SystemJS Modules

Understanding systemjs modules helps you work with single-spa confidently. Here you will learn the core ideas behind systemjs modules, see working code, and pick up best practices used on real teams.

SystemJS Modules Overview

SystemJS Modules 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 systemjs modules 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.

<script type="systemjs-importmap">
{
  "imports": {
    "@org/root-config": "//localhost:9000/org-root-config.js",
    "@org/navbar": "//localhost:8500/org-navbar.js"
  }
}
</script>

Import maps tell SystemJS where to load each micro frontend bundle at runtime.

SystemJS Modules Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to systemjs modules.

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 SystemJS Modules Works in Single-SPA

SystemJS Modules 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.

Import maps tell SystemJS where to load each micro frontend bundle at runtime.

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 SystemJS Modules

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

Key Takeaways

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

Pro Tip

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