Promise That Never Settles

Q: Does Express 5 auto-catch async errors?

Improved from Express 4 but still use try/catch + next(err) or express-async-errors pattern for safety.

Q: How to find which request hangs?

Log requestId at start and end of middleware chain; missing "end" log implicates route.

Q: Can AbortSignal help?

Yes - pass signal to fetch and DB drivers that support cancellation when client disconnects.

Q: Jest open handles?

--detectOpenHandles finds timers and servers left open after tests.

Q: Worker pool promises?

Parent awaits worker.postMessage reply with timeout; terminate worker on hang.

Q: Production mitigation?

HTTP server timeout + load balancer idle timeout + DB statement_timeout.

Requests hang when a Promise never resolves or rejects - missing callback, forgotten reject, abandoned DB client, or deadlock waiting on itself. In Express 5, the client times out while the server holds connections and pool slots.

Recipe

Quick-reference recipe card - copy-paste ready.

// BAD: manual Promise missing reject path
function fetchLegacy(): Promise<string> {
  return new Promise((resolve) => {
    legacyLib.getData((err: Error | null, data: string) => {
      if (!err) resolve(data);
      // err path never rejects - hangs forever on error
    });
  });
}
 
// GOOD: wrap with resolve + reject
function fetchLegacyFixed(): Promise<string> {
  return new Promise((resolve, reject) => {
    legacyLib.getData((err: Error | null, data: string) => {
      if (err) reject(err);
      else resolve(data);
    });
  });
}

// Timeout wrapper
async function withTimeout<T>(p: Promise<T>, ms: number): Promise<T> {
  let timer: NodeJS.Timeout;
  const timeout = new Promise<never>((_, reject) => {
    timer = setTimeout(() => reject(new Error(`timeout after ${ms}ms`)), ms);
  });
  try {
    return await Promise.race([p, timeout]);
  } finally {
    clearTimeout(timer!);
  }
}

When to reach for this:

p95 latency spikes with flat CPU (requests waiting, not computing)
Open connection count grows while RPS is steady
Integration test hangs until Jest timeout
After wrapping callback-style libraries in Promises

Working Example

import express from "express";
import { Pool } from "pg";
 
const pool = new Pool({ connectionString: process.env.DATABASE_URL, max: 10 });
const app = express();
 
// BUG: client never released if query throws before release in wrong order
app.get("/orders-bug/:id", async (req, res) => {
  const client = await pool.connect();
  const result = await client.query("SELECT * FROM orders WHERE id = $1", [req.params.id]);
  if (!result.rows[0]) {
    // forgot client.release() on early return - pool exhausts over time
    return res.status(404).json({ error: { code: "NOT_FOUND" } });
  }
  client.release();
  res.json({ data: result.rows[0] });
});
 
// FIX: try/finally always releases
app.get("/orders/:id", async (req, res, next) => {
  const client = await pool.connect();
  try {
    const result = await client.query("SELECT * FROM orders WHERE id = $1", [req.params.id]);
    if (!result.rows[0]) return res.status(404).json({ error: { code: "NOT_FOUND" } });
    res.json({ data: result.rows[0] });
  } finally {
    client.release();
  }
});
 
// BUG: missing await - fire and forget, errors unhandled
app.post("/notify-bug", (req, res) => {
  sendEmail(req.body.email); // returns Promise, not awaited
  res.status(202).json({ accepted: true });
});
 
async function sendEmail(to: string) {
  await new Promise((r) => setTimeout(r, 100));
  if (!to.includes("@")) throw new Error("invalid email");
}
 
// FIX
app.post("/notify", async (req, res, next) => {
  try {
    await sendEmail(req.body.email);
    res.status(202).json({ accepted: true });
  } catch (err) {
    next(err);
  }
});

What this demonstrates:

Early return without client.release() leaks pool connections
try/finally guarantees release even on 404
Missing await on async route handlers causes unhandled rejections
Express 5 async errors need next(err) or wrapper

Deep Dive

How It Works

Promise settles fulfilled or rejected; otherwise await blocks forever
Event loop stays alive if pending Promises or active handles exist
pg pool connect() waits when all clients checked out and never released
Express does not await returned Promises unless you use async middleware pattern with error propagation

Hang vs Slow

Signal	Hang (never settles)	Slow (settles late)
CPU	Low	May be high
Pool	Slots never return	Slots return after delay
Trace	Flat line on one await	DB query span long

Debugging Steps

1. node --inspect + pause during hang - inspect async stack
2. Log active pool: pool.totalCount, pool.idleCount, pool.waitingCount
3. trace_unhandled_rejections in staging
4. Add withTimeout around suspect external calls

TypeScript Notes

// Express 5: async handler typing still needs explicit error forward
import type { RequestHandler } from "express";
 
const handler: RequestHandler = async (req, res, next) => {
  try {
    await work();
    res.json({ ok: true });
  } catch (e) {
    next(e);
  }
};

Gotchas

Promise constructor only resolve branch - Callback APIs that error silently hang. Fix: Always reject(err).
forEach with async callback - Does not await; race and lost errors. Fix: for...of with await or Promise.all.
Transaction begin without commit/rollback - Connection held until timeout. Fix: try { ... commit } catch { rollback } finally { release }.
Deadlock on same connection - Await query while transaction open on same client incorrectly interleaved. Fix: One sequential flow per client.
Ignoring pool.waitingCount - Metric hits 10+ while idleCount 0. Fix: Release clients; lower concurrency.

