SystemJS

SystemJS Overview

At its core, systemjs 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 systemjs pays off across the whole codebase: fewer surprises, easier testing, and smoother onboarding. The snippet below is a solid starting point.

<script type="systemjs-importmap">
{
  "imports": {
    "@org/root-config": "//localhost:9000/org-root-config.js",
    "@org/navbar": "//localhost:8500/org-navbar.js"
  }
}
</script>

Import maps tell SystemJS where to load each micro frontend bundle at runtime.

SystemJS Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to systemjs.

How SystemJS Works in Single-SPA

SystemJS 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.

Import maps tell SystemJS where to load each micro frontend bundle at runtime.

• 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 SystemJS

For reliable micro frontends, systemjs should isolate failures and keep shared state minimal. Let each team own its app end to end while agreeing on a few shared contracts.

Common Mistakes

• Copying systemjs snippets without understanding what each line does.
• Skipping error handling and edge cases when wiring up systemjs.
• Leaving systemjs untested, so regressions slip into production.
• Over-engineering systemjs before you actually need the extra flexibility.

Key Takeaways

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