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What is AsyncLocalStorage and what problem does it solve?

Q: What is AsyncLocalStorage and what problem does it solve?

**TL;DR.** AsyncLocalStorage (from node:async_hooks ) stores data that **stays attached to a logical async chain** as it flows through callbacks, promises, and timers. It solves request-scoped context — a **request id**, user, or trace id available everywhere in a request — without threading parameters through every function call. run(store) { reqId } await db <r

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Understand the problem

Per-request context propagation across async boundaries.

nodejsasync-hookscontextobservability

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Per-request id available to any logger

import { AsyncLocalStorage } from 'node:async_hooks';
const als = new AsyncLocalStorage();

app.use((req, res, next) =>
  als.run({ reqId: crypto.randomUUID() }, () => next()));

function log(msg) {
  const { reqId } = als.getStore() ?? {};
  console.log(JSON.stringify({ reqId, msg }));   // no reqId param needed
}

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