developing-angular-apps

name	developing-angular-apps
description	Modern Angular (v20+) expert with deep knowledge of Signals, Standalone Components, Zoneless applications, SSR/Hydration, and reactive patterns.
risk	safe
source	self
date_added	2026-02-27

Angular Expert

Master modern Angular development with Signals, Standalone Components, Zoneless applications, SSR/Hydration, and the latest reactive patterns.

When to Use This Skill

• Building new Angular applications (v20+)
• Implementing Signals-based reactive patterns
• Creating Standalone Components and migrating from NgModules
• Configuring Zoneless Angular applications
• Implementing SSR, prerendering, and hydration
• Optimizing Angular performance
• Adopting modern Angular patterns and best practices

Do Not Use This Skill When

• Migrating from AngularJS (1.x) → use angular-migration skill
• Working with legacy Angular apps that cannot upgrade
• General TypeScript issues → use typescript-expert skill

Instructions

1. Assess the Angular version and project structure
2. Apply modern patterns (Signals, Standalone, Zoneless)
3. Implement with proper typing and reactivity
4. Validate with build and tests

Safety

• Always test changes in development before production
• Gradual migration for existing apps (don't big-bang refactor)
• Keep backward compatibility during transitions

---

Angular Version Timeline

Version	Release	Key Features
Angular 20	Q2 2025	Signals stable, Zoneless stable, Incremental hydration
Angular 21	Q4 2025	Signals-first default, Enhanced SSR
Angular 22	Q2 2026	Signal Forms, Selectorless components

---

1. Signals: The New Reactive Primitive

Signals are Angular's fine-grained reactivity system, replacing zone.js-based change detection.

Core Concepts

import { signal, computed, effect } from "@angular/core";

// Writable signal
const count = signal(0);

// Read value
console.log(count()); // 0

// Update value
count.set(5); // Direct set
count.update((v) => v + 1); // Functional update

// Computed (derived) signal
const doubled = computed(() => count() * 2);

// Effect (side effects)
effect(() => {
  console.log(`Count changed to: ${count()}`);
});

Signal-Based Inputs and Outputs

import { Component, input, output, model } from "@angular/core";

@Component({
  selector: "app-user-card",
  standalone: true,
  template: `
    <div class="card">
      <h3>{{ name() }}</h3>
      <span>{{ role() }}</span>
      <button (click)="select.emit(id())">Select</button>
    </div>
  `,
})
export class UserCardComponent {
  // Signal inputs (read-only)
  id = input.required<string>();
  name = input.required<string>();
  role = input<string>("User"); // With default

  // Output
  select = output<string>();

  // Two-way binding (model)
  isSelected = model(false);
}

// Usage:
// <app-user-card [id]="'123'" [name]="'John'" [(isSelected)]="selected" />

Signal Queries (ViewChild/ContentChild)

import {
  Component,
  viewChild,
  viewChildren,
  contentChild,
} from "@angular/core";

@Component({
  selector: "app-container",
  standalone: true,
  template: `
    <input #searchInput />
    <app-item *ngFor="let item of items()" />
  `,
})
export class ContainerComponent {
  // Signal-based queries
  searchInput = viewChild<ElementRef>("searchInput");
  items = viewChildren(ItemComponent);
  projectedContent = contentChild(HeaderDirective);

  focusSearch() {
    this.searchInput()?.nativeElement.focus();
  }
}

When to Use Signals vs RxJS

Use Case	Signals	RxJS
Local component state	✅ Preferred	Overkill
Derived/computed values	✅ computed()	combineLatest works
Side effects	✅ effect()	tap operator
HTTP requests	❌	✅ HttpClient returns Observable
Event streams	❌	✅ fromEvent, operators
Complex async flows	❌	✅ switchMap, mergeMap

---

2. Standalone Components

Standalone components are self-contained and don't require NgModule declarations.

