Agent skill
40-05-ts-singleton-patterns
TS: When something has one instance, use a module-level ref. Not a factory. Not a class. Not a closure. Prevents over-engineering and hidden coupling.
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SKILL.md
40.05 Singleton Patterns (TypeScript)
When there is exactly one instance of something for the lifetime of the app, represent it as a module-level ref. Period.
The Decision Tree
Is there ever more than one instance?
├── Yes → Factory or class (createEntitySync, createProjectileLifecycle)
└── No → Module-level ref
├── Set once at init → ref object (bridge.world, bridge.renderer)
├── Mutates over time → ref object (stdb.connection, stdb.connectionState)
└── Collection → exported Map/Set (shipEntities, positions)
The Patterns
Module-level ref object (mutable singleton state)
// client-state.ts
export const stdb = {
connection: null as DbConnection | null,
connectionState: "connecting" as SpacetimeConnectionState,
playerShipEntityId: null as string | null,
};
Set at runtime. Read from anywhere. One owner writes each field (see 40.01).
Module-level collection (shared data)
// client-state.ts
export const shipEntities = new Map<string, number>();
export const positions = new Map<string, PositionRow>();
No wrapper. No getter. Just the Map. Everyone who needs it imports it.
Constructor-time refs (set once, never change)
// client-state.ts
export const bridge = {
world: null as IWorld | null,
renderer: null as GameRenderer | null,
prediction: null as PlayerShipPrediction | null,
};
// spacetime-client.ts (at init)
bridge.world = options.world;
bridge.renderer = options.renderer;
These are null until init, then stable forever. The ! assertion in consumers is justified — these are only accessed after init.
Wrong Patterns
Factory for a singleton
// WRONG — factory creates one instance, returns it, done. Why the ceremony?
export function createEcsBridge(deps: { world: IWorld; renderer: GameRenderer }) {
return {
upsertShip: (id: string) => { deps.world... },
removeShip: (id: string) => { deps.renderer... },
};
}
// Caller: const bridge = createEcsBridge({ world, renderer });
// Nobody ever calls createEcsBridge again.
// RIGHT — module-level functions reading from a ref
export const bridge = { world: null as IWorld | null, renderer: null as GameRenderer | null };
export const upsertShip = (id: string) => { bridge.world!... };
export const removeShip = (id: string) => { bridge.renderer!... };
Same behavior. Less indirection. Testable by setting bridge.world directly. No mock factory needed.
Closure capture for "encapsulation"
// WRONG — closure makes testing impossible
function createClient() {
let playerShipId: string | null = null;
const getPlayerShipId = () => playerShipId;
// ... 2000 lines that read/write playerShipId
return { getPlayerShipId };
}
The closure doesn't protect anything — it just makes the state invisible. In a codebase with one client, "encapsulation" of its internals is cargo cult OOP.
Class for no reason
// WRONG — class with one instance, constructed once
class SpacetimeClient {
private connection: DbConnection | null = null;
private playerShipId: string | null = null;
// ... methods
}
const client = new SpacetimeClient(options);
This is a factory pattern with new instead of create. Same problem: state is trapped behind an instance. Use a ref object + standalone functions.
When Factories ARE Right
Factories are for things with multiple instances or parameterized behavior:
// RIGHT — multiple instances with different configs
const prediction = createPlayerShipPrediction(); // one per player
const projectileLifecycle = createProjectileLifecycle(); // one per entity type
// RIGHT — parameterized behavior
const visualSync = createVisualSync({
renderer, shipEntities, buildShipData, ...
});
Even here, if the factory is called exactly once and the result is stored in a ref — consider whether it's actually a singleton in disguise.
The Smell Test
Does this factory get called more than once?
If no — it's a singleton. Kill the factory. Use a ref.
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