Fallback UI

Understanding fallback ui helps you work with single-spa confidently. Here you will learn the core ideas behind fallback ui, see working code, and pick up best practices used on real teams.

Fallback UI Overview

Fallback UI 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 fallback ui 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.

Fallback UI Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to fallback ui.

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 Fallback UI Works in Single-SPA

Fallback UI 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 Fallback UI

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

Key Takeaways

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

Pro Tip

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