[Home](./index.md) > [@firebase/database](./database.md) > [onChildAdded](./database.onchildadded_1.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. Signature: ```typescript export declare function onChildAdded(query: Query, callback: (snapshot: DataSnapshot, previousChildName: string | null) => 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) => 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 null if it is the first child. | | options | [ListenOptions](./database.listenoptions.md) | An object that can be used to configure onlyOnce, which then removes the listener after its first invocation. | Returns: [Unsubscribe](./database.unsubscribe.md) A function that can be invoked to remove the listener.