SystemJS Import Maps

In this lesson you will learn systemjs import maps in single-spa, why it matters within systemjs, and how to use it correctly with clear, copy-ready examples.

SystemJS Import Maps Overview

SystemJS Import Maps 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 systemjs import maps focused and predictable. Start from the minimal example here, then layer in only the complexity your feature actually needs.

<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 Import Maps Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to systemjs import maps.

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 Import Maps Works in Single-SPA

SystemJS Import Maps 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 Import Maps

For reliable micro frontends, systemjs import maps 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 systemjs import maps snippets without understanding what each line does.
Skipping error handling and edge cases when wiring up systemjs import maps.
Leaving systemjs import maps untested, so regressions slip into production.
Over-engineering systemjs import maps before you actually need the extra flexibility.

Key Takeaways

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

Pro Tip

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