AWS Lambda Error – Unhandled Promise Rejection

#aws#lambda#devops#cloud

By ensuring all async calls are awaited, wrapping logic in try/catch, and avoiding unsupported async patterns, you prevent silent failures and keep your Lambda’s execution predictable and stable.

Problem

Your Node.js Lambda runs but abruptly fails with errors such as:

UnhandledPromiseRejectionWarning: Unhandled promise rejection

or:

Task timed out after X seconds
Cause: Runtime.UnhandledPromiseRejection

This happens when a Promise rejects and no valid .catch() or surrounding try/catch handles it.

• Missing await on asynchronous calls
• Functions returning before async work completes
• Errors thrown inside callbacks or background tasks

---

Clarifying the Issue

Lambda requires your handler to either resolve or reject its returned Promise. If your function exits early or leaves async work running in the background, the runtime cannot capture the error — and you get an unhandled rejection.

• A Promise rejection with no handler causes Lambda to crash
• Incomplete awaits cause requests to finish while async work is still pending
• Asynchronous callbacks (like setTimeout, event listeners) don’t bubble errors to the handler

---

Why It Matters

Unhandled rejections kill Lambda invocations immediately and often silently corrupt downstream work.

• External requests are fired but responses never recorded
• Partial writes or partial deletes occur
• Downstream retries multiply load unexpectedly

---

Key Terms

• Promise – A JavaScript construct representing asynchronous work
• Unhandled rejection – A Promise that errors without a .catch()
• Event loop – Node.js asynchronous scheduling mechanism

---

Steps at a Glance

1. Confirm the unhandled rejection in CloudWatch
2. Identify the async call that rejected
3. Check for missing await
4. Wrap handler logic in try/catch
5. Add .catch() to all Promise chains
6. Avoid unsupported async patterns
7. Re-test with a controlled invocation

---

Detailed Steps

Step 1: Confirm the unhandled rejection in CloudWatch.

Start by reviewing the recent logs:

aws logs tail /aws/lambda/my-function --since 5m

You should see something like:

UnhandledPromiseRejectionWarning: Error: Request failed

This confirms a Promise rejected with no handler. Next, find the failing async call.

---

Step 2: Identify the async call that rejected.

Look for the function that produced the error. Often it’s an HTTP request, AWS SDK call, or database call.

Now check whether that async function was awaited.

---

Step 3: Check for missing await.

Broken code example:

const data = fetch(url);  // missing await
return { ok: true };

Lambda returns before the Promise resolves. Fix it:

const data = await fetch(url);
return { ok: true };

Now wrap the handler.

---

Step 4: Wrap handler logic in try/catch.

Add a protective wrapper around your async handler:

exports.handler = async (event) => {
  try {
    const res = await doWork();
    return res;
  } catch (err) {
    console.error(err);
    throw err;
  }
};

Now secure remaining Promise chains.

---

Step 5: Add .catch() to all Promise chains.

Example broken pattern:

doThing().then(process);

If doThing() rejects, the error is lost. Fix it:

doThing()
  .then(process)
  .catch(err => {
    console.error(err);
    throw err;
  });

Now check for unsupported async patterns.

---

Step 6: Avoid unsupported async patterns.

The following do not propagate errors into the handler:

• setTimeout
• setInterval
• EventEmitter callbacks
• Detached async functions (fire-and-forget patterns)

If code uses these, errors must be manually caught and re-thrown. Now test the fix.

---

Step 7: Re-test with a controlled invocation.

aws lambda invoke \
  --function-name my-function \
  --payload '{}' \
  out.json

If no unhandled rejection appears in logs, the issue is resolved.

---

Pro Tips

Use async/await consistently.

Mixing .then() chains and await leads to subtle unhandled rejections.

Log early, log cleanly.

Printing the rejection cause in Step 4 dramatically simplifies debugging.

Never fire background async tasks.

Lambda may freeze or terminate before they complete.

---

Conclusion

Unhandled promise rejections occur when asynchronous work fails without proper error handling. By ensuring all async calls are awaited, wrapping logic in try/catch, and avoiding unsupported async patterns, you prevent silent failures and keep your Lambda’s execution predictable and stable.

---

Aaron Rose is a software engineer and technology writer at tech-reader.blog and the author of Think Like a Genius.