AWS Compute Blog

Using Node.js ES modules and top-level await in AWS Lambda

This post is written by Dan Fox, Principal Specialist Solutions Architect, Serverless.

AWS Lambda now enables the use of ECMAScript (ES) modules in Node.js 14 runtimes. This feature allows Lambda customers to use dependency libraries that are configured as ES modules, or to designate their own function code as an ES module. It provides customers the benefits of ES module features like import/export operators, language-level support for modules, strict mode by default, and improved static analysis and tree shaking. ES modules also enable top-level await, a feature that can lower cold start latency when used with Provisioned Concurrency.

This blog post shows how to use ES modules in a Lambda function. It also provides guidance on how to use top-level await with Provisioned Concurrency to improve cold start performance for latency sensitive workloads.

Designating a function handler as an ES module

You may designate function code as an ES module in one of two ways. The first way is to specify the “type” in the function’s package.json file. By setting the type to “module”, you designate all “.js” files in the package to be treated as ES modules. Set the “type” as “commonjs” to specify the package contents explicitly as CommonJS modules:

// package.json
  "name": "ec-module-example",
  "type": "module",
  "description": "This package will be treated as an ES module.",
  "version": "1.0",
  "main": "index.js",
  "author": "Dan Fox",
  "license": "ISC"

// index.js – this file will inherit the type from 
// package.json and be treated as an ES module.

import { double } from './lib.mjs';

export const handler = async () => {
    let result = double(6); // 12
    return result;

// lib.mjs

export function double(x) {
    return x + x;

The second way to designate a function as either an ES module or a CommonJS module is by using the file name extension. File name extensions override the package type directive.

File names ending in .cjs are always treated as CommonJS modules. File names ending in .mjs are always treated as ES modules. File names ending in .js inherit their type from the package. You may mix ES modules and CommonJS modules within the same package. Packages are designated as CommonJS by default:

// this file is named index.mjs – it will always be treated as an ES module
import { square } from './lib.mjs';

export async function handler() {
    let result = square(6); // 36
    return result;

// lib.mjs
export function square(x) {
    return x * x;

Understanding Provisioned Concurrency

When a Lambda function scales out, the process of allocating and initializing new runtime environments may increase latency for end users. Provisioned Concurrency gives customers more control over cold start performance by enabling them to create runtime environments in advance.

In addition to creating execution environments, Provisioned Concurrency also performs initialization tasks defined by customers. Customer initialization code performs a variety of tasks including importing libraries and dependencies, retrieving secrets and configurations, and initializing connections to other services. According to an AWS analysis of Lambda service usage, customer initialization code is the largest contributor to cold start latency.

Provisioned Concurrency runs both environment setup and customer initialization code. This enables runtime environments to be ready to respond to invocations with low latency and reduces the impact of cold starts for end users.

Reviewing the Node.js event loop

Node.js has an event loop that causes it to behave differently than other runtimes. Specifically, it uses a non-blocking input/output model that supports asynchronous operations. This model enables it to perform efficiently in most cases.

For example, if a Node.js function makes a network call, that request may be designated as an asynchronous operation and placed into a callback queue. The function may continue to process other operations within the main call stack without getting blocked by waiting for the network call to return. Once the network call is returned, the callback is run and then removed from the callback queue.

This non-blocking model affects the Lambda execution environment lifecycle. Asynchronous functions written in the initialization block of a Node.js Lambda function may not complete before handler invocation. In fact, it is possible for function handlers to be invoked with open items remaining in the callback queue.

Typically, JavaScript developers use the await keyword to instruct a function to block and force it to complete before moving on to the next step. However, await is not permitted in the initialization block of a CommonJS JavaScript function. This behavior limits the amount of asynchronous initialization code that can be run by Provisioned Concurrency before the invocation cycle.

