---
id: BaseCollectionConfig
title: BaseCollectionConfig
---

# Interface: BaseCollectionConfig\<T, TKey, TSchema, TUtils, TReturn\>

Defined in: [packages/db/src/types.ts:385](https://github.com/TanStack/db/blob/main/packages/db/src/types.ts#L385)

## Extended by

- [`CollectionConfig`](../CollectionConfig.md)
- [`LocalStorageCollectionConfig`](../LocalStorageCollectionConfig.md)

## Type Parameters

### T

`T` *extends* `object` = `Record`\<`string`, `unknown`\>

### TKey

`TKey` *extends* `string` \| `number` = `string` \| `number`

### TSchema

`TSchema` *extends* `StandardSchemaV1` = `never`

### TUtils

`TUtils` *extends* [`UtilsRecord`](../../type-aliases/UtilsRecord.md) = [`UtilsRecord`](../../type-aliases/UtilsRecord.md)

### TReturn

`TReturn` = `any`

## Properties

### autoIndex?

```ts
optional autoIndex: "eager" | "off";
```

Defined in: [packages/db/src/types.ts:434](https://github.com/TanStack/db/blob/main/packages/db/src/types.ts#L434)

Auto-indexing mode for the collection.
When enabled, indexes will be automatically created for simple where expressions.

#### Default

```ts
"eager"
```

#### Description

- "off": No automatic indexing
- "eager": Automatically create indexes for simple where expressions in subscribeChanges (default)

***

### compare()?

```ts
optional compare: (x, y) => number;
```

Defined in: [packages/db/src/types.ts:445](https://github.com/TanStack/db/blob/main/packages/db/src/types.ts#L445)

Optional function to compare two items.
This is used to order the items in the collection.

#### Parameters

##### x

`T`

The first item to compare

##### y

`T`

The second item to compare

#### Returns

`number`

A number indicating the order of the items

#### Example

```ts
// For a collection with a 'createdAt' field
compare: (x, y) => x.createdAt.getTime() - y.createdAt.getTime()
```

***

### gcTime?

```ts
optional gcTime: number;
```

Defined in: [packages/db/src/types.ts:414](https://github.com/TanStack/db/blob/main/packages/db/src/types.ts#L414)

Time in milliseconds after which the collection will be garbage collected
when it has no active subscribers. Defaults to 5 minutes (300000ms).

***

### getKey()

```ts
getKey: (item) => TKey;
```

Defined in: [packages/db/src/types.ts:409](https://github.com/TanStack/db/blob/main/packages/db/src/types.ts#L409)

Function to extract the ID from an object
This is required for update/delete operations which now only accept IDs

#### Parameters

##### item

`T`

The item to extract the ID from

#### Returns

`TKey`

The ID string for the item

#### Example

```ts
// For a collection with a 'uuid' field as the primary key
getKey: (item) => item.uuid
```

***

### id?

```ts
optional id: string;
```

Defined in: [packages/db/src/types.ts:398](https://github.com/TanStack/db/blob/main/packages/db/src/types.ts#L398)

***

### onDelete?

```ts
optional onDelete: DeleteMutationFn<T, TKey, TUtils, TReturn>;
```

Defined in: [packages/db/src/types.ts:583](https://github.com/TanStack/db/blob/main/packages/db/src/types.ts#L583)

Optional asynchronous handler function called before a delete operation

#### Param

Object containing transaction and collection information

#### Returns

Promise resolving to any value

#### Examples

```ts
// Basic delete handler
onDelete: async ({ transaction, collection }) => {
  const deletedKey = transaction.mutations[0].key
  await api.deleteTodo(deletedKey)
}
```

```ts
// Delete handler with multiple items
onDelete: async ({ transaction, collection }) => {
  const keysToDelete = transaction.mutations.map(m => m.key)
  await api.deleteTodos(keysToDelete)
}
```

```ts
// Delete handler with confirmation
onDelete: async ({ transaction, collection }) => {
  const mutation = transaction.mutations[0]
  const shouldDelete = await confirmDeletion(mutation.original)
  if (!shouldDelete) {
    throw new Error('Delete cancelled by user')
  }
  await api.deleteTodo(mutation.original.id)
}
```

```ts
// Delete handler with optimistic rollback
onDelete: async ({ transaction, collection }) => {
  const mutation = transaction.mutations[0]
  try {
    await api.deleteTodo(mutation.original.id)
  } catch (error) {
    // Transaction will automatically rollback optimistic changes
    console.error('Delete failed, rolling back:', error)
    throw error
  }
}
```

