An async request handler that throws kills Node's whole HTTP server by default

Fact

http.createServer(async (req, res) => …) ignores the promise the handler returns. Anything the handler throws and does not catch becomes an unhandled rejection, and under Node's default --unhandled-rejections=throw (since v15) that ends the process: one bad request drops every connection. Under warn the process survives, but that request is never answered and the client waits out its own timeout.

Why it matters

Handlers usually wrap routing and business logic in try/catch and then write the response after it. The write can throw too: res.setHeader(name, undefined) raises ERR_HTTP_INVALID_HEADER_VALUE, and JSON.stringify throws on a BigInt or a cycle. So the crash comes from the one line nobody guarded. Under node:test the same bug shows up as a hung request rather than a crash.

How to apply

• Keep the whole handler, the response write included, inside try/catch, with a last-resort 500: remove the headers set so far, or destroy the socket if they were already sent.
• Catch where the handler is registered: createServer((req, res) => handle(req, res).catch(log)).
• Or opt in to Node's own fallback: set events.captureRejections = true before creating the server. http.Server then answers a rejected handler with a bare 500 instead of crashing.
• Test the fallback: make the response-building code throw on purpose and assert the client gets a 500 promptly.