AWS Lambda: Runtime.UserCodeSyntaxError: Cannot use import statement outside
The Node.js runtime decides whether your handler is an ES module or CommonJS from the file extension and the nearest package.json, exactly as Node does locally. An index.js containing import statements is treated as CommonJS unless package.json says "type": "module". Locally a bundler or ts-node often hides this, and the package.json frequently never makes it into the deployment zip.
Quick fix
Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.
# What actually got deployed? Look for package.json next to the handler
aws lambda get-function --function-name my-fn --query Code.Location --output text | xargs curl -s -o fn.zip
unzip -l fn.zip
# Option 1: rename the handler file
mv index.js index.mjs # handler setting stays index.handler
# Option 2: declare ESM in package.json and include it in the zip
npm pkg set type=module
zip -r fn.zip index.js package.json node_modules
# Option 3: have your bundler emit CommonJS instead
npx esbuild src/index.ts --bundle --platform=node --format=cjs --outfile=dist/index.jsHow to diagnose Serverless errors
Serverless errors are shaped by hard platform limits: execution duration, deployment package size, memory, and concurrency. Unlike a server you control, these cannot be tuned away. The fix is to restructure the workload. VPC-attached functions add a second class of problem, because they lose default internet access and depend on ENI capacity and NAT configuration.
If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.
- Check the platform limit before changing code: timeout, package size and memory are all documented, and the error is usually the limit being reported literally.
- For timeouts, add timing logs around each external call. A function that times out is nearly always waiting on one downstream dependency.
- Reduce package size with layers, tree-shaking, and by excluding dev dependencies and the provider SDK that the runtime already includes.
- For VPC functions with no internet access, confirm there is a NAT Gateway or the relevant VPC endpoints. A function in a private subnet cannot reach the public internet by default.
- Measure cold start separately from warm invocation in your metrics, and use provisioned concurrency only after confirming cold starts are the actual problem.
Tools worth reaching for
CloudWatch Logs Insightsaws lambda get-function-configurationX-Ray / distributed tracingsam local invoke
Authoritative references
Primary documentation for this error, worth reading before applying any fix in production.
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Something missing or wrong?
This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.