[Home](./index.md) > [@firebase/database](./database.md) > [Query](./database.query.md) > [once](./database.query.once.md) ## Query.once() method Signature: ```typescript once( eventType: EventType, successCallback?: (a: DataSnapshot, b?: string | null) => any, failureCallbackOrContext?: ((a: Error) => void) | object | null, context?: object | null ): Promise; ``` ## Parameters | Parameter | Type | Description | | --- | --- | --- | | eventType | [EventType](./database.eventtype.md) | | | successCallback | (a: [DataSnapshot](./database.datasnapshot.md), b?: string \| null) => any | | | failureCallbackOrContext | ((a: Error) => void) \| object \| null | | | context | object \| null | | Returns: Promise<[DataSnapshot](./database.datasnapshot.md)>