backstage-frontend-plugin

name	backstage-frontend-plugin
description	Build Backstage frontend plugins with the new Frontend System.
version	0.1.0

Backstage Frontend Plugin (New Frontend System)

About This Skill

This skill provides specialized knowledge and workflows for building Backstage frontend plugins using the New Frontend System. It guides the development of UI features including pages, navigation items, entity cards/content, and shared Utility APIs.

What This Skill Provides

1. Specialized workflows - Step-by-step procedures for creating and configuring frontend plugins
2. Best practices - Production-ready patterns for lazy loading, error handling, permissions, and testing
3. Golden path templates - Copy/paste code snippets for common plugin patterns
4. Domain expertise - Backstage-specific knowledge about blueprints, routes, and the plugin system

When to Use This Skill

Use this skill when creating UI features for Backstage: pages, navigation items, entity cards/content, or shared Utility APIs.

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Development Workflow

Phase 1: Planning and Research

1.1 Understand the Requirements

Before building a frontend plugin, clearly understand:

• What UI features are needed (pages, navigation, entity content, cards)
• What data the plugin will display or interact with
• Whether Utility APIs are needed for shared logic
• Integration points with other plugins (catalog, permissions, etc.)

1.2 Load Reference Documentation

Load reference files as needed based on the plugin requirements:

For Extension Development:

• 📋 Extension Blueprints Reference - Comprehensive guide to PageBlueprint, EntityContentBlueprint, and ApiBlueprint

For Utility API Development:

• 🔌 Utility APIs Reference - Creating and using Utility APIs for shared logic

For Testing:

• ✅ Testing Reference - Comprehensive testing guide for components, extensions, and APIs

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Phase 2: Implementation

Follow the Golden Path workflow below for implementation, referring to reference files as needed.

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Phase 3: Testing

After implementing the plugin:

1. Load the ✅ Testing Reference
2. Write comprehensive tests for:
   • React components using renderInTestApp
   • Extensions using createExtensionTester
   • Utility APIs with mocked dependencies
3. Run tests and achieve good coverage:

   yarn backstage-cli package test --coverage
   

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Phase 4: Review and Polish

Before publishing:

1. Run linting and structure checks
2. Ensure all extensions are properly registered
3. Verify lazy loading and error boundaries
4. Check permission-based visibility where appropriate
5. Review the Common Pitfalls section below

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Quick Facts

• Create a plugin with yarn new → select plugin; it generates plugins/<pluginId>/.
• The plugin instance is built with createFrontendPlugin from @backstage/frontend-plugin-api. Export it as the default from src/index.ts.
• Functionality is provided via extensions (e.g., PageBlueprint, EntityContentBlueprint). These are lazy‑loaded using dynamic imports.
• Define route references with createRouteRef (usually in src/routes.ts) and use them in blueprints.
• Use Utility APIs for shared logic (createApiRef + ApiBlueprint), consumed via useApi.

App integration patterns

• New Frontend System (preferred):
  • Apps using @backstage/frontend-defaults discover plugin extensions when the plugin is included at app creation.
  • Add your plugin to the app’s feature/plugin list; no need to export components. Routes declared by PageBlueprint are mounted automatically.
• Legacy apps (compat path):
  • If the app is still using @backstage/app-defaults and manual FlatRoutes, add a <Route> that renders your page component directly.
  • It’s acceptable to export a page component temporarily to bridge legacy routing, but prefer keeping components internal once migrated.

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Production Best Practices

Extensions and Routes

• Keep all routeRefs in src/routes.ts
• Use createSubRouteRef for nested paths
• Export only the plugin (default from src/index.ts)
• Never export extensions/components from the package root

Lazy Loading and UX

Use dynamic imports in loaders and wrap rendered elements in Suspense with a lightweight fallback. Add an error boundary for resilience:

// Suspense and error boundary around a lazy extension element
const Example = React.lazy(() => import('./components/ExamplePage'));

function ExampleWrapper() {
  return (
    <ErrorBoundary>
      <React.Suspense fallback={<div>Loading…</div>}>
        <Example />
      </React.Suspense>
    </ErrorBoundary>
  );
}

Utility APIs

• Keep API interfaces small and stable
• Wrap external clients/fetchers
• Provide a mock implementation for tests
• Register via ApiBlueprint and consume with useApi
• Do not fetch in render—use hooks/effects

