Week 4: Component Fundamentals & Templates

1. Component Decorator Options

You've seen @Component a few times already; this week you learn it properly. Three options account for almost everything you'll configure day to day:

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

@Component({
  selector: 'app-rating-stars',
  templateUrl: './rating-stars.html',
  styleUrl: './rating-stars.css',
})
export class RatingStars {}

• selector — the HTML tag this component is used as, e.g. <app-rating-stars>
• templateUrl / template — an external file, or an inline string for very small components
• styleUrl / styleUrls — one or more stylesheets scoped to just this component (more on that scoping in Week 7)

Small, purely presentational components are often written with an inline template instead of a separate file — a judgment call you'll get a feel for as you write more of them.

2. Interpolation, Property & Event Binding

Three binding syntaxes cover almost every template you'll write:

<!-- Interpolation: render a value as text -->
<h2>{{ product.name }}</h2>

<!-- Property binding: set a DOM/component property -->
<img [src]="product.imageUrl" [alt]="product.name" />
<app-badge [count]="cartItemCount" />

<!-- Event binding: call a method when something happens -->
<button (click)="addToCart(product)">Add to cart</button>

<!-- Class and style bindings -->
<div [class.out-of-stock]="product.stock === 0"
     [style.opacity]="product.stock === 0 ? 0.5 : 1">
  {{ product.name }}
</div>

3. input(), output() & model()

These three functions are how a component receives data, emits events, and supports two-way binding — the signal-based replacement for the older @Input()/@Output() decorators.

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

@Component({
  selector: 'app-rating-stars',
  templateUrl: './rating-stars.html',
})
export class RatingStars {
  // Required input -- consumer MUST provide a value
  max = input.required<number>();

  // Optional input with a default
  readOnly = input(false);

  // Output: an event this component emits
  rated = output<number>();

  // model(): two-way bindable signal -- both an input and an output, paired
  value = model(0);

  select(stars: number) {
    if (this.readOnly()) return;
    this.value.set(stars);
    this.rated.emit(stars);
  }
}

<app-rating-stars
  [max]="5"
  [(value)]="productRating"
  (rated)="onRated($event)"
/>

[(value)] is Angular's "banana in a box" syntax — shorthand for binding [value] and listening to (valueChange) at once. model() generates that valueChange output for you automatically; you never write it by hand.

4. Template Reference Variables

A #name on any element gives you a handle to it — the DOM element, or the component/directive instance if there is one — usable anywhere else in that same template.

<input #searchBox type="text" (keyup.enter)="search(searchBox.value)" />
<button (click)="search(searchBox.value)">Search</button>

This is a lightweight escape hatch for simple cases — reading an input's current value without wiring up a full FormControl (which you'll learn properly in Week 12). For anything beyond "read this one value," reactive forms are the better tool.

5. ng-template & ng-container

ng-container groups elements — for a binding, or just for organization — without adding an extra element to the rendered DOM. It's invisible at runtime.

<ng-container [ngTemplateOutlet]="header"></ng-container>

<!-- No wrapping <div> in the rendered output -- useful when you need
     a binding target but a real DOM element would break your CSS layout -->

ng-template defines a block of markup that isn't rendered by default — something else decides when (or whether) to render it. You'll meet it again this way in Week 11, where @defer uses a similar "placeholder content" idea for lazy-loaded UI:

@for (item of items(); track item.id) {
  <li>{{ item.name }}</li>
} @empty {
  <ng-template #noItems>
    <li>No items yet.</li>
  </ng-template>
}

In practice, you'll write @if/@for's own @empty and @else blocks (Week 6) far more often than a raw ng-template — but recognizing it matters, because libraries and older codebases still use it directly.

6. Lifecycle Hooks & Signal-Based Alternatives

Angular calls specific methods on your component class at specific moments. The three you'll use most:

• ngOnInit() — once, after the component's inputs are first set
• ngOnChanges(changes) — whenever an @Input()-decorated property changes
• ngOnDestroy() — right before the component is removed, for cleanup (unsubscribing, clearing timers)

import { Component, OnInit, OnDestroy } from '@angular/core';

export class SearchBox implements OnInit, OnDestroy {
  private intervalId?: ReturnType<typeof setInterval>;

  ngOnInit() {
    this.intervalId = setInterval(() => this.refresh(), 30_000);
  }

  ngOnDestroy() {
    clearInterval(this.intervalId);
  }

  refresh() { /* ... */ }
}

With signals, ngOnChanges specifically is often unnecessary — effect() (Week 5) reacts to a signal input changing directly, without the boilerplate of comparing an old and new value yourself:

import { Component, input, effect } from '@angular/core';

export class SearchBox {
  query = input('');

  constructor() {
    effect(() => {
      const q = this.query(); // re-runs automatically whenever `query` changes
      console.log('Query changed to', q);
    });
  }
}

ngOnInit and ngOnDestroy remain useful and aren't going anywhere — it's specifically ngOnChanges that signals make largely redundant.

7. Hands-on Exercise

<input #filterBox type="text" />
<button (click)="logFilter(filterBox.value)">Log filter</button>

<ng-template #emptyState>
  <p>Nothing here yet.</p>
</ng-template>

<h3>Favorites</h3>
@for (item of favorites(); track item.id) {
  <p>{{ item.name }}</p>
} @empty {
  <ng-container [ngTemplateOutlet]="emptyState"></ng-container>
}

<h3>Recently viewed</h3>
@for (item of recentlyViewed(); track item.id) {
  <p>{{ item.name }}</p>
} @empty {
  <ng-container [ngTemplateOutlet]="emptyState"></ng-container>
}

8. Knowledge Check

Four quick questions. Expand each to check your answer.