Define Lambda function handler in Node.js

The Lambda function handler is the method in your function code that processes events. When your function is invoked, Lambda runs the handler method. Your function runs until the handler returns a response, exits, or times out.

This page describes how to work with Lambda function handlers in Node.js, including options for project setup, naming conventions, and best practices. This page also includes an example of a Node.js Lambda function that takes in information about an order, produces a text file receipt, and puts this file in an Amazon Simple Storage Service (Amazon S3) bucket. For information about how to deploy your function after writing it, see Deploy Node.js Lambda functions with .zip file archives or Deploy Node.js Lambda functions with container images.

Setting up your Node.js handler project

There are multiple ways to initialize a Node.js Lambda project. For example, you can create a standard Node.js project using npm, create an AWS SAM application, or create an AWS CDK application.

To create the project using npm:

npm init

This command initializes your project and generates a package.json file that manages your project's metadata and dependencies.

Your function code lives in a .js or .mjs JavaScript file. In the following example, we name this file index.mjs because it uses an ES module handler. Lambda supports both ES module and CommonJS handlers. For more information, see Designating a function handler as an ES module.

A typical Node.js Lambda function project follows this general structure:

/project-root
  ├── index.mjs — Contains main handler
  ├── package.json — Project metadata and dependencies
  ├── package-lock.json — Dependency lock file
  └── node_modules/ — Installed dependencies

Example Node.js Lambda function code

The following example Lambda function code takes in information about an order, produces a text file receipt, and puts this file in an Amazon S3 bucket.

import { S3Client, PutObjectCommand } from '@aws-sdk/client-s3';

// Initialize the S3 client outside the handler for reuse
const s3Client = new S3Client();

/**
 * Lambda handler for processing orders and storing receipts in S3.
 * @param {Object} event - Input event containing order details
 * @param {string} event.order_id - The unique identifier for the order
 * @param {number} event.amount - The order amount
 * @param {string} event.item - The item purchased
 * @returns {Promise<string>} Success message
 */
export const handler = async(event) => {
    try {
        // Access environment variables
        const bucketName = process.env.RECEIPT_BUCKET;
        if (!bucketName) {
            throw new Error('RECEIPT_BUCKET environment variable is not set');
        }

        // Create the receipt content and key destination
        const receiptContent = `OrderID: ${event.order_id}\nAmount: $${event.amount.toFixed(2)}\nItem: ${event.item}`;
        const key = `receipts/${event.order_id}.txt`;

        // Upload the receipt to S3
        await uploadReceiptToS3(bucketName, key, receiptContent);

        console.log(`Successfully processed order ${event.order_id} and stored receipt in S3 bucket ${bucketName}`);
        return 'Success';
    } catch (error) {
        console.error(`Failed to process order: ${error.message}`);
        throw error;
    }
};

/**
 * Helper function to upload receipt to S3
 * @param {string} bucketName - The S3 bucket name
 * @param {string} key - The S3 object key
 * @param {string} receiptContent - The content to upload
 * @returns {Promise<void>}
 */
async function uploadReceiptToS3(bucketName, key, receiptContent) {
    try {
        const command = new PutObjectCommand({
            Bucket: bucketName,
            Key: key,
            Body: receiptContent
        });

        await s3Client.send(command);
    } catch (error) {
        throw new Error(`Failed to upload receipt to S3: ${error.message}`);
    }
}

This index.mjs file contains the following sections of code:

For this function to work properly, its execution role must allow the s3:PutObject action. Also, ensure that you define the RECEIPT_BUCKET environment variable. After a successful invocation, the Amazon S3 bucket should contain a receipt file.

Handler naming conventions

When you configure a function, the value of the Handler setting is the file name and the name of the exported handler method, separated by a dot. The default for functions created in the console and for examples in this guide is index.handler. This indicates the handler method that's exported from the index.js or index.mjs file.

If you create a function in the console using a different file name or function handler name, you must edit the default handler name.

