URL-Based State

URL-Based State is an important part of building production-ready single-spa systems. This lesson explains what url-based state means, how it works, and how to apply it with practical examples you can reuse.

URL-Based State Overview

URL-Based State 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 url-based state 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.

URL-Based State Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to url-based state.

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 URL-Based State Works in Single-SPA

URL-Based State 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 URL-Based State

For reliable micro frontends, url-based state 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 url-based state snippets without understanding what each line does.
Skipping error handling and edge cases when wiring up url-based state.
Leaving url-based state untested, so regressions slip into production.
Over-engineering url-based state before you actually need the extra flexibility.

Key Takeaways

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

Pro Tip

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