ESM modules are finally gaining support for CJS. This means npm packages that use require will work in ESM land. Hooray 🎉.

I've used bundling with esBuild in the past with AWS Lambda. I didn't love the build step and how many npm packages wouldn't work in pure ESM land so I put it on the shelf.

But... now you can just deploy your ESM code, and things just work™, it's high time we start migrating older CJS code to ESM.

Let's jump into it.

This is a simple example demonstrating how to use ES Modules (ESM) with the Serverless Framework without requiring a build step.

• Why no bundle?
• How it works
• Our lambda code
• Our config
• Features
• Prerequisites
• Installation
• Usage

Why no bundle?

1. Bundling can mess with logging unless you setup sourcemaps.
2. It's super simple
3. You can read un-minified/compiled code in AWS console if you need to.

How it works

You simply need to set "type": "module" in your package.json

Then the serverless framework will package and deploy your unbundled ESM lambda function

This example repo demonstrates how to uses lodash-es as a direct dependency

Our lambda code

This is our lambda handler.

import { capitalize, reverse } from 'lodash-es'

export const handler = async (event) => {
  const message = 'hello world'
  const processedMessage = capitalize(message)
  const reversedMessage = reverse(message.split('')).join('')
  
  return {
    statusCode: 200,
    body: JSON.stringify({
      original: message,
      capitalized: processedMessage,
      reversed: reversedMessage
    })
  }
}

Our config

Below is a simple serverless.yml file with our runtime defined and a pointer to where our code lives in ./my-function.js.

service: my-esm-service

provider:
  name: aws
  runtime: nodejs22.x

functions:
  hello:
    handler: my-function.handler

# Optionally, you can exclude files from the packaged zip
package:
  patterns:
    - '!.gitignore'
    - '!.DS_Store'
    - '!README.md'

Features

• Native ES Module support using Node.js 22.x
• Uses lodash-es as an ESM-compatible dependency
• No bundling or transpilation required
• Simple Lambda function that processes text

Prerequisites

• Node.js 22.x or later
• Serverless Framework CLI installed (npm install -g serverless) or open serverless (npm install -g oss-serverless)
• AWS credentials configured

Installation

1. Clone the repository
2. Install dependencies:

npm install

Usage

1. Deploy the function:

serverless deploy

2. Invoke the function:

serverless invoke --function hello

3. Teardown

severless remove

Wrapping up

Thats about it. Nothing too fancy.

The code for this is in serverless-esm-functions-no-build-example repo.

Enjoy and happy coding.