Import Map Overrides

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

Import Map Overrides Overview

Import Map Overrides 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 import map overrides 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.

Import Map Overrides Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to import map overrides.

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 Import Map Overrides Works in Single-SPA

Import Map Overrides 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 Import Map Overrides

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

Key Takeaways

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

Pro Tip

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