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Simplifies writing unit tests for AWS Lambda functions using Node.js.


  • Verifies correct handler behavior
  • Mocks event types with code
  • Works asynchronously
  • Supports Promises
  • Verifies Node.js runtime version
  • AWS X-Ray support [experimental]
  • Detects resource leaks [experimental]
  • Easily integrates with test frameworks (Mocha and Jasmine)
  • Handlers can be loaded and removed after execution
  • Lightweight and won't impact performance
  • Maps the environment variable LAMBDA_TASK_ROOT to the application's root
  • Automatically loads .env files
  • Works with Node 10.x, and 12.x


Install via npm.

npm install lambda-tester --save-dev

Getting Started

Lambda handlers with support for callbacks use the typical Node.js asynchronous signature:

exports.handler = function( event, context, callback ) {

    callback( null, 'success!' );

The following example shows a simple case for validating that the Lambda (handler) was called successfully (i.e. callback( null, result ):

const LambdaTester = require( 'lambda-tester' );

const myHandler = require( '../index' ).handler;

describe( 'handler', function() {

	it( 'test success', async function() {

		await LambdaTester( myHandler )
			.event( { name: 'Fred' } )

If the handler calls callback( err ), then the test will fail.

Additionally, if one wanted to test for failure, then the following code would be used:

const LambdaTester = require( 'lambda-tester' );

const myHandler = require( '../index' ).handler;

describe( 'handler', function() {

	it( 'test failure', async function() {

		await LambdaTester( myHandler )
			.event( { name: 'Unknown' } )

Note: you must either return the LambdaTester instance back to the testing framework or use the await/async keywords.


Complete documentation can be found in our documentation page.

Projects Using lambda-tester

  • vandium - Secures and simplifies AWS Lambda handlers


We'd love to get feedback on how you're using lambda-tester and things we could add to make this tool better. Feel free to contact us at


Starting with version 4.0, lambda-tester supports node versions 10 and higher. If you require support for older versions of node, then use a previous version of lambda-tester.