• API Overview
• Package Entry Point
• createPulse()
  • Type parameters
  • Parameters
  • Returns
• PulseSchema
• PulseOptions
• HeartbeatOptions
• ReconnectOptions
• OutgoingMessage
• OutgoingTransform
• Pulse
  • channel(name)
  • connect()
  • disconnect(code?, reason?)
  • dispose()
  • on(event, handler)
  • once(event, handler)
  • send(event, payload)
  • wait(event, opts?)
  • room(name, opts?)
  • rooms
  • status
  • tap(handler, options?)
• PulseEvent
• PulseChannel
• RoomScope
  • RoomScopeBase
  • PresenceRoomScope
  • RoomOptions
• Errors
  • PulseError
  • PulseConnectionError
  • PulseProtocolError
  • PulseTimeoutError
  • PulseRoomTimeoutError
  • PulseAbortError
  • PulseDisposedError
• Channel and room definitions
  • ChannelDefinition
  • ChannelDefinitions
  • RoomDefinition
  • RoomDefinitions
• Utility types
  • MessageMap
  • EventKey
  • ServerEvents
  • ClientEvents
  • RoomMap
  • Unsubscribe
  • PulseStatus

API Overview

Symbol	Purpose	Execution mode	Common gotcha
createPulse()	Create a typed WebSocket session instance.	Sync (returns Pulse)	Does not open the connection — call connect().
Pulse	Main instance: channels, rooms, messaging, lifecycle.	Sync methods, async connect()/wait()	send() throws while disconnected.
PulseChannel	Scoped channel namespace with independent disposal.	Sync methods, async wait()	Each call returns a new scope; ref-counted subscription.
RoomScope	Ref-counted room membership with optional presence.	Sync methods, async joined	joined rejects on transport close or timeout.
PulseSchema	Declares server/client events, channels, and rooms.	Type-only	Infer all named scope types from this schema.
PulseOptions	Configuration: heartbeat, reconnect, transform.	Type-only	reconnect and heartbeat default to false.
PulseError	Base class for all Pulse errors.	Runtime	Check instanceof against subclasses.

Package Entry Point

Import	Purpose
@vielzeug/pulse	All public exports: createPulse, types, and error classes.

createPulse()

function createPulse<S extends PulseSchema = PulseSchema>(url: string, options?: PulseOptions): Pulse<S>

Creates a Pulse instance. The WebSocket is not opened until connect() is called.

Type parameters

Parameter	Constraint	Description
S	PulseSchema	Schema declaring server events, client events, channels, and rooms.

Parameters

Parameter	Type	Description
url	string	WebSocket URL.
options	PulseOptions	Optional configuration.

Returns

Pulse<S> — the Pulse instance.

---

PulseSchema

type PulseSchema = {
  server?: MessageMap;
  client?: MessageMap;
  channels?: ChannelDefinitions;
  rooms?: RoomDefinitions;
};

Declare all protocol surfaces once at construction. Named scopes infer their types from this schema.

Field	Type	Description
server	MessageMap	Root events the server sends.
client	MessageMap	Root events the client sends.
channels	ChannelDefinitions	Named channel schemas.
rooms	RoomDefinitions	Named room schemas with optional presence.

---

PulseOptions

type PulseOptions = {
  heartbeat?: boolean | HeartbeatOptions;
  protocols?: string | string[];
  reconnect?: boolean | ReconnectOptions;
  transform?: OutgoingTransform;
};

Option	Type	Default	Description
heartbeat	boolean | HeartbeatOptions	false	Ping/pong keep-alive.
protocols	string | string[]	—	Sub-protocols passed to the WebSocket constructor.
reconnect	boolean | ReconnectOptions	false	Auto-reconnect on unexpected close.
transform	OutgoingTransform	—	Transform or filter outgoing application messages.

---

HeartbeatOptions

type HeartbeatOptions = {
  interval?: number;
  timeout?: number;
};

Option	Type	Default	Description
interval	number	30_000	Interval between pings in ms.
timeout	number	5_000	How long to wait for a pong before treating the connection as dead.

