler: InMemoryEventHandler,
): void
```

```typescript theme={null}
import { eventBus, InMemoryEvent } from '@paralect/node-mongo';

const collectionName = 'users';

eventBus.on(`${collectionName}.created`, (data: InMemoryEvent<User>) => {
  try {
    const user = data.doc;

    console.log('user created', user);
  } catch (err) {
    logger.error(`${USERS}.created handler error: ${err}`);
  }
});

eventBus.on(`${collectionName}.updated`, (data: InMemoryEvent<User>) => {});

eventBus.on(`${collectionName}.deleted`, (data: InMemoryEvent<User>) => {});
```

In-memory events handler that listens for a CUD events.

**Parameters**

* eventName: `string`;\
  Events names to listen.\
  Valid format: `${collectionName}.created`, `${collectionName}.updated`, `${collectionName}.deleted`.
* handler: [`InMemoryEventHandler`](#inmemoryeventhandler);

**Returns** `void`.

### `eventBus.once`

```typescript theme={null}
once: (
  eventName: string,
  handler: InMemoryEventHandler,
): void
```

```typescript theme={null}
eventBus.once(`${USERS}.updated`, (data: InMemoryEvent<User>) => {
  try {
    const user = data.doc;

    console.log('user updated', user);
  } catch (err) {
    logger.error(`${USERS}.updated handler error: ${err}`);
  }
});
```

In-memory events handler that listens for a CUD events. **It will be called only once**.

**Parameters**

* eventName: `string`;\
  Events names to listen.\
  Valid format: `${collectionName}.created`, `${collectionName}.updated`, `${collectionName}.deleted`.
* handler: [`InMemoryEventHandler`](#inmemoryeventhandler);

**Returns** `void`.

### `eventBus.onUpdated`

```typescript theme={null}
onUpdated: (
  entity: string,
  properties: OnUpdatedProperties,
  handler: InMemoryEventHandler,
): void
```

```typescript theme={null}
import { eventBus, InMemoryEvent } from '@paralect/node-mongo';

eventBus.onUpdated('users', ['firstName', 'lastName'], async (data: InMemoryEvent<User>) => {
  try {
    await userService.atomic.updateOne(
      { _id: data.doc._id },
      { $set: { fullName: `${data.doc.firstName} ${data.doc.lastName}` } },
    );
  } catch (err) {
    console.log(`users onUpdated ['firstName', 'lastName'] handler error: ${err}`);
  }
});

eventBus.onUpdated('users', [{ fullName: 'John Wake', firstName: 'John' }, 'lastName'], () => {});

eventBus.onUpdated('users', ['oauth.google'], () => {});
```

In-memory events handler that listens for specific fields updates. It will be called when one of the provided `properties` updates.

**Parameters**

* entity: `string`;\
  Collection name for events listening.
* properties: [`OnUpdatedProperties`](#onupdatedproperties);\
  Properties whose update will trigger the event.
* handler: [`InMemoryEventHandler`](#inmemoryeventhandler);

**Returns** `void`.