External Dependencies

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

External Dependencies Overview

External Dependencies 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 external dependencies 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.

// shared utility module exposed via the import map
export const authService = {
  getToken: () => localStorage.getItem('token'),
};

// consumed by any micro frontend
import { authService } from '@org/utils';

Shared utility modules provide cross-app services like auth without tight coupling.

External Dependencies Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to external dependencies.

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 External Dependencies Works in Single-SPA

External Dependencies 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.

Shared utility modules provide cross-app services like auth without tight coupling.

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 External Dependencies

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

Key Takeaways

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

Pro Tip

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