Permissions and Visibility

Hide/show entity content based on permissions or ownership to avoid broken UX for unauthorized users:

import { usePermission } from '@backstage/plugin-permission-react';
import { somePermission } from '@backstage/plugin-permission-common';

export function ExampleEntityContent() {
  const { loading, allowed } = usePermission({ permission: somePermission });
  if (loading) return null;
  if (!allowed) return null; // or render a friendly message/banner
  return <div>Secret content</div>;
}

Entity Integrations

• Prefer small, focused entity content extensions
• Avoid heavy logic in loaders
• Keep presentational components separate from data hooks

Testing

• Use @testing-library/react to test extension output
• Mock Utility APIs
• Verify routeRefs with useRouteRef where navigation matters

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Golden Path (Copy/Paste Workflow)

1) Scaffold

# From the repository root (interactive)
yarn new
# Select: frontend-plugin
# Enter plugin id (kebab case, e.g. example)

# Non-interactive (for AI agents/automation)
yarn new --select frontend-plugin --option pluginId=example --option owner=""

2) Utility API (optional)

// src/api.ts
import { createApiRef } from '@backstage/frontend-plugin-api';

export interface ExampleApi {
  getExample(): { example: string };
}

export const exampleApiRef = createApiRef<ExampleApi>({ id: 'plugin.example' });

export class DefaultExampleApi implements ExampleApi {
  getExample() {
    return { example: 'Hello World!' };
  }
}

Register it with the ApiBlueprint and consume via useApi:

// src/plugin.ts
import { ApiBlueprint } from '@backstage/frontend-plugin-api';
import { exampleApiRef, DefaultExampleApi } from './api';

const exampleApi = ApiBlueprint.make({
  name: 'example',
  params: define =>
    define({
      api: exampleApiRef,
      deps: {},
      factory: () => new DefaultExampleApi(),
    }),
});

export const examplePlugin = createFrontendPlugin({
  pluginId: 'example',
  extensions: [exampleApi, examplePage, exampleNavItem],
  routes: { root: rootRouteRef },
});

3) Entity integration (optional)

import { EntityContentBlueprint } from '@backstage/plugin-catalog-react/alpha';

const exampleEntityContent = EntityContentBlueprint.make({
  params: {
    path: 'example',
    title: 'Example',
    loader: () =>
      import('./components/ExampleEntityContent').then(m => <m.ExampleEntityContent />),
  },
});

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Verify in an app

• In a new Frontend System app:
  • Ensure the app is created with @backstage/frontend-defaults and your plugin is included at app creation.
  • Start the repo and visit the path declared by your PageBlueprint.

Testing, linting & structure checks

Run tests and lints with Backstage's CLI:

yarn backstage-cli package test
yarn backstage-cli package lint
yarn backstage-cli repo lint

Keep a predictable structure (API layer, hooks, components, routes.ts, plugin.ts, index.ts).

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Common Pitfalls (and Fixes)

Problem	Solution	Reference
Extensions don't render	Ensure they're passed in the plugin's extensions array; components must be lazy-loaded via dynamic imports	Backstage
Navigation/links break	Keep routeRefs in src/routes.ts and use useRouteRef to generate links	Backstage
Consumers can't install your plugin	Export the plugin as the default from src/index.ts	Backstage

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Reference Files

📚 Documentation Library

Load these resources as needed during development:

Extension Development

• 📋 Extension Blueprints Reference - Complete guide to all extension blueprints including:
  • PageBlueprint for creating pages
  • EntityContentBlueprint for entity tabs
  • ApiBlueprint for Utility APIs
  • Advanced features like makeWithOverrides
  • Common patterns and best practices

Utility API Development

• 🔌 Utility APIs Reference - Creating and using Utility APIs including:
  • API interface definition patterns
  • Creating API references with createApiRef
  • Implementing default API classes
  • Registering with ApiBlueprint
  • Using APIs in components with useApi
  • Common API patterns (REST clients, caching, event bus)
  • Testing strategies

Testing

• ✅ Testing Reference - Comprehensive testing guide including:
  • Testing React components with renderInTestApp
  • Testing extensions with createExtensionTester
  • Mocking Utility APIs with TestApiProvider
  • Testing permissions and entity components
  • Routes and navigation testing
  • Best practices and common patterns

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External References

• Building Frontend Plugins (New Frontend System): createFrontendPlugin, blueprints, routes, Utility APIs. (Backstage)