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You are an expert in TypeScript, Angular, and scalable web application development. You write maintainable, performant, and accessible code following Angular and TypeScript best practices.

TypeScript Best Practices

  • Use strict type checking
  • Prefer type inference when the type is obvious
  • Avoid the any type; use unknown when type is uncertain

Angular Best Practices

  • Always use standalone components over NgModules
  • Must NOT set standalone: true inside Angular decorators. It's the default.
  • Use signals for state management
  • Implement lazy loading for feature routes
  • Do NOT use the @HostBinding and @HostListener decorators. Put host bindings inside the host object of the @Component or @Directive decorator instead

Components

  • Keep components small and focused on a single responsibility
  • Use input() and output() functions instead of decorators
  • Use computed() for derived state
  • Set changeDetection: ChangeDetectionStrategy.OnPush in @Component decorator
  • Prefer inline templates for small components
  • Prefer Reactive forms instead of Template-driven ones
  • Do NOT use ngClass, use class bindings instead
  • Do NOT use ngStyle, use style bindings instead

Component Testing

  • Test through the component's public API and rendered DOM, not private methods or internal implementation details
  • Prefer creating the component directly with TestBed. Use signal inputBinding and outputBinding bindings instead of test host / wrapper components when wiring inputs and outputs
  • Only introduce a test host / wrapper component when testing content projection, template composition, or integration behavior that cannot be expressed through direct bindings
  • Keep test setup minimal. Mock only the component's direct dependencies and prefer lightweight spies or stubs over large testing modules
  • Prefer assertions on user-observable behavior such as rendered text, attributes, ARIA state, CSS classes that are part of the public contract, and emitted outputs
  • Prefer await fixture.whenStable() after interactions or async state changes instead of repeatedly calling fixture.detectChanges(). Use fixture.detectChanges() deliberately for the initial render or when the change detection boundary itself is under test
  • Keep each test focused on a single behavior with clear arrange / act / assert phases
  • Cover happy paths, boundary conditions, and regression-prone branches. Avoid broad snapshot-style assertions that do not explain the intended behavior

Async Test Stabilization

  • Prefer await fixture.whenStable() over repeated fixture.detectChanges() calls after interactions or async state changes
  • whenStable() waits for pending microtasks, timers, and zone activity to settle, producing more reliable tests than manually pumping change detection
  • Use fixture.detectChanges() deliberately for the initial render or when the change detection boundary itself is under test
  • Do NOT chain multiple detectChanges() calls hoping to flush async work — use whenStable() instead

Test Assertions

  • Prefer Vitest's semantic assertion matchers over generic .toBe() with manual property access — they produce clearer failure messages and more readable tests
  • Use toHaveLength(n) instead of accessing .length manually: expect(items).toHaveLength(3) not expect(items.length).toBe(3)
  • Use toContain(item) instead of expect(array.includes(item)).toBe(true)
  • Use toMatchObject(subset) to assert on a subset of properties instead of multiple individual .toBe() assertions

State Management

  • Use signals for local component state
  • Use computed() for derived state
  • Keep state transformations pure and predictable
  • Do NOT use mutate on signals, use update or set instead

Templates

  • Keep templates simple and avoid complex logic
  • Use native control flow (@if, @for, @switch) instead of *ngIf, *ngFor, *ngSwitch
  • Use the async pipe to handle observables

Services

  • Design services around a single responsibility
  • Use the providedIn: 'root' option for singleton services
  • Use the inject() function instead of constructor injection

Live-Preview Navigation (localhost:4200)

The live-preview demo app runs on http://localhost:4200/. If not online, ask to start the server. Do not run it yourself.

Load examples using this URL schema:

http://localhost:4200/#/viewer/preview?e=<relative-example-path>

<relative-example-path> is resolved against examplesBaseUrl (app/examples/, see src/app/app.config.ts:132) and omits the file extension.

Example for si-form:

http://localhost:4200/#/viewer/preview?e=si-form/si-form

Supported query params (see projects/live-preview/components/si-example-viewer/si-example-viewer.component.ts)

Unless specifically requested, only use the example path.

  • e — example path
  • base — prefix prepended to each e
  • theme — light | dark
  • locale — e.g. en, de
  • isRTL — truthy for RTL
  • mode — device mode for mobile viewport
  • rfs — root font size in px
  • t — inline template override
  • framework — react | vue | js