Creating Standalone Components

import { Component } from "@angular/core";
import { CommonModule } from "@angular/common";
import { RouterLink } from "@angular/router";

@Component({
  selector: "app-header",
  standalone: true,
  imports: [CommonModule, RouterLink], // Direct imports
  template: `
    <header>
      <a routerLink="/">Home</a>
      <a routerLink="/about">About</a>
    </header>
  `,
})
export class HeaderComponent {}

Bootstrapping Without NgModule

// main.ts
import { bootstrapApplication } from "@angular/platform-browser";
import { provideRouter } from "@angular/router";
import { provideHttpClient } from "@angular/common/http";
import { AppComponent } from "./app/app.component";
import { routes } from "./app/app.routes";

bootstrapApplication(AppComponent, {
  providers: [provideRouter(routes), provideHttpClient()],
});

Lazy Loading Standalone Components

// app.routes.ts
import { Routes } from "@angular/router";

export const routes: Routes = [
  {
    path: "dashboard",
    loadComponent: () =>
      import("./dashboard/dashboard.component").then(
        (m) => m.DashboardComponent,
      ),
  },
  {
    path: "admin",
    loadChildren: () =>
      import("./admin/admin.routes").then((m) => m.ADMIN_ROUTES),
  },
];

---

3. Zoneless Angular

Zoneless applications don't use zone.js, improving performance and debugging.

Enabling Zoneless Mode

// main.ts
import { bootstrapApplication } from "@angular/platform-browser";
import { provideZonelessChangeDetection } from "@angular/core";
import { AppComponent } from "./app/app.component";

bootstrapApplication(AppComponent, {
  providers: [provideZonelessChangeDetection()],
});

Zoneless Component Patterns

import { Component, signal, ChangeDetectionStrategy } from "@angular/core";

@Component({
  selector: "app-counter",
  standalone: true,
  changeDetection: ChangeDetectionStrategy.OnPush,
  template: `
    <div>Count: {{ count() }}</div>
    <button (click)="increment()">+</button>
  `,
})
export class CounterComponent {
  count = signal(0);

  increment() {
    this.count.update((v) => v + 1);
    // No zone.js needed - Signal triggers change detection
  }
}

Key Zoneless Benefits

• Performance: No zone.js patches on async APIs
• Debugging: Clean stack traces without zone wrappers
• Bundle size: Smaller without zone.js (~15KB savings)
• Interoperability: Better with Web Components and micro-frontends

---

4. Server-Side Rendering & Hydration

SSR Setup with Angular CLI

ng add @angular/ssr

Hydration Configuration

// app.config.ts
import { ApplicationConfig } from "@angular/core";
import {
  provideClientHydration,
  withEventReplay,
} from "@angular/platform-browser";

export const appConfig: ApplicationConfig = {
  providers: [provideClientHydration(withEventReplay())],
};

Incremental Hydration (v20+)

import { Component } from "@angular/core";

@Component({
  selector: "app-page",
  standalone: true,
  template: `
    <app-hero />

    @defer (hydrate on viewport) {
      <app-comments />
    }

    @defer (hydrate on interaction) {
      <app-chat-widget />
    }
  `,
})
export class PageComponent {}

Hydration Triggers

Trigger	When to Use
on idle	Low-priority, hydrate when browser idle
on viewport	Hydrate when element enters viewport
on interaction	Hydrate on first user interaction
on hover	Hydrate when user hovers
on timer(ms)	Hydrate after specified delay

---

5. Modern Routing Patterns

Functional Route Guards

// auth.guard.ts
import { inject } from "@angular/core";
import { Router, CanActivateFn } from "@angular/router";
import { AuthService } from "./auth.service";

export const authGuard: CanActivateFn = (route, state) => {
  const auth = inject(AuthService);
  const router = inject(Router);

  if (auth.isAuthenticated()) {
    return true;
  }

  return router.createUrlTree(["/login"], {
    queryParams: { returnUrl: state.url },
  });
};

// Usage in routes
export const routes: Routes = [
  {
    path: "dashboard",
    loadComponent: () => import("./dashboard.component"),
    canActivate: [authGuard],
  },
];

Route-Level Data Resolvers

import { inject } from '@angular/core';
import { ResolveFn } from '@angular/router';
import { UserService } from './user.service';
import { User } from './user.model';

export const userResolver: ResolveFn<User> = (route) => {
  const userService = inject(UserService);
  return userService.getUser(route.paramMap.get('id')!);
};

// In routes
{
  path: 'user/:id',
  loadComponent: () => import('./user.component'),
  resolve: { user: userResolver }
}

// In component
export class UserComponent {
  private route = inject(ActivatedRoute);
  user = toSignal(this.route.data.pipe(map(d => d['user'])));
}

---

6. Dependency Injection Patterns

Modern inject() Function

import { Component, inject } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { UserService } from './user.service';

@Component({...})
export class UserComponent {
  // Modern inject() - no constructor needed
  private http = inject(HttpClient);
  private userService = inject(UserService);

  // Works in any injection context
  users = toSignal(this.userService.getUsers());
}

Injection Tokens for Configuration

import { InjectionToken, inject } from "@angular/core";

// Define token
export const API_BASE_URL = new InjectionToken<string>("API_BASE_URL");

// Provide in config
bootstrapApplication(AppComponent, {
  providers: [{ provide: API_BASE_URL, useValue: "https://api.example.com" }],
});

// Inject in service
@Injectable({ providedIn: "root" })
export class ApiService {
  private baseUrl = inject(API_BASE_URL);

  get(endpoint: string) {
    return this.http.get(`${this.baseUrl}/${endpoint}`);
  }
}

---

7. Component Composition & Reusability

Content Projection (Slots)

@Component({
  selector: 'app-card',
  template: `
    <div class="card">
      <div class="header">
        <!-- Select by attribute -->
        <ng-content select="[card-header]"></ng-content>
      </div>
      <div class="body">
        <!-- Default slot -->
        <ng-content></ng-content>
      </div>
    </div>
  `
})
export class CardComponent {}

// Usage
<app-card>
  <h3 card-header>Title</h3>
  <p>Body content</p>
</app-card>

Host Directives (Composition)

// Reusable behaviors without inheritance
@Directive({
  standalone: true,
  selector: '[appTooltip]',
  inputs: ['tooltip'] // Signal input alias
})
export class TooltipDirective { ... }

@Component({
  selector: 'app-button',
  standalone: true,
  hostDirectives: [
    {
      directive: TooltipDirective,
      inputs: ['tooltip: title'] // Map input
    }
  ],
  template: `<ng-content />`
})
export class ButtonComponent {}

---

8. State Management Patterns

Signal-Based State Service

import { Injectable, signal, computed } from "@angular/core";

interface AppState {
  user: User | null;
  theme: "light" | "dark";
  notifications: Notification[];
}

@Injectable({ providedIn: "root" })
export class StateService {
  // Private writable signals
  private _user = signal<User | null>(null);
  private _theme = signal<"light" | "dark">("light");
  private _notifications = signal<Notification[]>([]);

  // Public read-only computed
  readonly user = computed(() => this._user());
  readonly theme = computed(() => this._theme());
  readonly notifications = computed(() => this._notifications());
  readonly unreadCount = computed(
    () => this._notifications().filter((n) => !n.read).length,
  );

  // Actions
  setUser(user: User | null) {
    this._user.set(user);
  }

  toggleTheme() {
    this._theme.update((t) => (t === "light" ? "dark" : "light"));
  }

  addNotification(notification: Notification) {
    this._notifications.update((n) => [...n, notification]);
  }
}

Component Store Pattern with Signals

import { Injectable, signal, computed, inject } from "@angular/core";
import { HttpClient } from "@angular/common/http";
import { toSignal } from "@angular/core/rxjs-interop";

@Injectable()
export class ProductStore {
  private http = inject(HttpClient);

  // State
  private _products = signal<Product[]>([]);
  private _loading = signal(false);
  private _filter = signal("");

  // Selectors
  readonly products = computed(() => this._products());
  readonly loading = computed(() => this._loading());
  readonly filteredProducts = computed(() => {
    const filter = this._filter().toLowerCase();
    return this._products().filter((p) =>
      p.name.toLowerCase().includes(filter),
    );
  });

  // Actions
  loadProducts() {
    this._loading.set(true);
    this.http.get<Product[]>("/api/products").subscribe({
      next: (products) => {
        this._products.set(products);
        this._loading.set(false);
      },
      error: () => this._loading.set(false),
    });
  }

  setFilter(filter: string) {
    this._filter.set(filter);
  }
}

---

9. Forms with Signals (Coming in v22+)

Current Reactive Forms

import { Component, inject } from "@angular/core";
import { FormBuilder, Validators, ReactiveFormsModule } from "@angular/forms";

@Component({
  selector: "app-user-form",
  standalone: true,
  imports: [ReactiveFormsModule],
  template: `
    <form [formGroup]="form" (ngSubmit)="onSubmit()">
      <input formControlName="name" placeholder="Name" />
      <input formControlName="email" type="email" placeholder="Email" />
      <button [disabled]="form.invalid">Submit</button>
    </form>
  `,
})
export class UserFormComponent {
  private fb = inject(FormBuilder);

  form = this.fb.group({
    name: ["", Validators.required],
    email: ["", [Validators.required, Validators.email]],
  });

  onSubmit() {
    if (this.form.valid) {
      console.log(this.form.value);
    }
  }
}

Signal-Aware Form Patterns (Preview)

// Future Signal Forms API (experimental)
import { Component, signal } from '@angular/core';

@Component({...})
export class SignalFormComponent {
  name = signal('');
  email = signal('');

  // Computed validation
  isValid = computed(() =>
    this.name().length > 0 &&
    this.email().includes('@')
  );

  submit() {
    if (this.isValid()) {
      console.log({ name: this.name(), email: this.email() });
    }
  }
}

---

10. Performance Optimization

Change Detection Strategies

@Component({
  changeDetection: ChangeDetectionStrategy.OnPush,
  // Only checks when:
  // 1. Input signal/reference changes
  // 2. Event handler runs
  // 3. Async pipe emits
  // 4. Signal value changes
})

Defer Blocks for Lazy Loading

@Component({
  template: `
    <!-- Immediate loading -->
    <app-header />

    <!-- Lazy load when visible -->
    @defer (on viewport) {
      <app-heavy-chart />
    } @placeholder {
      <div class="skeleton" />
    } @loading (minimum 200ms) {
      <app-spinner />
    } @error {
      <p>Failed to load chart</p>
    }
  `
})

NgOptimizedImage

import { NgOptimizedImage } from '@angular/common';

@Component({
  imports: [NgOptimizedImage],
  template: `
    <img
      ngSrc="hero.jpg"
      width="800"
      height="600"
      priority
    />

    <img
      ngSrc="thumbnail.jpg"
      width="200"
      height="150"
      loading="lazy"
      placeholder="blur"
    />
  `
})

---

11. Testing Modern Angular

Testing Signal Components

import { ComponentFixture, TestBed } from "@angular/core/testing";
import { CounterComponent } from "./counter.component";

describe("CounterComponent", () => {
  let component: CounterComponent;
  let fixture: ComponentFixture<CounterComponent>;

  beforeEach(async () => {
    await TestBed.configureTestingModule({
      imports: [CounterComponent], // Standalone import
    }).compileComponents();

    fixture = TestBed.createComponent(CounterComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();
  });

  it("should increment count", () => {
    expect(component.count()).toBe(0);

    component.increment();

    expect(component.count()).toBe(1);
  });

  it("should update DOM on signal change", () => {
    component.count.set(5);
    fixture.detectChanges();

    const el = fixture.nativeElement.querySelector(".count");
    expect(el.textContent).toContain("5");
  });
});

Testing with Signal Inputs

import { ComponentFixture, TestBed } from "@angular/core/testing";
import { ComponentRef } from "@angular/core";
import { UserCardComponent } from "./user-card.component";

describe("UserCardComponent", () => {
  let fixture: ComponentFixture<UserCardComponent>;
  let componentRef: ComponentRef<UserCardComponent>;

  beforeEach(async () => {
    await TestBed.configureTestingModule({
      imports: [UserCardComponent],
    }).compileComponents();

    fixture = TestBed.createComponent(UserCardComponent);
    componentRef = fixture.componentRef;

    // Set signal inputs via setInput
    componentRef.setInput("id", "123");
    componentRef.setInput("name", "John Doe");

    fixture.detectChanges();
  });

  it("should display user name", () => {
    const el = fixture.nativeElement.querySelector("h3");
    expect(el.textContent).toContain("John Doe");
  });
});

---

Best Practices Summary

Pattern	✅ Do	❌ Don't
State	Use Signals for local state	Overuse RxJS for simple state
Components	Standalone with direct imports	Bloated SharedModules
Change Detection	OnPush + Signals	Default CD everywhere
Lazy Loading	@defer and loadComponent	Eager load everything
DI	inject() function	Constructor injection (verbose)
Inputs	input() signal function	@Input() decorator (legacy)
Zoneless	Enable for new projects	Force on legacy without testing

---

Resources

• Angular.dev Documentation
• Angular Signals Guide
• Angular SSR Guide
• Angular Update Guide
• Angular Blog

---

Common Troubleshooting

Issue	Solution
Signal not updating UI	Ensure OnPush + call signal as function count()
Hydration mismatch	Check server/client content consistency
Circular dependency	Use inject() with forwardRef
Zoneless not detecting changes	Trigger via signal updates, not mutations
SSR fetch fails	Use TransferState or withFetch()