Async context (s200/als)

Opt-in ambient access to the request Ctx — the supplement to s200's explicit ctx passing, for call stacks that cannot thread it.

Ambient ctx (s200/als)

import { als, currentCtx } from 's200/als';

use(app, als());                     // app-level: wraps the whole chain

get(app, '/x', async (ctx) => {
  await stepOne();                   // helpers need no ctx parameter
  return json(ctx, { user: currentCtx()?.state.user });
});

als() is a plain Middleware: it wraps everything below it in the platform's AsyncLocalStorage (node:async_hooks), seeded with the request's own ctx. currentCtx() then resolves that ctx from anywhere inside the wrapped extent — after any number of awaits, inside setTimeout callbacks, in deep utility or legacy code that never saw a parameter. Interleaved requests stay isolated (every chain runs in its own store); outside a request — before the middleware, after the chain settles, in a timer fired after the response — currentCtx() is undefined. Register it once, app-level; nested registrations are harmless (the inner one shadows), and one registration covers routes mounted later.

The storage is a module-level singleton — the simplest shape, and tree-shakable: importing currentCtx alone pulls exactly this entry, nothing else.

Export	Meaning
als()	The wrapping Middleware — storage.run(ctx, () => next()); takes no options
currentCtx()	`() => Ctx

Runtime boundary

node:async_hooks exists on Node and Bun. Cloudflare Workers has no async_hooks — importing s200/als fails there at bundle/boot time, so keep the entry out of edge bundles (a s200/cloudflare app simply never imports it). Explicit ctx passing needs no fallback: code that must run everywhere takes ctx as a parameter.