Environment Import Maps

Environment Import Maps sits at the heart of import maps in single-spa. This guide walks through the concept step by step, with examples, a cheatsheet, and common mistakes to avoid.

Environment Import Maps Overview

Environment 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 environment 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.

Environment Import Maps Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to environment 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 Environment Import Maps Works in Single-SPA

Environment 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 Environment Import Maps

For reliable micro frontends, environment 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

Skipping error handling and edge cases when wiring up environment import maps.
Leaving environment import maps untested, so regressions slip into production.
Over-engineering environment import maps before you actually need the extra flexibility.
Ignoring documentation, which makes environment import maps hard for the next developer to change.

Key Takeaways

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

Pro Tip

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