voluptasmollitia/docs-exp/database.onchildchanged.md

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[Home](./index.md) &gt; [@firebase/database](./database.md) &gt; [onChildChanged](./database.onchildchanged.md)

## onChildChanged() 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 `onChildChanged` event will be triggered when the data stored in a child (or any of its descendants) changes. Note that a single `child_changed` event may represent multiple changes to the child. The `DataSnapshot` passed to the callback will contain the new child contents. For ordering purposes, the callback is also 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 onChildChanged(query: Query, callback: (snapshot: DataSnapshot, previousChildName: string | null) => unknown, cancelCallback?: (error: Error) => unknown): 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. |

<b>Returns:</b>

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

A function that can be invoked to remove the listener.