registerApplication

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

registerApplication Overview

registerApplication 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 registerapplication 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/navbar',
  app: () => System.import('@org/navbar'),
  activeWhen: () => true,
});

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

start();

The root config registers each micro frontend and starts single-spa's routing.

registerApplication Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to registerapplication.

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 registerApplication Works in Single-SPA

registerApplication 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.

The root config registers each micro frontend and starts single-spa's routing.

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 registerApplication

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

Key Takeaways

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

Pro Tip

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