State Isolation

State Isolation sits at the heart of state management in single-spa. This guide walks through the concept step by step, with examples, a cheatsheet, and common mistakes to avoid.

State Isolation Overview

State 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 state isolation focused and predictable. Start from the minimal example here, then layer in only the complexity your feature actually needs.

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.

State Isolation Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to state 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 State Isolation Works in Single-SPA

State 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.

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 State Isolation

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

Key Takeaways

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

Pro Tip

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