Lambda Functions Unhandled Promise Rejection

What is Lambda Functions Unhandled Promise Rejection

Understanding AWS Lambda Functions

AWS Lambda is a serverless compute service that lets you run code without provisioning or managing servers. It executes your code only when needed and scales automatically, from a few requests per day to thousands per second. Lambda functions are triggered by events, such as changes to data in an Amazon S3 bucket or an update to a DynamoDB table.

Recognizing the Symptom: Unhandled Promise Rejection

When working with AWS Lambda, you might encounter the error: Unhandled Promise Rejection. This typically manifests as a runtime error in your logs, indicating that a promise was rejected but not handled properly. This can lead to unexpected behavior or crashes in your Lambda function.

Common Error Message

The error message might look something like this:

UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch().

Explaining the Issue: Unhandled Promise Rejection

In JavaScript, promises are used to handle asynchronous operations. An unhandled promise rejection occurs when a promise is rejected, but there is no .catch() method to handle the rejection. This can cause your Lambda function to fail unexpectedly, as the error is not properly managed.

Why It Happens

This issue often arises when developers forget to add error handling to promises, especially in complex asynchronous code. It can also occur if an error is thrown inside an async function without a try-catch block.

Steps to Fix the Unhandled Promise Rejection

To resolve this issue, ensure that all promises in your Lambda function have appropriate catch handlers. Here are the steps to fix it:

1. Identify Promises Without Catch Handlers

Review your Lambda function code to find all instances of promises. Look for promises that do not have a .catch() method chained to them.

2. Add Catch Handlers

For each promise, add a .catch() method to handle any potential rejections. For example:

myPromiseFunction() .then(result => { // handle success }) .catch(error => { console.error('Error occurred:', error); // handle error });

3. Use Try-Catch in Async Functions

If you are using async functions, wrap your code in a try-catch block to handle errors:

async function myAsyncFunction() { try { const result = await myPromiseFunction(); // handle success } catch (error) { console.error('Error occurred:', error); // handle error }}

4. Test Your Lambda Function

After making these changes, test your Lambda function to ensure that all promises are handled correctly and that the function behaves as expected.

Additional Resources

For more information on handling promises in JavaScript, check out the following resources:

MDN Web Docs: Using Promises AWS Lambda Official Documentation