Local Testing with SAM

Local Testing with SAM Overview

Local Testing with SAM lets you structure AWS Lambda work so it stays readable, testable, and easy to scale. Instead of ad-hoc code, you follow a clear pattern that other developers can recognise immediately.

The key is to keep local testing with sam focused and predictable. Start from the minimal example here, then layer in only the complexity your feature actually needs.

import { handler } from '../src/handler.mjs';

describe('handler', () => {
  it('returns a 200 response', async () => {
    const event = { httpMethod: 'GET', pathParameters: { id: '1' } };
    const result = await handler(event);
    expect(result.statusCode).toBe(200);
  });
});

Unit tests invoke the handler directly with a mock event and assert on the returned response.

Local Testing with SAM Example

// handler.mjs
export const handler = async (event, context) => {
  // 1. read input from the event
  // 2. do the work
  // 3. return a response (or throw on error)
};

• Start from a minimal Local Testing with SAM example and grow it only as needed.
• Keep configuration explicit so Local Testing with SAM behaves the same in every environment.
• Name things clearly so teammates understand your Local Testing with SAM at a glance.
• Add tests around Local Testing with SAM early to lock in expected behaviour.

AWS Lambda Cheatsheet

Handy reference for working with local testing with sam in AWS Lambda and Node.js.

How Local Testing with SAM Works in AWS Lambda

Local Testing with SAM runs inside the managed Lambda execution environment. AWS provisions a micro-VM, loads your Node.js code, runs any module-scope initialisation once, and then invokes your handler for each event.

Unit tests invoke the handler directly with a mock event and assert on the returned response.

• Handlers should be small and do one job well.
• Initialise SDK clients and config outside the handler to reuse them on warm starts.
• Return quickly and let event sources handle retries where possible.
• Emit structured logs so CloudWatch and X-Ray can correlate activity.

Practical Guidance for Local Testing with SAM

On real projects, local testing with sam works best when it is observable, secure, and cheap to run. Grant least-privilege IAM, validate every input, and keep the deployment package small.

Common Mistakes

• Skipping error handling and edge cases when wiring up local testing with sam.
• Leaving local testing with sam untested, so regressions slip into production.
• Over-engineering local testing with sam before you actually need the extra flexibility.
• Ignoring documentation, which makes local testing with sam hard for the next developer to change.

Key Takeaways

• Local Testing with SAM is a core part of working effectively with AWS Lambda.
• Start small and keep local testing with sam focused on a single responsibility.
• Apply consistent patterns so local testing with sam scales across your project.
• Test and document local testing with sam to keep it maintainable over time.