voluptasmollitia/docs-exp/database.onchildadded_2.md

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[Home](./index.md) &gt; [@firebase/database](./database.md) &gt; [onChildAdded](./database.onchildadded_2.md)

## onChildAdded() function

Listens for data changes at a particular location.

This is the primary way to read data from a Database. Your callback will be triggered for the initial data and again whenever the data changes. Invoke the returned unsubscribe callback to stop receiving updates. See [Retrieve Data on the Web](https://firebase.google.com/docs/database/web/retrieve-data) for more details.

An `onChildAdded` event will be triggered once for each initial child at this location, and it will be triggered again every time a new child is added. The `DataSnapshot` passed into the callback will reflect the data for the relevant child. For ordering purposes, it is passed a second argument which is a string containing the key of the previous sibling child by sort order, or `null` if it is the first child.

<b>Signature:</b>

```typescript
export declare function onChildAdded(query: Query, callback: (snapshot: DataSnapshot, previousChildName: string | null) => unknown, cancelCallback: (error: Error) => unknown, options: ListenOptions): Unsubscribe;
```

## Parameters

|  Parameter | Type | Description |
|  --- | --- | --- |
|  query | [Query](./database.query.md) | The query to run. |
|  callback | (snapshot: [DataSnapshot](./database.datasnapshot.md)<!-- -->, previousChildName: string \| null) =&gt; unknown | A callback that fires when the specified event occurs. The callback will be passed a DataSnapshot and a string containing the key of the previous child, by sort order, or <code>null</code> if it is the first child. |
|  cancelCallback | (error: Error) =&gt; unknown | An optional callback that will be notified if your event subscription is ever canceled because your client does not have permission to read this data (or it had permission but has now lost it). This callback will be passed an <code>Error</code> object indicating why the failure occurred. |
|  options | [ListenOptions](./database.listenoptions.md) | An object that can be used to configure <code>onlyOnce</code>, which then removes the listener after its first invocation. |

<b>Returns:</b>

[Unsubscribe](./database.unsubscribe.md)

A function that can be invoked to remove the listener.