Stop Passing req Everywhere — Express Middleware for Request Context Propagation

There's a pattern I've seen in almost every Express codebase I've worked in. It starts small.

You need to log a correlation ID inside a service function. So you add req as a parameter. Then the function that calls it needs req. Then the one above that. Within a few weeks, half your codebase takes req as a parameter even though it only ever reads one field from it.

// This is where it starts

async function processOrder(orderId: string, req: Request) {

  logger.info('Processing order', { correlationId: req.headers['x-correlation-id'] });

  await notifyWarehouse(orderId, req);       // now warehouse needs req too

  await sendEmail(orderId, req);             // and email

  await auditLog('order.processed', req);   // and audit

}

Every function signature is polluted. Tests need a mock req. Services are coupled to Express internals. It's a mess.

The fix is AsyncLocalStorage — a Node.js built-in (stable since v16) that lets you store data scoped to an async call chain and read it from anywhere inside that chain, without passing it through function parameters.

I wrapped it into