Browser Events

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

Browser Events Overview

Browser Events 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 browser events 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.

// 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.

Browser Events Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to browser 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 Browser Events Works in Single-SPA

Browser 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 Browser Events

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

Key Takeaways

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

Pro Tip

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