Angular Directives – Complete In-Depth Tutorial

🔷 What is a Directive in Angular?

A directive is a special instruction in Angular that changes the behavior, appearance, or structure of DOM elements.

👉 In simple words:

Directives teach HTML new tricks.

---

🧩 Types of Directives in Angular

Angular provides three types of directives:

1. Structural Directives

2. Attribute Directives

---

🧱 1️⃣ Structural Directives (Change DOM Structure)

📌 Structural directives add, remove, or manipulate elements in the DOM.

👉 They are identified by * (asterisk)

🔹 Why * is used?

The * tells Angular:

“This directive will change the structure of the DOM.”

Internally, Angular converts it into <ng-template>.

---

✅ Common Structural Directives

Conditionally adds/removes elements.

<p *ngIf="isLoggedIn">Welcome User</p>

✔ True → element exists
❌ False → element removed from DOM

---

🔸 *ngFor

Used to loop through collections.

<ul>
  <li *ngFor="let item of items; index as i">
    {{ i }} - {{ item }}
  </li>
</ul>

items = ['Angular', 'React', 'Vue'];

---

🔸 *ngSwitch

Used for multiple conditions.

<div [ngSwitch]="role">
  <p *ngSwitchCase="'admin'">Admin Panel</p>
  <p *ngSwitchCase="'user'">User Dashboard</p>
  <p *ngSwitchDefault>Guest</p>
</div>

---

import { Component } from '@angular/core';
@Component({
  selector: 'app-root',
  templateUrl: './app.component.html'
})
export class AppComponent {
  // Used for ngIf
  isLoggedIn: boolean = false;
  // Used for ngFor
  students: string[] = ['Rahul', 'Amit', 'Neha', 'Pooja'];
  toggleLogin() {
    this.isLoggedIn = !this.isLoggedIn;
  }
}
📄 Template File – app.component.html
🔹 Example 1: *ngIf (Condition Based Rendering)
<h2>*ngIf Example</h2>
<button (click)="toggleLogin()">
  Toggle Login
</button>
<p *ngIf="isLoggedIn">
  Welcome! You are logged in.
</p>
<p *ngIf="!isLoggedIn">
  Please login first.
</p>
<hr />

🔹 Example 2: 
*ngFor (Loop / List Rendering)
<h2>*ngFor Example</h2>
<ul>
  <li *ngFor="let student of students; index as i">
    {{ i + 1 }}. {{ student }}
  </li>
</ul>
<hr />

Example of NgSwitch:

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

@Component({

  selector: 'app-root',

  templateUrl: './app.component.html'

})

export class AppComponent {

  role: string = 'guest';

  changeRole(selectedRole: string) {

    this.role = selectedRole;

  }

}

📄 Template File – app.component.html

🔹 Role Selection Buttons

<h2>ngSwitch Example</h2>

<button (click)="changeRole('admin')">Admin</button>
<button (click)="changeRole('user')">User</button>
<button (click)="changeRole('guest')">Guest</button>



<div [ngSwitch]="role">

  <div *ngSwitchCase="'admin'">
    <h3>Admin Panel</h3>
    <p>You have full access.</p>
  </div>

  <div *ngSwitchCase="'user'">
    <h3>User Dashboard</h3>
    <p>Limited access.</p>
  </div>

  <div *ngSwitchDefault>
    <h3>Guest Page</h3>
    <p>Please login to continue.</p>
  </div>

</div>

🧠 Structural Directive Summary

Directive	Purpose
*ngIf	Conditional rendering
*ngFor	Loop rendering
*ngSwitch	Multiple conditions

---

🎨 2️⃣ Attribute Directives (Change Appearance/Behavior)

📌 Attribute directives do not remove elements
They modify properties, styles, or behavior of elements.

👉 Used without *

---

✅ Common Attribute Directives

🔹 ngClass

Dynamically add/remove CSS classes.

<p [ngClass]="{ active: isActive, error: hasError }">
  Status Text
</p>

---

🔹 ngStyle

Apply dynamic inline styles.

<p [ngStyle]="{ color: isError ? 'red' : 'green' }">
  Message
</p>

---

🔹 [disabled], [hidden]

Control element properties.

<button [disabled]="isSubmitting">Submit</button>

---

🧠 Attribute Directive Summary

Directive	Purpose
ngClass	CSS class binding
ngStyle	Inline style binding
[hidden]	Show/Hide (CSS)

---

⚠️ *ngIf vs [hidden] (Interview Favorite)

Feature	*ngIf	[hidden]
DOM Removal	✅ Yes	❌ No
Performance	Better	Less
Visibility	Element destroyed	CSS hidden

---