Build Configuration

Understanding build configuration helps you work with single-spa confidently. Here you will learn the core ideas behind build configuration, see working code, and pick up best practices used on real teams.

Build Configuration Overview

Build Configuration 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 build configuration 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.

Build Configuration Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to build configuration.

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 Build Configuration Works in Single-SPA

Build Configuration 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 Build Configuration

For reliable micro frontends, build configuration 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 build configuration.
Leaving build configuration untested, so regressions slip into production.
Over-engineering build configuration before you actually need the extra flexibility.
Ignoring documentation, which makes build configuration hard for the next developer to change.

Key Takeaways

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

Pro Tip

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