serverless-framework-test-plugin

Overview

The Serverless Framework (serverless.com/framework/docs (opens in new window)) is one of the older deploy-frameworks for Lambda. Its test-friendly ecosystem includes:

• serverless-offline - emulates API Gateway + Lambda locally; HTTP test surface.
• serverless invoke local - one-off invocation from CLI.
• Per-runtime test plugins (serverless-jest-plugin, serverless-mocha-plugin) - install runners aware of serverless project structure.

When to use

• Lambda projects deployed via Serverless Framework.
• Tests that exercise API Gateway routing locally.
• Existing serverless-jest / -mocha tooling.

Authoring

Install

npm install -g serverless
npm install --save-dev serverless-offline serverless-jest-plugin jest

serverless.yml plugin registration

plugins:
  - serverless-offline
  - serverless-jest-plugin

functions:
  hello:
    handler: handler.hello
    events:
      - http:
          path: hello
          method: get

serverless invoke local (single invocation)

serverless invoke local --function hello --data '{"name":"world"}'

Output: handler's return value (JSON), plus emulated Lambda log lines.

serverless-offline (local HTTP)

serverless offline --httpPort 3000

Now curl http://localhost:3000/hello exercises the full Serverless → API Gateway → Lambda route.

serverless-jest-plugin

Per npmjs.com/package/serverless-jest-plugin (opens in new window):

serverless create test --function hello   # Generates test scaffold
serverless invoke test                     # Runs tests

The scaffold uses Jest under the hood; tests follow standard Jest patterns:

import { hello } from '../handler';

test('returns greeting', async () => {
  const event = { queryStringParameters: { name: 'world' } };
  const response = await hello(event);
  expect(response.statusCode).toBe(200);
  expect(JSON.parse(response.body).message).toContain('Hello world');
});

Direct handler test (lightweight)

// handler.ts
export const hello = async (event: any) => ({
  statusCode: 200,
  body: JSON.stringify({ message: `Hello ${event.queryStringParameters?.name ?? 'World'}` }),
});

// handler.test.ts
import { hello } from './handler';

test('hello-world', async () => {
  const result = await hello({ queryStringParameters: {} });
  expect(result.statusCode).toBe(200);
});

This skips the Serverless plugin entirely - often the cleanest path.

Environment variables

serverless-offline reads serverless.yml env definitions:

provider:
  environment:
    DATABASE_URL: ${env:DATABASE_URL}

Tests should mirror this via process.env.DATABASE_URL in jest setup.

Running

npx jest                                  # Direct handler tests
serverless offline                        # Local HTTP emulator
serverless invoke local --function hello  # CLI one-off

CI integration

jobs:
  serverless-tests:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v5
      - uses: actions/setup-node@v4
      - run: npm ci
      - run: npx jest

For deployed-Lambda smoke tests:

      - run: serverless deploy --stage staging
      - run: npx jest tests/staging-smoke/

Anti-patterns

Limitations

• Serverless Framework is one deploy tool of many. SAM, CDK, Terraform also deploy Lambdas; this skill is Serverless-specific.
• serverless-offline mirrors API Gateway but isn't identical. Subtle differences in CORS, multi-value headers.
• Cold-start budget per cold-start-budget-reference. Local invocation doesn't simulate Lambda cold start.
• Plugin ecosystem maintenance varies. serverless-jest is community-maintained; serverless-offline is well-maintained.

References

• Serverless Framework docs: serverless.com/framework/docs (opens in new window).
• serverless-offline: github.com/dherault/serverless-offline (opens in new window).
• serverless-jest-plugin: npmjs.com/package/serverless-jest-plugin (opens in new window).
• Companion catalogs: cold-start-budget-reference, lambda-timeout-budget-reference.
• Sibling tools (equivalent for other frameworks): aws-sam-local-testing, lambda-test-tools-net.
• Builder: serverless-integration-test-builder.