***

### onInsert?

```ts
optional onInsert: InsertMutationFn<T, TKey, TUtils, TReturn>;
```

Defined in: [packages/db/src/types.ts:496](https://github.com/TanStack/db/blob/main/packages/db/src/types.ts#L496)

Optional asynchronous handler function called before an insert operation

#### Param

Object containing transaction and collection information

#### Returns

Promise resolving to any value

#### Examples

```ts
// Basic insert handler
onInsert: async ({ transaction, collection }) => {
  const newItem = transaction.mutations[0].modified
  await api.createTodo(newItem)
}
```

```ts
// Insert handler with multiple items
onInsert: async ({ transaction, collection }) => {
  const items = transaction.mutations.map(m => m.modified)
  await api.createTodos(items)
}
```

```ts
// Insert handler with error handling
onInsert: async ({ transaction, collection }) => {
  try {
    const newItem = transaction.mutations[0].modified
    const result = await api.createTodo(newItem)
    return result
  } catch (error) {
    console.error('Insert failed:', error)
    throw error // This will cause the transaction to fail
  }
}
```

```ts
// Insert handler with metadata
onInsert: async ({ transaction, collection }) => {
  const mutation = transaction.mutations[0]
  await api.createTodo(mutation.modified, {
    source: mutation.metadata?.source,
    timestamp: mutation.createdAt
  })
}
```

***

### onUpdate?

```ts
optional onUpdate: UpdateMutationFn<T, TKey, TUtils, TReturn>;
```

Defined in: [packages/db/src/types.ts:540](https://github.com/TanStack/db/blob/main/packages/db/src/types.ts#L540)

Optional asynchronous handler function called before an update operation

#### Param

Object containing transaction and collection information

#### Returns

Promise resolving to any value

#### Examples

```ts
// Basic update handler
onUpdate: async ({ transaction, collection }) => {
  const updatedItem = transaction.mutations[0].modified
  await api.updateTodo(updatedItem.id, updatedItem)
}
```

```ts
// Update handler with partial updates
onUpdate: async ({ transaction, collection }) => {
  const mutation = transaction.mutations[0]
  const changes = mutation.changes // Only the changed fields
  await api.updateTodo(mutation.original.id, changes)
}
```

```ts
// Update handler with multiple items
onUpdate: async ({ transaction, collection }) => {
  const updates = transaction.mutations.map(m => ({
    id: m.key,
    changes: m.changes
  }))
  await api.updateTodos(updates)
}
```

```ts
// Update handler with optimistic rollback
onUpdate: async ({ transaction, collection }) => {
  const mutation = transaction.mutations[0]
  try {
    await api.updateTodo(mutation.original.id, mutation.changes)
  } catch (error) {
    // Transaction will automatically rollback optimistic changes
    console.error('Update failed, rolling back:', error)
    throw error
  }
}
```

***

### schema?

```ts
optional schema: TSchema;
```

Defined in: [packages/db/src/types.ts:399](https://github.com/TanStack/db/blob/main/packages/db/src/types.ts#L399)

***

### startSync?

```ts
optional startSync: boolean;
```

Defined in: [packages/db/src/types.ts:425](https://github.com/TanStack/db/blob/main/packages/db/src/types.ts#L425)

Whether to eagerly start syncing on collection creation.
When true, syncing begins immediately. When false, syncing starts when the first subscriber attaches.

Note: Even with startSync=true, collections will pause syncing when there are no active
subscribers (typically when components querying the collection unmount), resuming when new
subscribers attach. This preserves normal staleTime/gcTime behavior.

#### Default

```ts
false
```

***

### syncMode?

```ts
optional syncMode: SyncMode;
```

Defined in: [packages/db/src/types.ts:454](https://github.com/TanStack/db/blob/main/packages/db/src/types.ts#L454)

The mode of sync to use for the collection.

#### Default

`eager`

#### Description

- `eager`: syncs all data immediately on preload
- `on-demand`: syncs data in incremental snapshots when the collection is queried
The exact implementation of the sync mode is up to the sync implementation.

***

### utils?

```ts
optional utils: TUtils;
```

Defined in: [packages/db/src/types.ts:585](https://github.com/TanStack/db/blob/main/packages/db/src/types.ts#L585)