AWS Lambda
Developer Guide

Lambda Function Handler (Node.js)

AWS Lambda invokes your Lambda function via a handler object. A handler represents the name of your Lambda function (and serves as the entry point that AWS Lambda uses to execute your function code. For example:

exports.myHandler = function(event, context) { ... }
  • myHandler – This is the name of the function AWS Lambda invokes. Suppose you save this code as helloworld.js. Then, myHandler is the function that contains your Lambda function code and helloworld is the name of the file that represents your deployment package. For more information, see Creating a Deployment Package (Node.js).

    AWS Lambda supports two invocation types:

    • RequestResponse, or synchronous execution: AWS Lambda returns the result of the function call to the client invoking the Lambda function. If the handler code of your Lambda function does not specify a return value, AWS Lambda will automatically return null for that value.

    • Event, or asynchronous execution: AWS Lambda will discard any results of the function call.


      If you discover that your Lambda function does not process the event using asynchronous invocation, you can investigate the failure using Dead Letter Queues.

      Event sources can range from a supported AWS service or custom applications that invoke your Lambda function. For examples, see Sample Events Published by Event Sources

  • context – AWS Lambda uses this parameter to provide details of your Lambda function's execution. For more information, see The Context Object (Node.js).

Using the Callback Parameter

The Node.js runtimes v6.10 and v8.10 support the optional callback parameter. You can use it to explicitly return information back to the caller. The general syntax is:

callback(Error error, Object result);


  • error – is an optional parameter that you can use to provide results of the failed Lambda function execution. When a Lambda function succeeds, you can pass null as the first parameter.

  • result – is an optional parameter that you can use to provide the result of a successful function execution. The result provided must be JSON.stringify compatible. If an error is provided, this parameter is ignored.

If you don't use callback in your code, AWS Lambda will call it implicitly and the return value is null.

When the callback is called (explicitly or implicitly), AWS Lambda continues the Lambda function invocation until the event loop is empty.

The following are example callbacks:

callback(); // Indicates success but no information returned to the caller. callback(null); // Indicates success but no information returned to the caller. callback(null, "success"); // Indicates success with information returned to the caller. callback(error); // Indicates error with error information returned to the caller.

AWS Lambda treats any non-null value for the error parameter as a handled exception.

Note the following:

  • Regardless of the invocation type specified at the time of the Lambda function invocation (see Invoke), the callback method automatically logs the string representation of non-null values of error to the Amazon CloudWatch Logs stream associated with the Lambda function.

  • If the Lambda function was invoked synchronously (using the RequestResponse invocation type), the callback returns a response body as follows:

    • If error is null, the response body is set to the string representation of result.

    • If the error is not null, the error value will be populated in the response body.


When the callback(error, null) (and callback(error)) is called, Lambda will log the first 256 KB of the error object. For a larger error object, AWS Lambda truncates the log and displays the text Truncated by Lambda next to the error object.

If you are using runtime version 8.10, you can include the async keyword:

exports.myHandler = async function(event, context) { ... // return information to the caller. }


Consider the following Node.js example code.

exports.myHandler = function(event, context, callback) { console.log("value1 = " + event.key1); console.log("value2 = " + event.key2); callback(null, "some success message"); // or // callback("some error type"); }

This example has one function, myHandler

In the function, the console.log() statements log some of the incoming event data to CloudWatch Logs. When the callback parameter is called, the Lambda function exits only after the event loop passed is empty.

If you want to use the async feature provided by the v8.10 runtime, consider the following code sample:

exports.myHandler = async function(event, context) { console.log("value1 = " + event.key1); console.log("value2 = " + event.key2); return "some success message”; // or // throw new Error(“some error type”); }

To upload and test this code as a Lambda function (console)

  1. In the console, create a Lambda function using the following information:

    • Use the hello-world blueprint.

    • The sample uses nodejs6.10 as the runtime but you can also select nodejs8.10. The code samples provided will work for any version.

    For instructions to create a Lambda function using the console, see Create a Simple Lambda Function.

  2. Replace the template code with the code provided in this section and create the function.

  3. Test the Lambda function using the Sample event template called Hello World provided in the Lambda console.