Custom Elements

Custom Elements Overview

Custom Elements 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 custom elements 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.

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.

Custom Elements Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to custom elements.

How Custom Elements Works in Single-SPA

Custom Elements 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 Custom Elements

For reliable micro frontends, custom elements should isolate failures and keep shared state minimal. Let each team own its app end to end while agreeing on a few shared contracts.

Common Mistakes

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

Key Takeaways

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