What is the AsyncLocalStorage API, and what problem does it solve for tracking context across asynchronous calls?

AsyncLocalStorage (from node:async_hooks) lets you store and retrieve data that stays associated with a single asynchronous execution chain (like one HTTP request) without needing to explicitly pass that data through every function call in between -- it's commonly used to propagate a request ID or user context through logging and downstream calls, replacing older, more fragile approaches to context propagation.

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

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

function log(message) {
  const store = als.getStore();
  console.log(`[${store?.requestId}] ${message}`);
}