---

ReconnectOptions

type ReconnectOptions = {
  delay?: number | ((attempt: number) => number);
  maxAttempts?: number;
};

Option	Type	Default	Description
delay	number | ((attempt: number) => number)	Full-jitter exponential backoff capped at 30 s	Delay between reconnect attempts in ms. attempt is zero-based.
maxAttempts	number	5	Maximum number of reconnect attempts after initial failure.

---

OutgoingMessage

type OutgoingMessage = { channel?: string; event: string; payload: unknown };

An outgoing application message before it is serialized.

---

OutgoingTransform

type OutgoingTransform = (message: Readonly<OutgoingMessage>) => OutgoingMessage | null;

Transform or filter outgoing application messages. Internal protocol frames (subscribe, join, leave, presence, ping) bypass this hook. Return null to drop the message.

---

Pulse

type Pulse<S extends PulseSchema = PulseSchema> = {
  // Channels
  channel<K extends keyof ChannelMap<S> & string>(
    name: K,
  ): PulseChannel<ChannelMap<S>[K]['server'], ChannelMap<S>[K]['client']>;

  // Connection
  connect(): Promise<void>;
  disconnect(code?: number, reason?: string): void;

  // Lifecycle
  readonly disposalSignal: AbortSignal;
  dispose(): void;
  readonly disposed: boolean;

  // Messaging
  on<K extends EventKey<ServerEvents<S>>>(event: K, handler: (payload: ServerEvents<S>[K]) => void): Unsubscribe;
  once<K extends EventKey<ServerEvents<S>>>(event: K, handler: (payload: ServerEvents<S>[K]) => void): Unsubscribe;
  send<K extends EventKey<ClientEvents<S>>>(event: K, payload: ClientEvents<S>[K]): void;
  wait<K extends EventKey<ServerEvents<S>>>(event: K, opts?: { signal?: AbortSignal; timeout?: number }): Promise<ServerEvents<S>[K]>;

  // Rooms
  room<K extends keyof RoomMap<S> & string>(name: K, opts?: RoomOptions): RoomScope<RoomMap<S>[K]>;
  readonly rooms: Readable<ReadonlySet<string>>;

  // Status
  readonly status: Readable<PulseStatus>;

  // Tap
  tap(handler: (event: PulseEvent) => void, options?: { signal?: AbortSignal }): () => void;

  [Symbol.dispose](): void;
};

channel(name)

Creates an isolated message namespace over the shared connection. Each call returns an independently disposable scope. The server subscription is reference-counted.

connect()

Explicitly opens the connection. Resolves after session restoration completes. Rejects if the connection closes before opening.

disconnect(code?, reason?)

Closes the connection without triggering reconnection. Default code is 1000.

dispose()

Permanently closes the connection and releases all resources. Idempotent.

on(event, handler)

Subscribes to a typed server event. Returns an unsubscribe function.

once(event, handler)

Subscribes once — auto-removes after first invocation.

send(event, payload)

Sends a typed event to the server. Throws PulseConnectionError unless the connection is open.

wait(event, opts?)

Resolves on the next emission of the given server event. Rejects when opts.signal aborts, the timeout elapses, or the instance is disposed.

room(name, opts?)

Creates a ref-counted room scope. The first scope sends join; the last disposal sends leave. When the room definition includes presence, the scope exposes reactive presence state.

rooms

Reactive set of rooms the client is currently a confirmed member of.

status

Reactive connection status: 'connecting' | 'open' | 'reconnecting' | 'closed'.

tap(handler, options?)

Subscribes to lifecycle events emitted by the Pulse instance. The handler receives a discriminated-union PulseEvent. Returns an unsubscribe function.

Parameter	Type	Description
handler	(event: PulseEvent) => void	Called for each lifecycle event.
options.signal	AbortSignal	Optional signal to stop the subscription.

const pulse = createPulse(url, { reconnect: true });
pulse.tap((event) => {
  if (event.type === 'error') console.error(event.error);
  if (event.type === 'status-change') console.log('status:', event.status);
});

---

PulseEvent

type PulseEvent =
  | { type: 'status-change'; status: PulseStatus }
  | { type: 'error'; error: PulseError }
  | { type: 'dispose' };

A discriminated union of lifecycle events emitted by a Pulse instance. Inspect event.type to narrow the payload.

type	Payload	When
status-change	status: PulseStatus	The connection status transitions.
error	error: PulseError	A typed transport or protocol error occurs.
dispose	—	The instance is disposed.