Alternatives

Alternative	Use When	Don't Use When
`util.promisify`	Node core callback APIs	Complex multi-arg callbacks
`async`/`await` only	Greenfield code	Legacy callback lib without promisify
`p-timeout` package	Standard timeout helper	You prefer 10-line wrapper
Reactive streams	Complex event pipelines	Simple CRUD routes

FAQs

Does Express 5 auto-catch async errors?

Improved from Express 4 but still use try/catch + next(err) or express-async-errors pattern for safety.

How to find which request hangs?

Log requestId at start and end of middleware chain; missing "end" log implicates route.

Fastify behavior?

Fastify awaits async handlers and maps rejections to error handler - still release DB clients in finally.

What is pool waitingCount?

Requests queued for a free client. Sustained > 0 means leak or pool too small.

Can AbortSignal help?

Yes - pass signal to fetch and DB drivers that support cancellation when client disconnects.

Promise.race pitfalls?

Losing slow request still runs unless aborted - combine with AbortController.

Jest open handles?

--detectOpenHandles finds timers and servers left open after tests.

graphql hangs?

Resolver awaiting another resolver on same dataloaders without batching - check N+1 and deadlock.

Worker pool promises?

Parent awaits worker.postMessage reply with timeout; terminate worker on hang.

Production mitigation?

HTTP server timeout + load balancer idle timeout + DB statement_timeout.

Debugging Basics - inspector attach
Connection Pool Exhaustion - pool metrics
Unhandled Rejection Production Outage - missing await fallout
Timeouts Everywhere - boundary timeouts
Error Handling in Express - async errors

Node.js API Documentation - Complete reference for core modules and APIs.
Node.js Best Practices (GitHub) - Curated production best practices.
Stack Overflow – Node.js Tag - Q&A for real-world problems.
Node.js Cookbook, 5th Edition - Practical server-side recipes.
Node.js Design Patterns, 4th Edition - Production-grade application patterns.

Stack versions: This page was written for Node.js 24.18.0 (Active LTS), npm 10+, TypeScript 5.6+, Express 5, Fastify 5, and NestJS 11.

Recipe

Quick-reference recipe card - copy-paste ready.

// BAD: manual Promise missing reject path
function fetchLegacy(): Promise<string> {
  return new Promise((resolve) => {
    legacyLib.getData((err: Error | null, data: string) => {
      if (!err) resolve(data);
      // err path never rejects - hangs forever on error
    });
  });
}
 
// GOOD: wrap with resolve + reject
function fetchLegacyFixed(): Promise<string> {
  return new Promise((resolve, reject) => {
    legacyLib.getData((err: Error | null, data: string) => {
      if (err) reject(err);
      else resolve(data);
    });
  });
}

// Timeout wrapper
async function withTimeout<T>(p: Promise<T>, ms: number): Promise<T> {
  let timer: NodeJS.Timeout;
  const timeout = new Promise<never>((_, reject) => {
    timer = setTimeout(() => reject(new Error(`timeout after ${ms}ms`)), ms);
  });
  try {
    return await Promise.race([p, timeout]);
  } finally {
    clearTimeout(timer!);
  }
}

When to reach for this:

p95 latency spikes with flat CPU (requests waiting, not computing)
Open connection count grows while RPS is steady
Integration test hangs until Jest timeout
After wrapping callback-style libraries in Promises

Working Example

import express from "express";
import { Pool } from "pg";
 
const pool = new Pool({ connectionString: process.env.DATABASE_URL, max: 10 });
const app = express();
 
// BUG: client never released if query throws before release in wrong order
app.get("/orders-bug/:id", async (req, res) => {
  const client = await pool.connect();
  const result = await client.query("SELECT * FROM orders WHERE id = $1", [req.params.id]);
  if (!result.rows[0]) {
    // forgot client.release() on early return - pool exhausts over time
    return res.status(404).json({ error: { code: "NOT_FOUND" } });
  }
  client.release();
  res.json({ data: result.rows[0] });
});
 
