Layout Engine

In this lesson you will learn layout engine in single-spa, why it matters within core concepts, and how to use it correctly with clear, copy-ready examples.

Layout Engine Overview

Layout Engine 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 layout engine 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.

<single-spa-router>
  <nav>
    <application name="@org/navbar"></application>
  </nav>
  <route path="checkout">
    <application name="@org/checkout"></application>
  </route>
</single-spa-router>

The single-spa layout engine declares where applications render for each route.

Layout Engine Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to layout engine.

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 Layout Engine Works in Single-SPA

Layout Engine 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.

The single-spa layout engine declares where applications render for each route.

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 Layout Engine

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

Key Takeaways

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

Pro Tip

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