Create PubSub

A tiny Event Emitter and Observable Store for JavaScript apps. You destructure a pair of named functions instead of passing event-name strings around, so there are no event names to keep in sync and your editor completes them for you.

• Tiny. The ES Module build is 264 bytes, or 138 brotli-compressed, with zero dependencies.
• No string event names. const [gameStarted, onGameStarted] = createPubSub(), and you pick both names.
• Emitter or store. Pass an initial value and you also get a getter, plus the previous value on every update.
• Runs everywhere. Browser, Node and Deno, as an ES Module.
• Optional extras. A usePubSub React hook and an Immer-backed variant, in separate entry points.

const [pub, sub] = createPubSub<string>();

sub((data) => console.log(`Hello ${data}!`));

pub("World"); // Prints "Hello World!".

It's written in vanilla JavaScript, so it's framework-agnostic.

Install

The package is available in the NPM Registry. Install it using the following command:

npm install create-pubsub

Import

// Import as an ES Module.
import { createPubSub } from "create-pubsub";

// Or require it from CommonJS, on Node 20.19+ or 22.12+.
const { createPubSub } = require("create-pubsub");

// Or import it from a URL, with no install step. Works in Deno too.
import { createPubSub } from "https://esm.sh/create-pubsub";

<!-- In a plain HTML page, that same import needs a module script. -->
<script type="module">
  import { createPubSub } from "https://esm.sh/create-pubsub";
</script>

Usage

As Emitter/Listener

For each event you want to track, create a new PubSub.

const [pub, sub] = createPubSub<Type>();

As Store

To store a value, pass an initial value and set a third element while destructuring it.

const [pub, sub, get] = createPubSub(initialValue);

Examples

Example: Naming Functions

Name the 'pub' and 'sub' functions as you wish. The idea is to avoid relying on strings representing the events names. The following snippet shows different ways to name an event which indicates the game started:

const [publishGameStarted, subscribeToGameStarted] = createPubSub();

const [gameStarted, onGameStarted] = createPubSub();

const [dispatchGameStarted, listenGameStartedEvent] = createPubSub();

Example: Signalling

You can also publish events with no data, just for signalling:

const [emitPageIsReady, whenPageIsReady] = createPubSub();

whenPageIsReady(() => {
  // Do something with the page, which is now ready.
});

emitPageIsReady();

Example: Unsubscribing

And you can unsubscribe at any moment, by invoking the function returned when you subscribe:

const [publish, subscribe] = createPubSub();

const unsubscribe = subscribe((numberReceived) => {
  console.log(numberReceived);

  if (numberReceived === 2) unsubscribe();
});

publish(1); // Prints 1.
publish(2); // Prints 2 and unsubscribe.
publish(3); // Nothing is printed.

Example: Chaining Events

const [emitAssetsLoaded, onAssetsLoaded] = createPubSub();
const [emitGameStarted, onGameStarted] = createPubSub();

onGameStarted(() => {
  // Setup world, characters, etc. And possibly chain more events.
});

onAssetsLoaded(() => {
  // Initializes the game, load last saved session, etc.
  emitGameStarted();
});

emitAssetsLoaded();

Example: Storing Data

const [set, sub, get] = createPubSub("red");

console.log(get()); // Prints "red".

set("blue"); // Sets the store to "blue", but nothing is printed.

sub((state) => console.log(state)); // Subscribe to next store updates.

set("green"); // Sets the store to "green" and prints it.

console.log(get()); // Prints "green".

Example: Action & Reaction

You also receive the previous value along with the new one, so you can check whether it has changed or not since last time it was set, for example:

const [updatePlayer, onPlayerChanged, getPlayer] = createPubSub({
  name: "Player1",
  level: 5,
  hp: 33,
  mana: 92,
});

onPlayerChanged((playerState, previousPlayerState) => {
  if (playerState.level > previousPlayerState.level) {
    // Player leveled up! Let's display the level-up dialog.
  }
});

updatePlayer({ ...getPlayer(), level: 6, hp: 40, mana: 100 });

Example: State Management

Using it as a store and reacting to other events:

const [setValue, watchValue, getValue] = createPubSub(0);

const [dispatchIncremented, onIncremented] = createPubSub();

const [dispatchDecremented, onDecremented] = createPubSub();

onIncremented(() => setValue(getValue() + 1));

onDecremented(() => setValue(getValue() - 1));

watchValue((state) => console.log(state));

dispatchIncremented(); // Prints 1.
dispatchIncremented(); // Prints 2.
dispatchDecremented(); // Prints 1.

Example: Working with Immutable Data

For creating a PubSub instance that makes use of Immer, import the createImmerPubSub function from create-pubsub/immer and the Publish function will then provide the draft which you can mutate to generate the new state.

import { createImmerPubSub } from "create-pubsub/immer";

const [updateColorsList, onColorsListUpdated, getColorsList] =
  createImmerPubSub([
    { name: "White", code: { r: 255, g: 255, b: 255 } },
    { name: "Gray", code: { r: 128, g: 128, b: 128 } },
  ]);

onColorsListUpdated((currentColorsList, previousColorsList) => {
  console.log(currentColorsList);
  // Prints:
  // [
  //   { name: "White", code: { r: 255, g: 255, b: 255 } },
  //   { name: "Green", code: { r: 0, g: 128, b: 0 } },
  // ]

  console.log(previousColorsList);
  // Prints:
  // [
  //   { name: "White", code: { r: 255, g: 255, b: 255 } },
  //   { name: "Gray", code: { r: 128, g: 128, b: 128 } },
  // ]
});

updateColorsList((colorsList) => {
  const color = colorsList.find((color) => color.name === "Gray");
  if (color) {
    color.name = "Green";
    color.code.r = 0;
    color.code.b = 0;
  }
});

console.log(getColorsList());
// Prints:
// [
//   { name: "White", code: { r: 255, g: 255, b: 255 } },
//   { name: "Green", code: { r: 0, g: 128, b: 0 } },
// ]

Example: React Hook

For linking a PubSub instance with a React element, import the usePubSub hook from create-pubsub/react and use it inside the component, similar to React's useState. It uses useSyncExternalStore, so it requires React 18 or later.

Create the PubSub instance outside the component, as in the example below. The hook subscribes to the instance you pass it, so creating a new one on every render makes it re-subscribe on every render.

import { createPubSub } from "create-pubsub";
import { usePubSub } from "create-pubsub/react";

const counterPubSub = createPubSub(0);

const ReactButton = () => {
  const [count, setCount] = usePubSub(counterPubSub);

  return <button onClick={() => setCount(count + 1)}>Count: {count}</button>;
};