---

PulseChannel

type PulseChannel<TServer extends MessageMap = MessageMap, TClient extends MessageMap = MessageMap> = {
  readonly disposalSignal: AbortSignal;
  readonly disposed: boolean;
  readonly name: string;
  dispose(): void;
  on<K extends EventKey<TServer>>(event: K, handler: (payload: TServer[K]) => void): Unsubscribe;
  once<K extends EventKey<TServer>>(event: K, handler: (payload: TServer[K]) => void): Unsubscribe;
  send<K extends EventKey<TClient>>(event: K, payload: TClient[K]): void;
  wait<K extends EventKey<TServer>>(event: K, opts?: { signal?: AbortSignal; timeout?: number }): Promise<TServer[K]>;
  [Symbol.dispose](): void;
};

---

RoomScope

type RoomScope<R extends RoomDefinition = RoomDefinition> = R extends { presence: infer P }
  ? P extends undefined
    ? RoomScopeBase
    : PresenceRoomScope<P>
  : RoomScopeBase;

A room scope. When the room definition includes presence, the scope is a PresenceRoomScope; otherwise it is a RoomScopeBase.

RoomScopeBase

type RoomScopeBase = {
  readonly disposalSignal: AbortSignal;
  readonly disposed: boolean;
  readonly name: string;
  readonly joined: Promise<void>;
  dispose(): void;
  [Symbol.dispose](): void;
};

PresenceRoomScope

type PresenceRoomScope<T = unknown> = RoomScopeBase & {
  readonly presence: Readable<ReadonlyMap<string, T>>;
  updatePresence(state: T): void;
  onJoin(handler: (memberId: string, state: T) => void): Unsubscribe;
  onLeave(handler: (memberId: string) => void): Unsubscribe;
};

Member	Type	Description
presence	Readable<ReadonlyMap<string, T>>	Reactive map of memberId → state.
updatePresence(state)	(state: T) => void	Broadcast this client's presence state. Throws PulseConnectionError unless open.
onJoin(handler)	(handler) => Unsubscribe	Called whenever a new member joins with their initial state.
onLeave(handler)	(handler) => Unsubscribe	Called whenever a member leaves.

RoomOptions

type RoomOptions = {
  signal?: AbortSignal;
  timeout?: number;
};

Option	Type	Description
signal	AbortSignal	Aborts the join, rejecting joined with PulseAbortError.
timeout	number	Join timeout in ms. Rejects joined with PulseRoomTimeoutError.

---

Errors

All errors extend PulseError.

PulseError

Base class for all Pulse errors.

PulseConnectionError

Transport failure, send while disconnected, or room join rejected on close.

PulseProtocolError

Malformed frame or server error frame.

PulseTimeoutError

wait() timed out before the server event arrived.

PulseRoomTimeoutError

Room scope joined timed out before the server confirmed membership.

PulseAbortError

wait() or room joined aborted via AbortSignal.

PulseDisposedError

Operation attempted after disposal.

---

Channel and room definitions

ChannelDefinition

type ChannelDefinition = { client: MessageMap; server: MessageMap };

ChannelDefinitions

type ChannelDefinitions = Record<string, ChannelDefinition>;

RoomDefinition

type RoomDefinition = { presence?: unknown };

RoomDefinitions

type RoomDefinitions = Record<string, RoomDefinition>;

---

Utility types

MessageMap

type MessageMap = Record<string, unknown>;

EventKey

type EventKey<T extends MessageMap> = keyof T & string;

ServerEvents

type ServerEvents<S extends PulseSchema> = S extends { server: infer M extends MessageMap } ? M : MessageMap;

Extract server events from a schema, defaulting to an empty map.

ClientEvents

type ClientEvents<S extends PulseSchema> = S extends { client: infer M extends MessageMap } ? M : MessageMap;

Extract client events from a schema, defaulting to an empty map.

RoomMap

type RoomMap<S extends PulseSchema> = S extends { rooms: infer R extends RoomDefinitions } ? R : RoomDefinitions;

Extract room definitions from a schema, defaulting to an empty map.

Unsubscribe

type Unsubscribe = () => void;

PulseStatus

type PulseStatus = 'connecting' | 'open' | 'reconnecting' | 'closed';