Introducing mutex-forge: Mutex & Semaphore for async JavaScript

javascript dev.to August 13, 2026

JavaScript is single-threaded, but async code still races. Two functions can await at the same time and both update shared state — caches, balances, worker message flows. Those bugs are hard to reproduce.

I built mutex-forge to bring classic mutex and semaphore patterns to async JavaScript and TypeScript.

What it does

Mutex — exclusive access to a critical section
Semaphore — limit how many jobs run at once (pools, rate limits)
runExclusive — acquire, run, and release automatically
withTimeout — fail if the lock takes too long
tryAcquire — non-blocking lock attempts
TypeScript types included

Example: prevent overlapping updates


javascript
const { Mutex } = require('mutex-forge');

const mutex = new Mutex();
let balance = 0;

async function credit(amount) {
  await mutex.runExclusive(async () => {
    const current = balance;
    await someAsyncWork();
    balance = current + amount;
  });
}

await Promise.all([credit(10), credit(20)]);
// balance === 30

Source: dev.to