Single-SPA Migration

Migration is an important part of building production-ready single-spa systems. This lesson explains what migration means, how it works, and how to apply it with practical examples you can reuse.

Migration Overview

At its core, migration 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 migration 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.

Migration Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to migration.

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 Migration Works in Single-SPA

Migration 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 Migration

For reliable micro frontends, migration 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

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

Key Takeaways

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

Pro Tip

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