Run One Application Locally

In this lesson you will learn run one application locally in single-spa, why it matters within local development, and how to use it correctly with clear, copy-ready examples.

Run One Application Locally Overview

Run One Application Locally 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 run one application locally 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.

import { registerApplication, start } from 'single-spa';

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

start();

single-spa orchestrates multiple framework apps on one page through a root config.

Run One Application Locally Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to run one application locally.

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 Run One Application Locally Works in Single-SPA

Run One Application Locally 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.

single-spa orchestrates multiple framework apps on one page through a root config.

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 Run One Application Locally

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

Key Takeaways

Run One Application Locally is a core part of working effectively with single-spa.
Start small and keep run one application locally focused on a single responsibility.
Apply consistent patterns so run one application locally scales across your project.
Test and document run one application locally to keep it maintainable over time.

Pro Tip

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