AWS Lambda Node 18.x ESM Module simple import of .js file

Question

I got following error in as ESM Project on AWS Lambda Node 18.x:

{ 
  "errorType": "Error",
  "errorMessage": "Cannot find module '/var/task/src/storefront/api/checkout/preview' imported from /var/task/src/storefront/api/checkout/index.js",
  "code": "ERR_MODULE_NOT_FOUND",
...
}

The error refers to this simple barelling in index.js:

export { preview } from './preview';
...

Adding .js to the export fixes the error:

export { preview } from './preview.js';

So not a path reference or permission problem. But that knowledge doesn't help me much because my code is automatically compiled from typescript to esm javascript and I would think that import without .js is standard. Anyone an idea why lambda makes this problems and how to fix it?

Here my package.json:

{
  "name": "checkout",
  "type": "module",
  "main": "index.js",
  "version": "0.1.0",
  "engines": {
    "node": ">=18.0.0"
  },
 ...

tsconfig.js:

{
  "compilerOptions": {
    "preserveConstEnums": true,
    "strictNullChecks": true,
    "sourceMap": true,
    "target": "es2022",
    "outDir": ".build",
    "moduleResolution": "node",
    "lib": [
      "es2022"
    ],
    "skipLibCheck": true,
    "rootDir": "./",
    "typeRoots": [
      "./node_modules/@types",
      "./@types"
    ]
  },
  "include": [
    "./src/**/*"
  ],
    "exclude": [   
    "node_modules"
  ]
}

I deployed with serverless.yml (runtime: nodejs18.x) and serverless-typescript plugin.

Answer 1

Looks similar to this question:

Cannot use ES6 on AWS Lambda function; how to import ES6 module within Lambda

TLDR:

AWS Lambda needs the handler to be configured in the form of filename.methodname, it will always use the .js extension, which makes it not possible to use a different file extension for your entry point.