Routes and Applications

Routes and Applications is an important part of building production-ready single-spa systems. This lesson explains what routes and applications means, how it works, and how to apply it with practical examples you can reuse.

Routes and Applications Overview

Routes and Applications 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 routes and applications 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.

registerApplication({
  name: '@org/checkout',
  app: () => System.import('@org/checkout'),
  activeWhen: (location) => location.pathname.startsWith('/checkout'),
});

activeWhen decides which route ranges a micro frontend owns.

Routes and Applications Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to routes and applications.

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 Routes and Applications Works in Single-SPA

Routes and Applications 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.

activeWhen decides which route ranges a micro frontend owns.

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 Routes and Applications

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

Key Takeaways

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

Pro Tip

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