express 5 async error handling: working versions (tested 2026-10-18)

{
  "name": "express5-async-error-handling",
  "private": true,
  "type": "module",
  "scripts": {
    "test": "node --import tsx --test test/*.test.ts"
  },
  "dependencies": {
    "express": "5.3.0",
    "supertest": "7.3.1"
  },
  "devDependencies": {
    "@types/node": "26.6.5",
    "tsx": "4.23.15"
  }
}

import express from "express";
import type { NextFunction, Request, Response } from "express";

export function createApp() {
  const app = express();
  app.use(express.json());

  const orders = express.Router({ mergeParams: true });
  orders.get("/:orderId", (req: Request, res: Response) => {
    const { userId, orderId } = req.params;
    res.json({ userId, orderId });
  });

  app.use("/users/:userId/orders", orders);

  app.get("/boom", async () => {
    throw new Error("kaboom");
  });

  app.use((req: Request, res: Response) => {
    res.status(404).json({ error: "not_found", path: req.path });
  });

  app.use((err: Error, _req: Request, res: Response, _next: NextFunction) => {
    res.status(500).json({ error: "internal_error", message: err.message });
  });

  return app;
}

import { test } from "node:test";
import assert from "node:assert/strict";
import request from "supertest";
import { createApp } from "../src/app.ts";

test("resolves async handler with router-level params", async () => {
  const app = createApp();
  const res = await request(app).get("/users/u1/orders/o2");
  assert.equal(res.status, 200);
  assert.deepEqual(res.body, { userId: "u1", orderId: "o2" });
});

test("rejected async handler reaches error middleware without try/catch", async () => {
  const app = createApp();
  const res = await request(app).get("/boom");
  assert.equal(res.status, 500);
  assert.deepEqual(res.body, { error: "internal_error", message: "kaboom" });
});

test("unknown route returns JSON 404", async () => {
  const app = createApp();
  const res = await request(app).get("/nope");
  assert.equal(res.status, 404);
  assert.deepEqual(res.body, { error: "not_found", path: "/nope" });
});