Error UI

In this lesson you will learn error ui in single-spa, why it matters within layout engine, and how to use it correctly with clear, copy-ready examples.

Error UI Overview

At its core, error ui is about doing one thing well inside your single-spa project. Once you understand the pattern, you can apply it consistently across features and teams.

Good error ui pays off across the whole codebase: fewer surprises, easier testing, and smoother onboarding. The snippet below is a solid starting point.

<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.

Error UI Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to error 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 Error UI Works in Single-SPA

Error 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 Error UI

For reliable micro frontends, error 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

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

Key Takeaways

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

Pro Tip

Bookmark this error ui pattern and reuse it. Consistency across your single-spa codebase is worth more than clever one-off solutions.