Function createLocalOnlySync

packages/db/src/local-only.ts:294–382

* Internal function to create Local-only sync configuration with transaction confirmation * * This captures the sync functions and provides synchronous confirmation of operations. * It creates a loopback sync that immediately confirms all optimistic operations, * making them permanent in the col

(
  initialData?: Array<T>,
)

 * @returns Object with sync configuration and confirmOperationsSync function
 */
function createLocalOnlySync<T extends object, TKey extends string | number>(
  initialData?: Array<T>,
) {
  // Capture sync functions and collection for transaction confirmation
  let syncBegin: (() => void) | null = null
  let syncWrite: ((message: { type: OperationType; value: T }) => void) | null =
    null
  let syncCommit: (() => void) | null = null
  let collection: Collection<T, TKey, LocalOnlyCollectionUtils> | null = null

  const sync: SyncConfig<T, TKey> = {
    /**
     * Sync function that captures sync parameters and applies initial data
     * @param params - Sync parameters containing begin, write, and commit functions
     * @returns Unsubscribe function (empty since no ongoing sync is needed)
     */
    sync: (params) => {
      const { begin, write, commit, markReady } = params

      // Capture sync functions and collection for later use
      syncBegin = begin
      syncWrite = write
      syncCommit = commit
      collection = params.collection
      params.collection._state.isLocalOnly = true

      // Apply initial data if provided
      if (initialData && initialData.length > 0) {
        // Mark initial data as local so $origin is 'local' for local-only collections
        for (const item of initialData) {
          const key = params.collection.getKeyFromItem(item)
          params.collection._state.pendingLocalChanges.add(key)
        }

        begin()
        initialData.forEach((item) => {
          write({
            type: `insert`,
            value: item,
          })
        })
        commit()
      }

      // Mark collection as ready since local-only collections are immediately ready
      markReady()

      // Return empty unsubscribe function - no ongoing sync needed
      return () => {}
    },
    /**
     * Get sync metadata - returns empty object for local-only collections
     * @returns Empty metadata object
     */
    getSyncMetadata: () => ({}),
  }

  /**