Application Preloading

In this lesson you will learn application preloading in single-spa, why it matters within performance, and how to use it correctly with clear, copy-ready examples.

Application Preloading Overview

At its core, application preloading is about doing one thing well inside your single-spa project. Once you understand the pattern, you can apply it consistently across features and teams.

Good application preloading pays off across the whole codebase: fewer surprises, easier testing, and smoother onboarding. The snippet below is a solid starting point.

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.

Application Preloading Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to application preloading.

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 Application Preloading Works in Single-SPA

Application Preloading 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 Application Preloading

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

Key Takeaways

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

Pro Tip

Bookmark this application preloading pattern and reuse it. Consistency across your single-spa codebase is worth more than clever one-off solutions.