Defining and accessing the input event object

JSON is the most common and standard input format for Lambda functions. In this example, the function expects an input similar to the following:

{
    "order_id": "12345",
    "amount": 199.99,
    "item": "Wireless Headphones"
}

When working with Lambda functions in Node.js, you can define the expected shape of the input event using JSDoc annotations. In this example, we define the input structure in the handler's JSDoc comment:

/**
 * Lambda handler for processing orders and storing receipts in S3.
 * @param {Object} event - Input event containing order details
 * @param {string} event.order_id - The unique identifier for the order
 * @param {number} event.amount - The order amount
 * @param {string} event.item - The item purchased
 * @returns {Promise<string>} Success message
 */

After you define these types in your JSDoc comment, you can access the fields of the event object directly in your code. For example, event.order_id retrieves the value of order_id from the original input.

Valid handler patterns for Node.js functions

We recommend that you use async/await to declare the function handler instead of using callbacks. Async/await is a concise and readable way to write asynchronous code, without the need for nested callbacks or chaining promises. With async/await, you can write code that reads like synchronous code, while still being asynchronous and non-blocking.

Using async/await (recommended)

The async keyword marks a function as asynchronous, and the await keyword pauses the execution of the function until a Promise is resolved. The handler accepts the following arguments:

Here are the valid signatures for the async/await pattern:

Using callbacks

Callback handlers can use the event, context, and callback arguments. Here are the valid signatures:

The callback function expects an Error and a response, which must be JSON-serializable. The function continues to execute until the event loop is empty or the function times out. The response isn't sent to the invoker until all event loop tasks are finished. If the function times out, an error is returned instead. You can configure the runtime to send the response immediately by setting context.callbackWaitsForEmptyEventLoop to false.

import https from "https";
let url = "https://aws.amazon.com/";

export const handler = (event, context, callback) => {
  https.get(url, (res) => {
    callback(null, res.statusCode);
  }).on("error", (e) => {
    callback(Error(e));
  });
};

Using the SDK for JavaScript v3 in your handler

Often, you’ll use Lambda functions to interact with or make updates to other AWS resources. The simplest way to interface with these resources is to use the AWS SDK for JavaScript. All supported Lambda Node.js runtimes include the SDK for JavaScript version 3. However, we strongly recommend that you include the AWS SDK clients that you need in your deployment package. This maximizes backward compatibility during future Lambda runtime updates. Only rely on the runtime-provided SDK when you can't include additional packages (for example, when using the Lambda console code editor or inline code in an AWS CloudFormation template).

To add SDK dependencies to your function, use the npm install command for the specific SDK clients that you need. In the example code, we used the Amazon S3 client. Add this dependency by running the following command in the directory that contains your package.json file:

npm install @aws-sdk/client-s3

In the function code, import the client and commands that you need, as the example function demonstrates:

import { S3Client, PutObjectCommand } from '@aws-sdk/client-s3';

Then, initialize an Amazon S3 client:

const s3Client = new S3Client();

In this example, we initialized our Amazon S3 client outside of the main handler function to avoid having to initialize it every time we invoke our function. After you initialize your SDK client, you can then use it to make API calls for that AWS service. The example code calls the Amazon S3 PutObject API action as follows:

const command = new PutObjectCommand({
    Bucket: bucketName,
    Key: key,
    Body: receiptContent
});

Accessing environment variables

In your handler code, you can reference any environment variables by using process.env. In this example, we reference the defined RECEIPT_BUCKET environment variable using the following lines of code:

// Access environment variables
const bucketName = process.env.RECEIPT_BUCKET;
if (!bucketName) {
    throw new Error('RECEIPT_BUCKET environment variable is not set');
}

Using global state

Lambda runs your static code during the initialization phase before invoking your function for the first time. Resources created during initialization stay in memory between invocations, so you can avoid having to create them every time you invoke your function.

In the example code, the S3 client initialization code is outside the handler. The runtime initializes the client before the function handles its first event, and the client remains available for reuse across all invocations.

Code best practices for Node.js Lambda functions

Follow these guidelines when building Lambda functions: