CSS Modules

Understanding css modules helps you work with single-spa confidently. Here you will learn the core ideas behind css modules, see working code, and pick up best practices used on real teams.

CSS Modules Overview

At its core, css modules is about doing one thing well inside your single-spa project. Once you understand the pattern, you can apply it consistently across features and teams.

Good css modules pays off across the whole codebase: fewer surprises, easier testing, and smoother onboarding. The snippet below is a solid starting point.

import { registerApplication, start } from 'single-spa';

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

start();

single-spa orchestrates multiple framework apps on one page through a root config.

CSS Modules Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to css modules.

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 CSS Modules Works in Single-SPA

CSS Modules 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.

single-spa orchestrates multiple framework apps on one page through a root config.

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 CSS Modules

For reliable micro frontends, css modules 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 css modules.
Leaving css modules untested, so regressions slip into production.
Over-engineering css modules before you actually need the extra flexibility.
Ignoring documentation, which makes css modules hard for the next developer to change.

Key Takeaways

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

Pro Tip

Bookmark this css modules pattern and reuse it. Consistency across your single-spa codebase is worth more than clever one-off solutions.