Custom Events

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

Custom Events Overview

Custom Events 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 custom events focused and predictable. Start from the minimal example here, then layer in only the complexity your feature actually needs.

// shared utility module exposed via the import map
export const authService = {
  getToken: () => localStorage.getItem('token'),
};

// consumed by any micro frontend
import { authService } from '@org/utils';

Shared utility modules provide cross-app services like auth without tight coupling.

Custom Events Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to custom events.

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 Custom Events Works in Single-SPA

Custom Events 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.

Shared utility modules provide cross-app services like auth without tight coupling.

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 Custom Events

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

Key Takeaways

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

Pro Tip

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