Improving cold start performance with top-level await

With ES modules, developers may use top-level await within their functions. This allows developers to use the await keyword in the top level of the file. With this feature, Node.js functions may now complete asynchronous initialization code before handler invocations. This maximizes the effectiveness of Provisioned Concurrency as a mechanism for limiting cold start latency.

Consider a Lambda function that retrieves a parameter from the AWS Systems Manager Parameter Store. Previously, using CommonJS syntax, you place the await operator in the body of the handler function:

// method1 – CommonJS

// CommonJS require syntax
const { SSMClient, GetParameterCommand } = require("@aws-sdk/client-ssm"); 

const ssmClient = new SSMClient();
const input = { "Name": "/configItem" };
const command = new GetParameterCommand(input);
const init_promise = ssmClient.send(command);

exports.handler = async () => {
    const parameter = await init_promise; // await inside handler

    const response = {
        "statusCode": 200,
        "body": parameter.Parameter.Value
    return response;

When you designate code as an ES module, you can use the await keyword at the top level of the code. As a result, the code that makes a request to the AWS Systems Manager Parameter Store now completes before the first invocation:

// method2 – ES module

// ES module import syntax
import { SSMClient, GetParameterCommand } from "@aws-sdk/client-ssm"; 

const ssmClient = new SSMClient();
const input = { "Name": "/configItem" }
const command = new GetParameterCommand(input);
const parameter = await ssmClient.send(command); // top-level await

export async function handler() {
    const response = {
        statusCode: 200,
        "body": parameter.Parameter.Value
    return response;

With on-demand concurrency, an end user is unlikely to see much difference between these two methods. But when you run these functions using Provisioned Concurrency, you may see performance improvements. Using top-level await, Provisioned Concurrency fetches the parameter during its startup period instead of during the handler invocation. This reduces the duration of the handler execution and improves end user response latency for cold invokes.

Performing benchmark testing

You can perform benchmark tests to measure the impact of top level await. I have created a project that contains two Lambda functions, one that contains an ES module and one that contains a CommonJS module.

Both functions are configured to respond to a single API Gateway endpoint. Both functions retrieve a parameter from AWS Systems Manager Parameter Store and are configured to use Provisioned Concurrency. The ES module uses top-level await to retrieve the parameter. The CommonJS function awaits the parameter retrieval in the handler.

Example architecture

Before deploying the solution, you need:

To deploy:

  1. From a terminal window, clone the git repo:
    git clone
  2. Change directory:
    cd ./aws-lambda-es-module-performance-benchmark
  3. Build the application:
    sam build
  4. Deploy the application to your AWS account:
    sam deploy --guided
  5. Take note of the API Gateway URL in the Outputs section.
    Deployment outputs

This post uses a popular open source tool Artillery to provide load testing. To perform load tests:

  1. Open config.yaml document in the /load_test directory and replace the target string with the URL of the API Gateway:
    target: “Put API Gateway url string here”
  2. From a terminal window, navigate to the /load_test directory:
    cd load_test
  3. Download and install dependencies:
    npm install
  4. Begin load test for the CommonJS function.
  5. Begin load test for ES module function.

Reviewing the results

Test results

Here is a side-by-side comparison of the results of two load tests of 600 requests each. The left shows the results for the CommonJS module and the right shows the results for the ES module. The p99 response time reflects the cold start durations when the Lambda service scales up the function due to load. The p99 for the CommonJS module is 603 ms while the p99 for the ES module is 340.5 ms, a performance improvement of 43.5% (262.5 ms) for the p99 of this comparison load test.

Cleaning up

To delete the sample application, use the latest version of the AWS SAM CLI and run:

sam delete


Lambda functions now support ES modules in Node.js 14.x runtimes. ES modules support await at the top-level of function code. Using top-level await maximizes the effectiveness of Provisioned Concurrency and can reduce the latency experienced by end users during cold starts.

This post demonstrates a sample application that can be used to perform benchmark tests that measure the impact of top-level await.

For more serverless content, visit Serverless Land.