AWS SDK for JavaScript
Developer Guide for SDK v2

Loading Credentials in Node.js from IAM Roles for EC2

If you run your Node.js application on an Amazon EC2 instance, you can leverage IAM roles for Amazon EC2 to automatically provide credentials to the instance. If you configure your instance to use IAM roles, the SDK automatically selects the IAM credentials for your application, eliminating the need to manually provide credentials.

For more information on adding IAM roles to an Amazon EC2 instance, see IAM Roles for Amazon EC2.