Web Component Library

Web Component Library sits at the heart of design systems in single-spa. This guide walks through the concept step by step, with examples, a cheatsheet, and common mistakes to avoid.

Web Component Library Overview

At its core, web component library 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 web component library pays off across the whole codebase: fewer surprises, easier testing, and smoother onboarding. The snippet below is a solid starting point.

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.

Web Component Library Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to web component library.

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 Web Component Library Works in Single-SPA

Web Component Library 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 Web Component Library

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

Key Takeaways

Web Component Library is a core part of working effectively with single-spa.
Start small and keep web component library focused on a single responsibility.
Apply consistent patterns so web component library scales across your project.
Test and document web component library to keep it maintainable over time.

Pro Tip

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