angular-best-practices-legacy

name	angular-best-practices-legacy
description	Angular 12-16 performance optimization guidelines with NgModules, RxJS patterns, and classic template syntax (*ngIf, *ngFor). Use when maintaining or working with pre-v17 Angular codebases. Triggers on tasks involving Angular components, services, modules, or performance improvements in legacy projects.
license	MIT
metadata	{"author":"community","version":"1.0.0"}

Angular Best Practices (Legacy: v12-16)

Comprehensive performance optimization guide for Angular 12-16 applications using NgModule-based architecture and RxJS-centric patterns. Contains 30+ rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.

When to Apply

Reference these guidelines when:

• Maintaining Angular 12-16 codebases
• Using NgModule-based architecture
• Working with *ngIf, *ngFor directives
• Using RxJS for state management
• Using class-based HTTP interceptors
• Reviewing code for performance issues

Key Patterns (v12-16)

• NgModules - Feature modules for organization
• BehaviorSubject - Reactive state management
• Subject + takeUntil - Subscription cleanup pattern
• Class-based interceptors - HTTP handling with implements HttpInterceptor
• **ngFor / ngIf - Structural directives with trackBy
• loadChildren - Module-based lazy loading

Rule Categories by Priority

Priority	Category	Impact	Prefix
1	Change Detection	CRITICAL	change-
2	Bundle & Lazy Loading	CRITICAL	bundle-
3	RxJS Optimization	HIGH	rxjs-
4	Template Performance	HIGH	template-
5	Dependency Injection	MEDIUM-HIGH	di-
6	HTTP & Caching	MEDIUM	http-
7	Forms Optimization	MEDIUM	forms-
8	General Performance	LOW-MEDIUM	perf-

Quick Reference

1. Change Detection (CRITICAL)

• change-rxjs-state - Use BehaviorSubject with OnPush for reactive state
• change-onpush - Use OnPush change detection strategy
• change-detach-reattach - Detach change detector for heavy operations
• change-run-outside-zone - Run non-UI code outside NgZone

2. Bundle & Lazy Loading (CRITICAL)

• bundle-ngmodule - Organize code into feature NgModules
• bundle-scam - Use Single Component Angular Modules pattern
• bundle-lazy-routes - Lazy load routes with loadChildren
• bundle-preload - Preload routes for perceived speed
• bundle-no-barrel-imports - Avoid barrel files, use direct imports

3. RxJS Optimization (HIGH)

• rxjs-async-pipe - Use async pipe instead of manual subscriptions
• rxjs-takeuntil - Use takeUntil with destroy$ Subject for cleanup
• rxjs-share-replay - Share observables to avoid duplicate requests
• rxjs-operators - Use efficient RxJS operators
• rxjs-mapping-operators - Use correct mapping operators (switchMap vs exhaustMap)
• rxjs-no-nested-subscribe - Avoid nested subscriptions

4. Template Performance (HIGH)

• template-trackby - Use trackBy function with *ngFor
• template-pure-pipes - Use pure pipes for expensive transformations
• template-ng-optimized-image - Use NgOptimizedImage for image optimization
• template-no-function-calls - Avoid function calls in templates
• template-virtual-scroll - Use virtual scrolling for large lists

5. Dependency Injection (MEDIUM-HIGH)

• di-provided-in-root - Use providedIn: 'root' for singleton services
• di-injection-token - Use InjectionToken for non-class dependencies
• di-factory-providers - Use factory providers for complex initialization

6. HTTP & Caching (MEDIUM)

• http-interceptors - Use class-based interceptors for cross-cutting concerns
• http-transfer-state - Use TransferState for SSR hydration

7. Forms Optimization (MEDIUM)

• forms-reactive - Use reactive forms instead of template-driven
• forms-typed - Use typed FormGroup for type safety

8. General Performance (LOW-MEDIUM)

• perf-memory-leaks - Prevent memory leaks (timers, listeners, subscriptions)
• perf-web-workers - Offload heavy computation to Web Workers
• arch-smart-dumb-components - Use Smart/Dumb component pattern

How to Use

Read individual rule files for detailed explanations and code examples:

rules/change-rxjs-state.md
rules/bundle-ngmodule.md
rules/rxjs-takeuntil.md

Each rule file contains:

• Brief explanation of why it matters
• Incorrect code example with explanation
• Correct code example with explanation
• Additional context and references

Full Compiled Document

For the complete guide with all rules expanded: AGENTS.md