Black Sheep Code

Black box contract tests with jest - adding performance tests

In this post we'll investigate adding performance tests to our previous contract tests.

Step 1 - Let's make the application slow.

First, we'll make it so that when we GET /pets it'll take 1000ms per pet that exists.

The commit here achieves this.

func (s *DefaultApiService) FindPets(ctx context.Context, tags []string, limit int32) (ImplResponse, error) {
	// TODO - update FindPets with the required logic for this service method.
	// Add api_default_service.go to the .openapi-generator-ignore to avoid overwriting this service implementation when updating open api generation.
	values := []*Pet{}
	for _, value := range pets {
		values = append(values, value)

+	time.Sleep(time.Duration(len(values) * 1000 * int(time.Millisecond)))
	return Response(200, values), nil


Write a performance test - jest-bench

Let's try jest-bench first.

This commit here sets up jest-bench in the repo, but I don't implement performance tests.

Basically jest-bench looks like it is more for performance testing micro optimisations of JavaScript, rather than this black box contract tests that we're trying to do here.

Also, see this Github issue, very helpful interaction with the package maintainer.

An interlude - a Jest based solution for load testing might not be appropriate.

Just a looking at the K6 documentation:

JavaScript is not generally well suited for high performance. To achieve maximum performance, the tool itself is written in Go, embedding a JavaScript runtime allowing for easy test scripting.

This is a pretty good point. No doubt that in fake scenario that I'm manufacturing here, I'm going to be able to demonstrate some kind of 'performance test', regardless of tool used.

But in a real world, designed-for-scale application, jest tests might not be able to simulate the load required to demonstrate performance issues.

Let's give it a go anyway.

Write a performance test - jest-timing-action

jest-timing-action is a tool that will create a PR showing the performance diff in a given PR. It's based on jest-timing-reporter by the same author, which is custom jest reporter.

So it's not quite a 'performance snapshot, fail if the performance has degraded' tool, although it looks very likely that this is possible, given that the tool does generate snapshots!

This commit here adds jest-timing-reporter and commits the snapshot.

Next, we add the jest-timing-action as a Github action (commit).

We'll now create a performance test (commit). This test looks much the same as other tests, we're just making more API calls:

(This PR also reduces the delay to 100ms, just for my own experience writing these).

describe("A performance test, get when there are 10 pets", () => {

    it("Works as expected", async () => {

        const initialResult = await petsApi.findPetsRaw({});
        const initialResultBody = await initialResult.value();

        const proms = new Array(10).fill(true).map(async (v,i) => {
            const apiResult1 = await petsApi.addPetRaw({
                pet: {
                    id: i+1,
                    name: "Fido"

            const apiResultBody = await apiResult1.value();

        await Promise.all(proms); 

        const newState = await petsApi.findPetsRaw({});
        const newStateBody = await newState.value();

We'll now create a PR that worsens performance, and voila! The github actions reports on the degraded performance.


I'm really happy with the approach of jest-timing-reporter. What I particularly like about it is that it doesn't require us to write our performance tests in a different way.

It's possible that in a real application the limits of this approach will quickly become apparent. But until then, this is a very easy solution to implement.

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