How to test Middy handler with mocked middleware using Jest?

[jgilewski-kone]

Hi,

I have a folloiwng handler:

export const handler = middy()
  .use(captureLambdaHandler(tracer))
  .use(injectLambdaContext(logger))
  .use(httpEventNormalizer())
  .use(httpHeaderNormalizer())
  .use(
    jsonBodyParser({
      disableContentTypeError: true, // Skip throwing 415 when Content-Type is invalid
    }),
  )
  .use(httpSecurityHeaders())
  .use(httpErrorHandler())
  .use(
    ssm({
      fetchData: {
        EXAMPLE_PARAM_VALUE: env.AWS_PARAMETER_STORE_EXAMPLE_PARAM,
      },
      setToContext: true,
      cache: false, // Switch off for tests
      //cacheExpiry: 15 * 60 * 1000,
    }),
  )
  .handler(lambdaHandler);

and I would like to test the handler mocking .use(ssm({...})) with jest.

First I was fighting with Jest and ESM module setup and I finally I get to the point where I was thinking to mock getInternal function in the following way:

import { jest } from "@jest/globals";
import { APIGatewayProxyEventV2, Context } from "aws-lambda";

const getInternal = jest.fn();
jest.unstable_mockModule("@middy/util", () => ({
  // eslint-disable-next-line @typescript-eslint/no-empty-object-type
  ...(jest.requireActual("@middy/util") as {}),
  getInternal,
}));

import { handler } from "../handler";

const createMockEvent = (
  data?: Partial<APIGatewayProxyEventV2>,
): APIGatewayProxyEventV2 => {
...
};

describe("MiddyRouter", () => {
  test("handler", async () => {
    const event = createMockEvent();
    const context = {} as never;

    getInternal.mockResolvedValue({
      EXAMPLE_PARAM_VALUE: "exampleValue",
    } as never);

    const result = await handler(event, context);

    expect(result).toEqual({});
  });
});

But from this error, it looks like the original implementation of getInternal is called (not my mock). I have no idea why.

    Error: Failed to resolve internal values
        at getInternal (/Users/jgi/Development/my_project/node_modules/@middy/util/index.js:83:11)
        at processTicksAndRejections (node:internal/process/task_queues:105:5)
        at async ssmMiddlewareBefore (/Users/jgi/Development/my_project/node_modules/@middy/ssm/index.js:181:20)
        at async runMiddlewares (/Users/jgi/Development/my_project/node_modules/@middy/core/index.js:230:17)
        at async runRequest (/Users/jgi/Development/my_project/node_modules/@middy/core/index.js:154:5)
        at async Object.<anonymous> (/Users/jgi/Development/my_project/lambdas/middy-router/src/__tests__/handler.test.ts:76:20) {
      [cause]: { package: '@middy/util', data: [ [CredentialsProviderError] ] }
    }

Do you guys have any example how to properly test Middy handler with mocking selected use middleware?
Any help and examples will be much appreciated.

Best wishes
JGI

[willfarrell]

Sorry to hear you're having troubles. I don't personally use jest, so I can't help there. But, if you take a look at the unit tests for middlewares like ssm, you'll see that the tests mock the AWS Client using aws-sdk-client-mock.

[jgilewski-kone]

I tried that too. ssm midleware is initializing the middleware in the tests so they can provide mocked AwsClient (as here) from aws-sdk-client-mock.
My ssm middleware is already configured in the handler I tested and is not using the mocked client.