Contract Testing

Contract Testing sits at the heart of testing in single-spa. This guide walks through the concept step by step, with examples, a cheatsheet, and common mistakes to avoid.

Contract Testing Overview

Contract Testing is a building block you will reach for often in single-spa. It keeps related logic together and makes your intent obvious to reviewers and future maintainers.

When you learn contract testing properly, you avoid the guesswork that leads to bugs and rework. The example below shows the shape you will use in most real single-spa projects.

import * as app from './org-checkout.js';

test('exports single-spa lifecycles', () => {
  expect(typeof app.bootstrap).toBe('function');
  expect(typeof app.mount).toBe('function');
  expect(typeof app.unmount).toBe('function');
});

Verify each micro frontend exports valid single-spa lifecycle functions.

Contract Testing Example

registerApplication({
  name: '@org/app',
  app: () => System.import('@org/app'),
  activeWhen: ['/app'],
});
start();

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

Single-SPA Cheatsheet

Core single-spa APIs related to contract testing.

API	Example	Purpose
registerApplication	`registerApplication({ name, app, activeWhen })`	Register a micro frontend
activeWhen	`activeWhen: ['/checkout']`	Route ownership
start	`start()`	Begin routing
bootstrap	`export async function bootstrap()`	One-time setup
mount	`export async function mount(props)`	Render the app
unmount	`export async function unmount(props)`	Clean up the app
import map	`systemjs-importmap`	Locate app bundles

How Contract Testing Works in Single-SPA

Contract Testing is part of how single-spa lets multiple applications — even in different frameworks — coexist on one page. A root config registers each app and controls when it is active.

Verify each micro frontend exports valid single-spa lifecycle functions.

A root config registers apps and calls start().
Each app exports bootstrap, mount, and unmount lifecycles.
activeWhen decides which routes each app owns.
Import maps resolve each app's bundle at runtime.

Practical Guidance for Contract Testing

For reliable micro frontends, contract testing should isolate failures and keep shared state minimal. Let each team own its app end to end while agreeing on a few shared contracts.

Concern	Recommendation
Isolation	One app's crash should not break others
Shared state	Prefer shared utility modules over globals
Routing	Keep activeWhen rules explicit and non-overlapping
Deployment	Release via import-map updates per app

Common Mistakes

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

Key Takeaways

Contract Testing is a core part of working effectively with single-spa.
Start small and keep contract testing focused on a single responsibility.
Apply consistent patterns so contract testing scales across your project.
Test and document contract testing to keep it maintainable over time.

Pro Tip

Pair contract testing with automated tests from day one. It is far cheaper to catch single-spa regressions in CI than in production.