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A Pub/Sub state manager you'll love... or hate


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A Pub/Sub state manager you'll love... or hate


  • Familiar
    Inspired by Immer and Redux
  • Composable
    Attach/Detach action handlers as needed. Components can bring their own handlers!
  • Immutability
    Modifications or mutations within a handler do not affect state until completion
  • Full TypeScript Support
    Utilize TypeScript for in-action guards and validations
  • Extremely lightweight
    Weighs only 623 bytes, including dependencies!


$ npm install --save vegemite


Check out the /examples directory for more patterns.

import vegemite from 'vegemite';

const todomvc = vegemite({
	next: 1,
	todos: [],

// handler-type: mutate
todomvc.on('todo:add', (state, data) => {
	let nxt =;
		id: nxt,
		done: false,
		title: data.title,

// handler-type: return object
todomvc.on('todo:toggle', (state, id) => {
	let todos = => {
		if ( === id) item.done = !item.done;
		return item;

	return { next:, todos };

// handler-type: return Promise
todomvc.on('todo:assign', async (state, data) => {
	await httpie.send('POST', `/api/todos/${}/assign`, {
		body: { user: data.userid }

	state.todos = => {
		if ( === {
			item.assigned = item.assigned || [];
		return item;

// Add a global "listener" (side effect)
// This runs _after_ state has been updated
todomvc.listen((state, prev) => {
	console.log('~> this is the NEW state:', state);
	console.log('~> this was the OLD state:', prev);



Returns Vegemite

Returns a Vegemite instance, pre-filled with an initial state object.


Type: State extends any

Any initial data of your choosing.


Returns: State

A getter function that returns a snapshot of the current state.

Vegemite.on(event, handler)

Returns: Function

Assign a new action handler for a specific event. Returns an unsubscriber function that, when called, will detach this handler.

Important: When more than one handler exists for an event, they are executed in the order that they were assigned.

let store = vegemite({ count: 5 });

let detach = store.on('count:add', (state, event) => {
	state.count += event.value;

await store.dispatch('count:add', { value: 10 });
console.log(store.state); //=> { count: 15 }

detach(); // deactivated

await store.dispatch('count:add', { value: 3 });
console.log(store.state); //=> { count: 15 } (unchanged)


Type: String

The event/topic name.


Type: Handler

See Handlers below.

Vegemite.listen(event, subscriber)

Returns: Function

Attach a new subscription — when no event is specified, then the subscriber will listen to all events.

Returns an unsubscriber function that, when called, will detach this subscriber.

Important: When more than one subscriber exists for an event (or globally), they are executed in the order that they were assigned.

let store = vegemite({ count: 5 });

store.on('increment', state => {

let unlisten = store.listen('increment', (state, prevState) => {
	console.log(`heard increment: ${prevState.count} ~> ${state.count}`);

await store.dispatch('increment');
//=> heard increment: 5 ~> 6

unlisten(); // deactivated

await store.dispatch('increment');
// (silence)


Type: String

The event/topic name, if any.


Type: Subscriber

See Subscribers below.

Vegemite.set(state, event?)

Returns: void

Forcibly set the instance's state.

When set() is run, all related subscribers/listeners are called.
Optionally pass an event to trigger a specific set of listeners.

Note: This is automatically called at the end of a dispatch() chain.


Type: State

The new State data.


Type: String

The event/topic on whose behalf you're committing.

Vegemite.dispatch(event, eventData)

Returns: Promise<void>

Sends a new message to an event/topic.

When dispatch() is called, all related action handlers (via on()) are run to produce a new state. Once this chain is complete, set() is invoked with the new state, which will trigger any listeners for the event (in addition to global listeners).

Attention TypeScript Users:
For events that do not require eventData, you must dispatch with an undefined payload, or similar.


Type: String

The event/topic to target.


Type: any

Any data to send the topic.

Important: Your eventData is sent to your event/topic's handlers as is!


Action handlers can be thought of as "producers" or "reducers" – they are functions that, when matched, are invoked to help create a new state.

Action handlers belong to a specific event and are assigned via on(). Multiple handlers may exist for a single event topic. When this is the case, the list of handlers will be executed in the order that they were assigned. This allows handlers to access previous handlers' results and use those as part of its own computation, if desired.

Every handler receives the current state as its first parameter and then any eventData that was sent to the event (via dispatch()). Handlers may return a new state object or they may choose to mutate the state in place and forgo a return entirely. A Promise may also be returned that resolves either to a new state or to a mutation.

type Handler<T, X> = (state: T, eventData: X) => Promise<T | void> | T | void;

While an event's handlers are running (dubbed the "dispatch chain"), you are free to mutate the state and eventData object as much as you'd like — mutations have no effect on the instance's actual state during this time! Once the "dispatch chain" has resolved (aka, all handlers have completed), Vegemite will promote the chain's final state object as the instance's new actual state (via set(state, event)).

At this point, the instance's state has been updated, which means ctx.state will reflect the current changes. Additionally, any subscribers (via listen()) are enqueued for action.


Subscribers, or listeners, run after the instance has updated its internal state. These can be thought of as "callbacks" or side-effects that, when matched, should be alerted about a change in state.

All subscribers receive the current state as its first parameter and the previous state as its second parameter. You can use this information to infer what, specifically, changed between the two state objects. The return value from subscribers has no effect anywhere.

type Listener<T> = (state: T, prevState: T) => any;

Subscribers can listen in on a specific event topic, or they can listen to everything (aka, "global" listeners).

When an event's "dispatch chain" has resolved, any subscribers for that event will be queued in the order that they were assigned. Additionally, all "global" subscribers, if any, will be executed in the order that they were defined. However, please note that global subscribers always execute before event-specific subscribers.

TypeScript Support

Vegemite is made to work with an "EventMap" — this is an interface whose keys are the names of your events and whose values are the event-datas you expect to pass along as the message. With this information alone, TypeScript and vegemite can ensure that all of your event publishers and subscribers are passing or expecting the correct data types.

Additionally, vegemite requires a State descriptor — this is a separate interface that describes what you want this vegemite instance to contain at all times. With this information, TypeScript can ensure your actions/handlers are abiding by the rules & not causing mischief.

interface Todo {
	id: number;
	done: boolean;
	title: string;
	assigned?: string[];

interface Assignment {
	id: Todo['id'];
	userid: string;

interface EventMap {
	'todo:add': Todo['title'];
	'todo:toggle': Todo['id'];
	'todo:assign': Assignment;
	// ...

interface State {
	next: number;
	todos: Todo[];

const todomvc = vegemite<EventMap, State>({
	next: 1,
	todos: []

With the setup above, TypeScript can help us ensure that the todo:add and todo:toggle topics will only send or receive their respective Todo attributes, whereas the todo:assign topic will always send or receive a more complex Assignment object.

Meanwhile, any manipulations within the State will be protected, ensuring that state.todos only contains valid Todo objects and is always a number.


MIT © Luke Edwards


A Pub/Sub state manager you'll love... or hate