UnhandledPromiseRejection 🟩 Node.js

Node.js: [UnhandledPromiseRejection] — This error originated either by throwing inside of an async function without a catch block

A promise rejected and nothing handled it; since Node 15 this crashes the process.

Seen on: Node.js JavaScript

Meaning

The real error is the one printed with the rejection (the reason). Missing await, missing .catch(), or async callbacks passed to APIs that ignore returned promises (forEach, event emitters, Express 4 handlers) are the usual sources.

Common causes

  • Async function called without await or .catch()
  • async callback in forEach / setTimeout / event listener
  • Express 4 async route without error forwarding
  • Promise.all where one promise rejects unhandled

⚡ Quick fix

  1. Read the reason printed with the warning — that is the actual bug
  2. await or .catch() every promise
  3. Use for…of with await instead of forEach(async)
  4. Express 4: wrap async handlers or upgrade to Express 5

Detailed fix by platform

Node.js

  1. Last-resort logging (still fix the source):
    javascript
    process.on('unhandledRejection', (reason) => {
  console.error('Unhandled rejection:', reason);
  process.exitCode = 1;
});
// Express 4:
const wrap = fn => (req, res, next) => Promise.resolve(fn(req, res, next)).catch(next);
app.get('/users', wrap(async (req, res) => res.json(await db.users())));

How to diagnose

  1. Reason — What error/stack is attached?
  2. Caller — Which call is missing await/catch?

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