Framework Isolation

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

Framework Isolation Overview

Framework Isolation lets you structure single-spa work so it stays readable, testable, and easy to scale. Instead of ad-hoc code, you follow a clear pattern that other developers can recognise immediately.

The key is to keep framework isolation focused and predictable. Start from the minimal example here, then layer in only the complexity your feature actually needs.

// shared utility module exposed via the import map
export const authService = {
  getToken: () => localStorage.getItem('token'),
};

// consumed by any micro frontend
import { authService } from '@org/utils';

Shared utility modules provide cross-app services like auth without tight coupling.

Framework Isolation Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to framework isolation.

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 Framework Isolation Works in Single-SPA

Framework Isolation 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.

Shared utility modules provide cross-app services like auth without tight coupling.

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 Framework Isolation

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

Key Takeaways

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

Pro Tip

When you get stuck on framework isolation, reduce it to the smallest reproducible example first — most single-spa issues become obvious once the noise is gone.