Unit Testing

Unit Testing is an important part of building production-ready single-spa systems. This lesson explains what unit testing means, how it works, and how to apply it with practical examples you can reuse.

Unit Testing Overview

Unit 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 unit 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.

Unit Testing Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to unit 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 Unit Testing Works in Single-SPA

Unit 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 Unit Testing

For reliable micro frontends, unit 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

Copying unit testing snippets without understanding what each line does.
Skipping error handling and edge cases when wiring up unit testing.
Leaving unit testing untested, so regressions slip into production.
Over-engineering unit testing before you actually need the extra flexibility.

Key Takeaways

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

Pro Tip

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