Single-or-Array Overload Pattern

Accept both single items and arrays, normalize internally, delegate to array-only implementation.

Quick Reference

// Option 1: Explicit overloads (cleaner IDE signatures)
function create(item: T): Promise<Result<T, E>>;
function create(items: T[]): Promise<Result<T[], E>>;
function create(itemOrItems: T | T[]): Promise<Result<T | T[], E>> {
	const items = Array.isArray(itemOrItems) ? itemOrItems : [itemOrItems];
	return createInternal(items);
}

// Option 2: Union type (less boilerplate)
function create(itemOrItems: T | T[]): Promise<Result<void, E>> {
	const items = Array.isArray(itemOrItems) ? itemOrItems : [itemOrItems];
	// ... implementation
}

The Structure

1. Public API: Accepts T | T[]
2. Normalization: Array.isArray() check, wrap single in array
3. Internal function: Only handles arrays

// Public: flexible API
function createServer(clientOrClients: Client | Client[], options?: Options) {
	const clients = Array.isArray(clientOrClients)
		? clientOrClients
		: [clientOrClients];
	return createServerInternal(clients, options);
}

// Internal: array-only, all real logic here
function createServerInternal(clients: Client[], options?: Options) {
	// Implementation only handles arrays
}

Naming Conventions

Parameter	Normalized Variable
recordingOrRecordings	recordings
clientOrClients	clients
itemOrItems	items
paramsOrParamsArray	paramsArray

When to Use

Good fit:

• CRUD operations (create, update, delete)
• Batch processing APIs
• Factory functions accepting dependencies
• Any "do this to one or many" scenario

Skip when:

• Single vs batch have different semantics
• Return types vary significantly
• Array version needs different options

Codebase Examples

Server Factory (packages/epicenter/src/server/server.ts)

function createServer(
	client: AnyWorkspaceClient,
	options?: ServerOptions,
): ReturnType<typeof createServerInternal>;
function createServer(
	clients: AnyWorkspaceClient[],
	options?: ServerOptions,
): ReturnType<typeof createServerInternal>;
function createServer(
	clientOrClients: AnyWorkspaceClient | AnyWorkspaceClient[],
	options?: ServerOptions,
) {
	const clients = Array.isArray(clientOrClients)
		? clientOrClients
		: [clientOrClients];
	return createServerInternal(clients, options);
}

Database Service (apps/whispering/src/lib/services/isomorphic/db/web.ts)

delete: async (recordingOrRecordings) => {
  const recordings = Array.isArray(recordingOrRecordings)
    ? recordingOrRecordings
    : [recordingOrRecordings];
  const ids = recordings.map((r) => r.id);
  return tryAsync({
    try: () => db.recordings.bulkDelete(ids),
    catch: (error) => DbServiceErr({ message: `Error deleting: ${error}` }),
  });
},

Query Mutations (apps/whispering/src/lib/query/isomorphic/db.ts)

delete: defineMutation({
  mutationFn: async (recordings: Recording | Recording[]) => {
    const recordingsArray = Array.isArray(recordings)
      ? recordings
      : [recordings];

    for (const recording of recordingsArray) {
      services.db.recordings.revokeAudioUrl(recording.id);
    }

    const { error } = await services.db.recordings.delete(recordingsArray);
    if (error) return Err(error);
    return Ok(undefined);
  },
}),

Anti-Patterns

Don't: Separate functions for single vs array

// Harder to maintain, users must remember two APIs
function createRecording(recording: Recording): Promise<...>;
function createRecordings(recordings: Recording[]): Promise<...>;

Don't: Force arrays everywhere

// Awkward for single items
createRecordings([recording]); // Ugly

Don't: Duplicate logic in overloads

// BAD: Logic duplicated
function create(item: T) {
	return db.insert(item); // Duplicated
}
function create(items: T[]) {
	return db.bulkInsert(items); // Different code path
}

// GOOD: Single implementation
function create(itemOrItems: T | T[]) {
	const items = Array.isArray(itemOrItems) ? itemOrItems : [itemOrItems];
	return db.bulkInsert(items); // One code path
}

References

• Full article — detailed explanation with more examples