Shadow DOM

Shadow DOM sits at the heart of web components in single-spa. This guide walks through the concept step by step, with examples, a cheatsheet, and common mistakes to avoid.

Shadow DOM Overview

Shadow DOM 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 shadow dom focused and predictable. Start from the minimal example here, then layer in only the complexity your feature actually needs.

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.

Shadow DOM Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to shadow dom.

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 Shadow DOM Works in Single-SPA

Shadow DOM 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 Shadow DOM

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

Key Takeaways

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

Pro Tip

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