// FIX: try/finally always releases
app.get("/orders/:id", async (req, res, next) => {
  const client = await pool.connect();
  try {
    const result = await client.query("SELECT * FROM orders WHERE id = $1", [req.params.id]);
    if (!result.rows[0]) return res.status(404).json({ error: { code: "NOT_FOUND" } });
    res.json({ data: result.rows[0] });
  } finally {
    client.release();
  }
});
 
// BUG: missing await - fire and forget, errors unhandled
app.post("/notify-bug", (req, res) => {
  sendEmail(req.body.email); // returns Promise, not awaited
  res.status(202).json({ accepted: true });
});
 
async function sendEmail(to: string) {
  await new Promise((r) => setTimeout(r, 100));
  if (!to.includes("@")) throw new Error("invalid email");
}
 
// FIX
app.post("/notify", async (req, res, next) => {
  try {
    await sendEmail(req.body.email);
    res.status(202).json({ accepted: true });
  } catch (err) {
    next(err);
  }
});

What this demonstrates:

Early return without client.release() leaks pool connections
try/finally guarantees release even on 404
Missing await on async route handlers causes unhandled rejections
Express 5 async errors need next(err) or wrapper

Deep Dive

How It Works

Promise settles fulfilled or rejected; otherwise await blocks forever
Event loop stays alive if pending Promises or active handles exist
pg pool connect() waits when all clients checked out and never released
Express does not await returned Promises unless you use async middleware pattern with error propagation

Hang vs Slow

Signal	Hang (never settles)	Slow (settles late)
CPU	Low	May be high
Pool	Slots never return	Slots return after delay
Trace	Flat line on one await	DB query span long

Debugging Steps

1. node --inspect + pause during hang - inspect async stack
2. Log active pool: pool.totalCount, pool.idleCount, pool.waitingCount
3. trace_unhandled_rejections in staging
4. Add withTimeout around suspect external calls

TypeScript Notes

// Express 5: async handler typing still needs explicit error forward
import type { RequestHandler } from "express";
 
const handler: RequestHandler = async (req, res, next) => {
  try {
    await work();
    res.json({ ok: true });
  } catch (e) {
    next(e);
  }
};

Gotchas

Promise constructor only resolve branch - Callback APIs that error silently hang. Fix: Always reject(err).
forEach with async callback - Does not await; race and lost errors. Fix: for...of with await or Promise.all.
Transaction begin without commit/rollback - Connection held until timeout. Fix: try { ... commit } catch { rollback } finally { release }.
Deadlock on same connection - Await query while transaction open on same client incorrectly interleaved. Fix: One sequential flow per client.
Ignoring pool.waitingCount - Metric hits 10+ while idleCount 0. Fix: Release clients; lower concurrency.

Alternatives

Alternative	Use When	Don't Use When
`util.promisify`	Node core callback APIs	Complex multi-arg callbacks
`async`/`await` only	Greenfield code	Legacy callback lib without promisify
`p-timeout` package	Standard timeout helper	You prefer 10-line wrapper
Reactive streams	Complex event pipelines	Simple CRUD routes

FAQs

Does Express 5 auto-catch async errors?

Improved from Express 4 but still use try/catch + next(err) or express-async-errors pattern for safety.

How to find which request hangs?

Log requestId at start and end of middleware chain; missing "end" log implicates route.

Fastify behavior?

Fastify awaits async handlers and maps rejections to error handler - still release DB clients in finally.

What is pool waitingCount?

Requests queued for a free client. Sustained > 0 means leak or pool too small.

Can AbortSignal help?

Yes - pass signal to fetch and DB drivers that support cancellation when client disconnects.

Promise.race pitfalls?

Losing slow request still runs unless aborted - combine with AbortController.

Jest open handles?

--detectOpenHandles finds timers and servers left open after tests.

graphql hangs?

Resolver awaiting another resolver on same dataloaders without batching - check N+1 and deadlock.

Worker pool promises?

Parent awaits worker.postMessage reply with timeout; terminate worker on hang.

Production mitigation?

HTTP server timeout + load balancer idle timeout + DB statement_timeout.

Debugging Basics - inspector attach
Connection Pool Exhaustion - pool metrics
Unhandled Rejection Production Outage - missing await fallout
Timeouts Everywhere - boundary timeouts
Error Handling in Express - async errors

Node.js API Documentation - Complete reference for core modules and APIs.
Node.js Best Practices (GitHub) - Curated production best practices.
Stack Overflow – Node.js Tag - Q&A for real-world problems.
Node.js Cookbook, 5th Edition - Practical server-side recipes.
Node.js Design Patterns, 4th Edition - Production-grade application patterns.

Stack versions: This page was written for Node.js 24.18.0 (Active LTS), npm 10+, TypeScript 5.6+, Express 5, Fastify 5, and NestJS 11.

Promise That Never Settles

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Promise That Never Settles