useWorkerizedReducer is like useReducer, but the reducer runs in a worker. This makes it possible to place long-running computations in the reducer without affecting the responsiveness of the app.


// worker.js
import { initWorkerizedReducer } from "use-workerized-reducer";

  "counter", // Name of the reducer
  async (state, action) => {
    // Reducers can be async!
    // Manipulate `state` directly. ImmerJS will take
    // care of maintaining referential equality.
    switch (action.type) {
      case "increment":
        state.counter += 1;
      case "decrement":
        state.counter -= 1;
        throw new Error();

// main.js
import { render, h, Fragment } from "preact";
import { useWorkerizedReducer } from "use-workerized-reducer/preact";

// Spin up the worker running the reducers.
const worker = new Worker(new URL("./worker.js", import.meta.url), {
  type: "module",

function App() {
  // A worker can contain multiple reducers, each with a unique name.
  // `busy` is true if any action is still being processed.
  const [state, dispatch, busy] = useWorkerizedReducer(
    "counter", // Reducer name
    { counter: 0 } // Initial state

  return (
      Count: {state.counter}
      <button disabled={busy} onclick={() => dispatch({ type: "decrement" })}>
      <button disabled={busy} onclick={() => dispatch({ type: "increment" })}>

render(<App />, document.querySelector("main"));

Browser Support

useWorkerizedReducer works in all browsers. Firefox requires a polyfill.

(Currently, useWorkerizedReducer relies on WritableStream, which is available everywhere except Firefox. If you want to support Firefox, I recommend the web-streams-polyfill.)


useWorkerizedReducer takes care of bringing the functionality of useReducer to a worker. It bridges the gap between worker and main thread by duplicating the reducer’s state to the main thread. The reducer manipulates the state object in the worker, and through ImmerJS only patches will be postMessage()’d to keep the main thread’s copy of the state up-to-date.

Due to the communication with a worker, useWorkerizedReducer is inherently asynchronous. In fact, part of the motivation was to enable long-running reducers, which means considerable time can pass between a dispatch() call and the subsequent state change. useWorkerizedReducer will fully finish processing an action before starting the next one, even if the reducer is async.

If a reducer is still running, the busy variable returned by useWorkerizedReducer will be set to true.


Exported methods

useWorkerizedReducer(worker, name, initialState): [State, DispatchFunc, isBusy];

isBusy will be true until the initialState has been successfully replicated to the worker. Afterwards, isBusy is true when there actions still being processed, false otherwise.

initWorkerizedReducer(name, reducerFunc, localState?);

name is the name of the reducer, which has to be identical to the name passed into useWorkerizedReducer. reducerFunc is a function of type (state, action, localState) => void | Promise<void>. It behaves the same as the reducer function you pass to the vanilla useReducer hook. In contrast to the reducer functions from the vanilla useReducer hook, it is important to manipulate the state object directly. ImmerJS is recording the operations performend on the object to generate a patch set. Creating copies of the object will not yield the desired effect. Since the modifications to state have to be transferred back to the main thread, the state object can only hold structured cloneable values.

localState is optional, and is a function of type (initialState) => LocalState. It will be called when a new reducer is being created and is expected to return a new local state instance. Local state will not be transferred to the main thread and therefore can hold references to values that are not structured cloneable, like functions or errors.

Convenience exports

For React:

import { ... } from "use-workerized-reducer/react";

For Preact:

import { ... } from "use-workerized-reducer/preact";

If, for some reason, you don’t want to use either of those, you can use the generic export. Note that useWorkerizedReducer takes 3 extra parameters, which have to be the useState, useEffect and useMemo hook in that order.

import { ... } from "use-workerized-reducer";



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