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The OnDisconnect class lets you register write operations that the server will execute if the client disconnects unexpectedly. This is commonly used for presence systems, cleanup tasks, and ensuring data consistency when a user goes offline. Obtain an OnDisconnect instance via DatabaseReference.onDisconnect(). 
const presenceRef = db.ref('status/alice');

presenceRef.onDisconnect().set('offline');
await presenceRef.set('online');
All OnDisconnect methods return a Promise that resolves once the server has acknowledged and registered the operation. The operation itself executes later, only if the client disconnects.

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Methods

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set

set(value: any): Promise<void>
Registers a set operation to run on disconnect. The specified value will replace any data at this location when the client disconnects.
ParameterTypeDescription
valueanyThe value to write on disconnect. Pass null to delete the data.
const ref = db.ref('rooms/room1/members/alice');

await ref.onDisconnect().set(null);
await ref.set({ name: 'Alice', joinedAt: ServerValue.TIMESTAMP });

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update

update(values: object): Promise<void>
Registers an update operation to run on disconnect. Only the specified children are modified; other children at this location are left intact.
ParameterTypeDescription
valuesobjectAn object containing the children to update on disconnect.
await db.ref('users/alice').onDisconnect().update({
  status: 'offline',
  lastSeen: ServerValue.TIMESTAMP,
});

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remove

remove(): Promise<void>
Registers a remove operation to run on disconnect. All data at this location (and its children) will be deleted when the client disconnects. 
const memberRef = db.ref('rooms/room1/members/alice');

await memberRef.onDisconnect().remove();
await memberRef.set({ name: 'Alice' });

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setWithPriority

setWithPriority(value: any, priority: string | number | null): Promise<void>
Registers a set operation with a priority to run on disconnect.
ParameterTypeDescription
valueanyThe value to write on disconnect.
prioritystring | number | nullThe priority to assign.
await db.ref('status/alice').onDisconnect().setWithPriority('offline', 0);

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cancel

cancel(): Promise<void>
Cancels all previously registered OnDisconnect operations at this location. The server will no longer perform any disconnect writes for this reference. 
const ref = db.ref('status/alice');

// Register a disconnect handler
await ref.onDisconnect().set('offline');

// Later, cancel it
await ref.onDisconnect().cancel();

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Usage Pattern

A typical presence system combines onDisconnect with a normal write to track whether a user is online. 
async function setupPresence(db: LarkDatabase, userId: string): Promise<void> {
  const statusRef = db.ref(`status/${userId}`);
  const connectedRef = db.ref('.info/connected');

  connectedRef.on('value', async (snapshot) => {
    if (!snapshot.val()) {
      return;
    }

    await statusRef.onDisconnect().set({
      state: 'offline',
      lastSeen: ServerValue.TIMESTAMP,
    });

    await statusRef.set({
      state: 'online',
      lastSeen: ServerValue.TIMESTAMP,
    });
  });
}