Single-SPA Applications

Applications sits at the heart of core concepts in single-spa. This guide walks through the concept step by step, with examples, a cheatsheet, and common mistakes to avoid.

Applications Overview

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 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.

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.

Applications Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to 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 Applications Works in Single-SPA

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.

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 Applications

For reliable micro frontends, 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

Skipping error handling and edge cases when wiring up applications.
Leaving applications untested, so regressions slip into production.
Over-engineering applications before you actually need the extra flexibility.
Ignoring documentation, which makes applications hard for the next developer to change.

Key Takeaways

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

Pro Tip

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