AWS 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. When the handler exits or returns a response, it becomes available to handle another event.

The following example function logs the contents of the event object and returns the location of the logs.

Note

This page shows examples of both CommonJS and ES module handlers. To learn about the difference between these two handler types, see Designating a function handler as an ES module.

CommonJS module handler

exports.handler = async function (event, context) {
  console.log("EVENT: \n" + JSON.stringify(event, null, 2));
  return context.logStreamName;
};

ES module handler

export const handler = async (event, context) => {
  console.log("EVENT: \n" + JSON.stringify(event, null, 2));
  return context.logStreamName;
};

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 in the console and for examples in this guide is index.handler. This indicates the handler method that's exported from the index.js file.

The runtime passes arguments to the handler method. The first argument is the event object, which contains information from the invoker. The invoker passes this information as a JSON-formatted string when it calls Invoke, and the runtime converts it to an object. When an AWS service invokes your function, the event structure varies by service.

The second argument is the context object, which contains information about the invocation, function, and execution environment. In the preceding example, the function gets the name of the log stream from the context object and returns it to the invoker.

You can also use a callback argument, which is a function that you can call in non-async handlers to send a response. We recommend that you use async/await instead of callbacks. Async/await provides improved readability, error handling, and efficiency. For more information about the differences between async/await and callbacks, see Using callbacks.

Using async/await

If your code performs an asynchronous task, use the async/await pattern to make sure that the handler finishes running. Async/await is a concise and readable way to write asynchronous code in Node.js, 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.

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

Note

Make sure to wait for asynchronous events to complete. If the function returns before async events are complete, the function might fail or cause unexpected behavior in your application. This can happen when a forEach loop contains an async event. forEach loops expect a synchronous call. For more information, see Array.prototype.forEach() in the Mozilla documentation.

CommonJS module handler

Example – HTTP request with async/await

const https = require("https");
let url = "https://aws.amazon.com/";

exports.handler = async function (event) {
  let statusCode;
  await new Promise(function (resolve, reject) {
    https.get(url, (res) => {
        statusCode = res.statusCode;
        resolve(statusCode);
      }).on("error", (e) => {
        reject(Error(e));
      });
  });
  console.log(statusCode);
  return statusCode;
};

ES module handler

Example – HTTP request with async/await

This example uses fetch, which is available in the nodejs18.x runtime.

const url = "https://aws.amazon.com/";

export const handler = async(event) => {
    try {
        // fetch is available with Node.js 18
        const res = await fetch(url);
        console.info("status", res.status);
        return res.status;
    }
    catch (e) {
        console.error(e);
        return 500;
    }
};

The next example uses async/await to list your Amazon Simple Storage Service buckets.

Note

Before using this example, make sure that your function's execution role has Amazon S3 read permissions.

CommonJS module handler

Example – AWS SDK v2 with async/await

This example uses the AWS SDK for JavaScript v2, which is available in the Node.js 12, 14, and 16 runtimes.

const AWS = require('aws-sdk')
const s3 = new AWS.S3()
   
exports.handler = async function(event) {
  const buckets = await s3.listBuckets().promise()
  return buckets
}

ES module handler

Example – AWS SDK v3 with async/await

This example uses the AWS SDK for JavaScript v3, which is available in the nodejs18.x runtime.

import {S3Client, ListBucketsCommand} from '@aws-sdk/client-s3';
const s3 = new S3Client({region: 'us-east-1'});
   
export const handler = async(event) => {
  const data = await s3.send(new ListBucketsCommand({}));
  return data.Buckets;
        
};

Using callbacks

We recommend that you use async/await to declare the function handler instead of using callbacks. Async/await is a better choice for several reasons:

• Readability: Async/await code is easier to read and understand than callback code, which can quickly become difficult to follow and result in callback hell.

• Debugging and error handling: Debugging callback-based code can be difficult. The call stack can become hard to follow and errors can easily be swallowed. With async/await, you can use try/catch blocks to handle errors.

• Efficiency: Callbacks often require switching between different parts of the code. Async/await can reduce the number of context switches, resulting in more efficient code.

When you use callbacks in your handler, 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.

The callback function takes two arguments: an Error and a response. The response object must be compatible with JSON.stringify.

The following example function checks a URL and returns the status code to the invoker.

CommonJS module handler

Example – HTTP request with callback

const https = require("https");
let url = "https://aws.amazon.com/";

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

ES module handler

Example – HTTP request with callback

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

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

In the next example, the response from Amazon S3 is returned to the invoker as soon as it's available. The timeout running on the event loop is frozen, and it continues running the next time the function is invoked.

Note

Before using this example, make sure that your function's execution role has Amazon S3 read permissions.

CommonJS module handler

Example – AWS SDK v2 with callbackWaitsForEmptyEventLoop

This example uses the AWS SDK for JavaScript v2, which is available in the Node.js 12, 14, and 16 runtimes.

const AWS = require("aws-sdk");
const s3 = new AWS.S3();

exports.handler = function (event, context, callback) {
  context.callbackWaitsForEmptyEventLoop = false;
  s3.listBuckets(null, callback);
  setTimeout(function () {
    console.log("Timeout complete.");
  }, 5000);
};

ES module handler

Example – AWS SDK v3 with callbackWaitsForEmptyEventLoop

This example uses the AWS SDK for JavaScript v3, which is available in the nodejs18.x runtime.

import AWS from "@aws-sdk/client-s3";
const s3 = new AWS.S3({});

export const handler = function (event, context, callback) {
  context.callbackWaitsForEmptyEventLoop = false;
  s3.listBuckets({}, callback);
  setTimeout(function () {
    console.log("Timeout complete.");
  }, 5000);
};