[API reference](https://evolu.dev/docs/api-reference) › [@evolu/common](https://evolu.dev/docs/api-reference/common) › [Type](https://evolu.dev/docs/api-reference/common/Type) › Typed Defined in: [packages/common/src/Type.ts:3491](https://github.com/evoluhq/evolu/blob/e7144e2bbe9069362b62dec1b64a8aa922b8f1b0/packages/common/src/Type.ts#L3491) Base interface for objects with a discriminant `type` property. This enables [discriminated unions](https://www.typescriptlang.org/docs/handbook/2/narrowing.html#discriminated-unions) (also known as tagged unions) — a pattern where TypeScript uses a literal `type` field to narrow union types automatically. ## Why Discriminated Unions? Discriminated unions model states that are **mutually exclusive**. Instead of optional fields and boolean flags that can combine into invalid configurations, each variant is a distinct type. This makes illegal states unrepresentable — invalid combinations cannot exist, so bugs cannot create them. Benefits: - **Self-documenting** — Union cases immediately show all possible states - **Compile-time safety** — TypeScript enforces handling all cases - **Refactoring-friendly** — Adding a new state breaks code that doesn't handle it ### Example ```ts // Bad: optional fields allow invalid states (no contact info at all) interface Contact { readonly email?: Email; readonly phone?: Phone; } // Good: discriminated union makes "at least one" explicit interface EmailOnly extends Typed<"EmailOnly"> { readonly email: Email; } interface PhoneOnly extends Typed<"PhoneOnly"> { readonly phone: Phone; } interface EmailAndPhone extends Typed<"EmailAndPhone"> { readonly email: Email; readonly phone: Phone; } type ContactInfo = EmailOnly | PhoneOnly | EmailAndPhone; ``` ```ts interface Pending extends Typed<"Pending"> { readonly createdAt: DateIso; } interface Shipped extends Typed<"Shipped"> { readonly trackingNumber: TrackingNumber; } interface Delivered extends Typed<"Delivered"> { readonly deliveredAt: DateIso; } interface Cancelled extends Typed<"Cancelled"> { readonly reason: CancellationReason; } type OrderState = Pending | Shipped | Delivered | Cancelled; // TypeScript enforces exhaustiveness via return type const getStatusMessage = (state: OrderState): string => { switch (state.type) { case "Pending": return "Order placed"; case "Shipped": return `Shipped: ${state.trackingNumber}`; case "Delivered": return `Delivered on ${state.deliveredAt.toLocaleDateString()}`; case "Cancelled": return `Cancelled: ${state.reason}`; } }; // For void functions, use exhaustiveCheck to ensure all cases are handled const logState = (state: OrderState): void => { switch (state.type) { case "Pending": console.log("Order placed"); break; case "Shipped": console.log(`Shipped: ${state.trackingNumber}`); break; case "Delivered": console.log(`Delivered on ${state.deliveredAt.toLocaleDateString()}`); break; case "Cancelled": console.log(`Cancelled: ${state.reason}`); break; default: exhaustiveCheck(state); } }; ``` ## Why `type` (and not e.g. `_tag`)? Underscore-prefixing is meant to avoid clashing with domain properties, but proper discriminated union design means the discriminant IS the domain concept — there's no clash to avoid. The `type` prop name also aligns with [Type](https://evolu.dev/docs/api-reference/common/Type/interfaces/Type)'s name. If an entity has a meaningful "type" (like product category), model it as the discriminant itself: ```ts interface Electronics extends Typed<"Electronics"> { voltage: Voltage; } interface Clothing extends Typed<"Clothing"> { size: Size; } type Product = Electronics | Clothing; ``` ## See - [exhaustiveCheck](https://evolu.dev/docs/api-reference/common/Function/functions/exhaustiveCheck) to ensure all cases are handled in void functions. - [typed](https://evolu.dev/docs/api-reference/common/Type/functions/typed) for runtime-validated typed objects. ## Extended by - [`DecryptWithXChaCha20Poly1305Error`](https://evolu.dev/docs/api-reference/common/Crypto/interfaces/DecryptWithXChaCha20Poly1305Error) - [`UnknownError`](https://evolu.dev/docs/api-reference/common/Error/interfaces/UnknownError) - [`Some`](https://evolu.dev/docs/api-reference/common/Option/interfaces/Some) - [`None`](https://evolu.dev/docs/api-reference/common/Option/interfaces/None) - [`Done`](https://evolu.dev/docs/api-reference/common/Result/interfaces/Done) - [`SqlIdentifier`](https://evolu.dev/docs/api-reference/common/Sqlite/interfaces/SqlIdentifier) - [`RawSql`](https://evolu.dev/docs/api-reference/common/Sqlite/interfaces/RawSql) - [`RunStateRunning`](https://evolu.dev/docs/api-reference/common/Task/interfaces/RunStateRunning) - [`RunStateDisposing`](https://evolu.dev/docs/api-reference/common/Task/interfaces/RunStateDisposing) - [`RunStateSettled`](https://evolu.dev/docs/api-reference/common/Task/interfaces/RunStateSettled) - [`RetryError`](https://evolu.dev/docs/api-reference/common/Task/interfaces/RetryError) - [`WebSocketSendError`](https://evolu.dev/docs/api-reference/common/WebSocket/interfaces/WebSocketSendError) - [`WebSocketConnectError`](https://evolu.dev/docs/api-reference/common/WebSocket/interfaces/WebSocketConnectError) - [`WebSocketConnectionError`](https://evolu.dev/docs/api-reference/common/WebSocket/interfaces/WebSocketConnectionError) - [`WebSocketConnectionCloseError`](https://evolu.dev/docs/api-reference/common/WebSocket/interfaces/WebSocketConnectionCloseError) - [`AppOwner`](https://evolu.dev/docs/api-reference/common/local-first/Owner/interfaces/AppOwner) - [`ShardOwner`](https://evolu.dev/docs/api-reference/common/local-first/Owner/interfaces/ShardOwner) - [`SharedOwner`](https://evolu.dev/docs/api-reference/common/local-first/Owner/interfaces/SharedOwner) - [`SharedReadonlyOwner`](https://evolu.dev/docs/api-reference/common/local-first/Owner/interfaces/SharedReadonlyOwner) - [`OwnerWebSocketTransport`](https://evolu.dev/docs/api-reference/common/local-first/Owner/interfaces/OwnerWebSocketTransport) - [`ProtocolHeader`](https://evolu.dev/docs/api-reference/common/local-first/Protocol/interfaces/ProtocolHeader) - [`ProtocolVersionError`](https://evolu.dev/docs/api-reference/common/local-first/Protocol/interfaces/ProtocolVersionError) - [`ProtocolInvalidDataError`](https://evolu.dev/docs/api-reference/common/local-first/Protocol/interfaces/ProtocolInvalidDataError) - [`ProtocolWriteKeyError`](https://evolu.dev/docs/api-reference/common/local-first/Protocol/interfaces/ProtocolWriteKeyError) - [`ProtocolWriteError`](https://evolu.dev/docs/api-reference/common/local-first/Protocol/interfaces/ProtocolWriteError) - [`ProtocolQuotaError`](https://evolu.dev/docs/api-reference/common/local-first/Protocol/interfaces/ProtocolQuotaError) - [`ProtocolSyncError`](https://evolu.dev/docs/api-reference/common/local-first/Protocol/interfaces/ProtocolSyncError) - [`ProtocolTimestampMismatchError`](https://evolu.dev/docs/api-reference/common/local-first/Protocol/interfaces/ProtocolTimestampMismatchError) - [`ApplyProtocolMessageAsClientResponse`](https://evolu.dev/docs/api-reference/common/local-first/Protocol/interfaces/ApplyProtocolMessageAsClientResponse) - [`ApplyProtocolMessageAsClientNoResponse`](https://evolu.dev/docs/api-reference/common/local-first/Protocol/interfaces/ApplyProtocolMessageAsClientNoResponse) - [`ApplyProtocolMessageAsClientBroadcast`](https://evolu.dev/docs/api-reference/common/local-first/Protocol/interfaces/ApplyProtocolMessageAsClientBroadcast) - [`ApplyProtocolMessageAsRelayResult`](https://evolu.dev/docs/api-reference/common/local-first/Protocol/interfaces/ApplyProtocolMessageAsRelayResult) - [`StorageQuotaError`](https://evolu.dev/docs/api-reference/common/local-first/Storage/interfaces/StorageQuotaError) - [`TimestampDriftError`](https://evolu.dev/docs/api-reference/common/local-first/Timestamp/interfaces/TimestampDriftError) - [`TimestampCounterOverflowError`](https://evolu.dev/docs/api-reference/common/local-first/Timestamp/interfaces/TimestampCounterOverflowError) - [`TimestampTimeOutOfRangeError`](https://evolu.dev/docs/api-reference/common/local-first/Timestamp/interfaces/TimestampTimeOutOfRangeError) ## Properties ### type ```ts readonly type: T; ``` Defined in: [packages/common/src/Type.ts:3492](https://github.com/evoluhq/evolu/blob/e7144e2bbe9069362b62dec1b64a8aa922b8f1b0/packages/common/src/Type.ts#L3492)