Writing Tests - ServerlessSpy

Writing Tests

Creating ServerlessSpyListener

Include the TypeScript file that was generated when deploying

import { ServerlessSpyEvents } from '../serverlessSpyEvents/ServerlessSpyEvents';

Get ServerlessSpy WebSocket URL from CloudFormation output

const exportLocation = path.join(__dirname, 'cdkOutput.json');
const output = JSON.parse(fs.readFileSync(exportLocation).toString())[

Initialize the ServerlessSpyListener

let serverlessSpyListener: ServerlessSpyListener<ServerlessSpyEvents>;
beforeEach(async () => {
  serverlessSpyListener =
    await createServerlessSpyListener<ServerlessSpyEvents>({
      serverlessSpyWsUrl: output.ServerlessSpyWsUrl,

Close the ServerlessSpyListener

afterEach(async () => {

Writing test

There are several ways that you can use to write a test.

Waiting for the event to occur

await serverlessSpyListener.waitForXXX<TestData>()


await serverlessSpyListener.waitForXXX()

WaitFor methods are generated with TypeScript. You should name your resources in CDK in a way that these methods are easy to read.

Please note that there is no order enforced on events. The same event can match with multiple WaitFor calls. When WaitFor is called, it checks for a matching event in the bucket of all received events since initializing ServerlessSpyListener. The matching event will not be removed. If the event is not found, it starts to wait for one. So if you write await serverlessSpyListener.waitForXXX<TestData>() twice, because you expect events to occur twice, it will not work. The first event will satisfy both WaitFor calls.

TestData is an optional generic argument that makes all parameters strongly typed. If that is useful, decide based on your use case.

Filtering the events

It makes a lot of sense to filter a message by some condition, like ID.

await serverlessSpyListener.waitForSnsTopicMyTopic<TestData>({
  condition: (d) => === id,

This way, you can run tests in parallel and handle cases when a similar event occurs multiple times as part of the same test.

đź’ˇRecommendation: Try not to use conditions for validating the event. Just find the event with the help of conditions and then validate it with Jest. Tests are easier to debug this way because the error shows which part of the data does not match. But if you like using just conditions, so tests are easy to read, go ahead.

Extracting the event data with getData()

const event = (await serverlessSpyListener.waitForXXX<TestData>()).getData();

You can validate that event with Jest, or any other testing framework like you always do:


Using ServerlessSpy integration with Jest

The previous example can be done in a much slicker way:

(await serverlessSpyListener.waitForXXX<TestData>()).toMatchObject(...);

Tracing event through the same Lambda call

Tests for the same Lambda request can be chained together:

  await (
    await (
      await serverlessSpyListener.waitForFunctionMyLambdaRequest<TestData>({
        condition: (d) => ===,
        request: myData,
      request: myData,
      console: {
        message: 'My console log message',
        optionalParams: [myData],
  request: myData,
  response: {
    message: `${myData.message} ServerlessSpy`,

That is useful when you expect a Lambda request that will produce some data. You can also validate intermediate steps of processing with console log events.

more on spying on Lambda