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Multithreading is a tiny runtime that allows you to execute JavaScript functions on separate threads. It is designed to be as simple and fast as possible, and to be used in a similar way to regular functions.

With a minified size of only 4.5kb, it has first class support for Node.js, Deno and the browser. It can also be used with any framework or library such as React, Vue or Svelte.

Depending on the environment, it uses Worker Threads or Web Workers. In addition to ES6 generators to make multithreading as simple as possible.

The State of Multithreading in JavaScript

JavaScript's single-threaded nature means that tasks are executed one after the other, leading to potential performance bottlenecks and underutilized CPU resources. While Web Workers and Worker Threads offer a way to offload tasks to separate threads, managing the state and communication between these threads is often complex and error-prone.

This project aims to solve these challenges by providing an intuitive Web Worker abstraction that mirrors the behavior of regular JavaScript functions. This way it feels like you're executing a regular function, but in reality, it's running in parallel on a separate threads.


npm install multithreading


Basic example

import { threaded } from "multithreading";

const add = threaded(function* (a, b) {
  return a + b;

console.log(await add(5, 10)); // 15

The add function is executed on a separate thread, and the result is returned to the main thread when the function is done executing. Consecutive invocations will be automatically executed in parallel on separate threads.

Example with shared state

import { threaded, $claim, $unclaim } from "multithreading";

const user = {
  name: "john",
  balance: 0,

const add = threaded(async function* (amount) {
  yield user; // Add user to dependencies

  await $claim(user); // Wait for write lock

  user.balance += amount;

  $unclaim(user); // Release write lock

await Promise.all([

console.log(user.balance); // 15

This example shows how to use a shared state across multiple threads. It introduces the concepts of claiming and unclaiming write access using $claim and $unclaim. This is to ensure that only one thread can write to a shared state at a time.

Always $unclaim() a shared state after use, otherwise the write lock will never be released and other threads that want to write to this state will be blocked indefinitely.

The yield statement is used to specify external dependencies, and must be defined at the top of the function.

Example with external functions

import { threaded, $claim, $unclaim } from "multithreading";

// Some external function
function add (a, b) {
  return a + b;

const user = {
  name: "john",
  balance: 0,

const addBalance = threaded(async function* (amount) {
  yield user;
  yield add; // Add external function to dependencies

  await $claim(user);

  user.balance = add(user.balance, amount);


await Promise.all([

console.log(user.balance); // 15

In this example, the add function is used within the multithreaded addBalance function. The yield statement is used to declare external dependencies. You can think of it as a way to import external data, functions or even packages.

As with previous examples, the shared state is managed using $claim and $unclaim to guarantee proper synchronization and prevent data conflicts.

External functions like add cannot have external dependencies themselves. All variables and functions used by an external function must be declared within the function itself.

Using imports from external packages

When using external modules, you can dynamically import them by using yield "some-package". This is useful when you want to use other packages within a threaded function.

import { threaded } from "multithreading";

const getId = threaded(function* () {
   * @type {import("uuid")}
  const { v4 } = yield "uuid"; // Import other package

  return v4();

console.log(await getId()); // 1a107623-3052-4f61-aca9-9d9388fb2d81

You can also import external modules in a variety of other ways:

const { v4 } = yield "npm:uuid"; // Using npm specifier (available in Deno)
const { v4 } = yield ""; // From CDN url (available in browser and Deno)

Enhanced Error Handling

Errors inside threads are automatically injected with a pretty stack trace. This makes it easier to identify which line of your function caused the error, and in which thread the error occured.

Example of an enhanced stack trace