veritatisdoloremque/docs-exp/database.query.on.md

Monorepo for the Firebase JavaScript SDK

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[Home](./index.md) &gt; [@firebase/database](./database.md) &gt; [Query](./database.query.md) &gt; [on](./database.query.on.md)

## Query.on() method

<b>Signature:</b>

```typescript
on(
    eventType: EventType,
    callback: (a: DataSnapshot, b?: string | null) => any,
    cancelCallbackOrContext?: ((a: Error) => any) | object | null,
    context?: object | null
  ): (a: DataSnapshot, b?: string | null) => any;
```

## Parameters

|  Parameter | Type | Description |
|  --- | --- | --- |
|  eventType | [EventType](./database.eventtype.md) |  |
|  callback | (a: [DataSnapshot](./database.datasnapshot.md)<!-- -->, b?: string \| null) =&gt; any |  |
|  cancelCallbackOrContext | ((a: Error) =&gt; any) \| object \| null |  |
|  context | object \| null |  |

<b>Returns:</b>

(a: [DataSnapshot](./database.datasnapshot.md)<!-- -->, b?: string \| null) =&gt; any