scanCollections.ts

const mappedInv$ = 
      // Gets all docs
      this.invService.getAll().pipe(
        // Stream each element of the array
        mergeMap((inventory) => from(inventory)),
        // Using each element of the array, do your next action
        mergeMap((inventoryItem: InventoryItem) =>
          // This use case involved getting more details from a difference collection
          this.itemService.getAsync(inventoryItem.itemId).pipe(
            take(1),
            map((item) => ({ ...inventoryItem, item } as InventoryItem))
          )
        ),
        // Collect all the results back into one array
        // Scan will collect over time, it may be worth using reduce()
        scan(
          (inventory: Inventory, inventoryItem) =>
            inventory.find((i) => i.id === inventoryItem.id)
              ? inventory
              : [...inventory, inventoryItem],
          []
        )
      );