How to solve "Cannot use import statement outside a module" error from AWS Lambda deployed with CDK

Question

I am using the AWS CDK (v2.140.0) to deploy a single lambda function written in ES6 javascript and with a Node v20.x runtime. The root of the project contains a package.json file in which I am defining "type": "module". I am using aws CDK and SDK packages but otherwise am not consuming any 3rd party libraries in my application code.

My file structure looks like:

bin
  cdk.js
lib
  my-stack.js
src
  index.js
package.json
cdk.json

I am able to successfully synth and deploy the lambda to AWS. The lambda runs on an Eventbridge schedule and is invoked at the correct times.

However, I am getting the following error in Cloudwatch: SyntaxError: Cannot use import statement outside a module

My index.js file is written as:

import { foo } from './foo-service.js';

export const handler = async () => {
  // do stuff with foo
};

I have gone down a few rabbit holes and tried most of the suggestions from this SO post without any success.

• changing my index.js file to a index.mjs
• using the "node" specific CDK construct
• creating an additional package.json file within the src directory and defining "type": "module"

Any insight or guidance is much appreciated.

Answer 1

Ultimately my issue came from following an AWS tutorial in which the lambda function was contained entirely within a single index.js file, and in my case my handler required modules outside of that file. I needed to change my cdk implementation to:

    // BEFORE
    const lambdaFn = new Lambda.Function(this, 'lambda-id', {
      runtime: Lambda.Runtime.NODEJS_20_X,
      handler: 'index.handler',
      code: new Lambda.InlineCode(fs.readFileSync('./src/index.js', { encoding: 'utf-8' }))
    });

    // AFTER
    const lambdaFn = new Lambda.Function(this, 'lambda-id', {
      runtime: Lambda.Runtime.NODEJS_20_X,
      handler: 'index.handler',
      code: Lambda.Code.fromAsset('./src', { exclude: ['*.js'] })
    });