Loading UI

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

Loading UI Overview

Loading UI 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 loading ui focused and predictable. Start from the minimal example here, then layer in only the complexity your feature actually needs.

<single-spa-router>
  <nav>
    <application name="@org/navbar"></application>
  </nav>
  <route path="checkout">
    <application name="@org/checkout"></application>
  </route>
</single-spa-router>

The single-spa layout engine declares where applications render for each route.

Loading UI Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to loading 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 Loading UI Works in Single-SPA

Loading 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.

The single-spa layout engine declares where applications render for each route.

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 Loading UI

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

Key Takeaways

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

Pro Tip

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