Selector of Type
The HTML tag name is the basis for selects. App-root as an example. Since this kind of selection is the standard for all Angular components, you probably already know it.

Attribute Selector

• The selector property in the @Component decorator specifies the selector for the component. In this case, we're using an attribute selector [app-attribute-selector].
• To use the directive, you add the app-attribute-selector attribute to any HTML element in your template.
• The content within that element will be replaced by the template defined in the directive.

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

    @Component({
      selector: '[app-attribute-selector]',
      standalone: true,
      imports: [],
      templateUrl: './attribute-selector.component.html',
      styleUrl: './attribute-selector.component.scss'
    })
    export class AttributeSelectorComponent {

    }

component view code
<p>attribute-selector works!</p>

Attribute selector with value

• The selector property in the @Component decorator specifies the selector for the component. In this case, we're using an attribute selector [app-attribute-selector-with-value=” test”].
• To use the directive, you add the app-attribute-selector-with-value=” test” attribute to any HTML element in your template.
• The content within that element will be replaced by the template defined in the directive.

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

    @Component({
      selector: '[app-attribute-selector-with-value="test"]',
      standalone: true,
      imports: [],
      templateUrl: './attribute-selector-with-value.component.html',
      styles: ``
    })
    export class AttributeSelectorWithValueComponent {

    }

component view code
<p>attribute-selector-with-value works!</p>

The selector with multiple attributes
The selector property in the @Component decorator specifies the selector for the component. In this case, we're using an attribute selector [app-multiple-attribute-selector="test"][is-active].
To use the directive, you add the app-multiple-attribute-selector="test" is-active attribute to any HTML element in your template.
The content within that element will be replaced by the template defined in the directive.
    import { Component } from '@angular/core';

    @Component({
      selector: '[app-multiple-attribute-selector="test"][is-active]',
      standalone: true,
      imports: [],
      templateUrl: './multiple-attribute-selector.component.html',
      styleUrl: './multiple-attribute-selector.component.scss'
    })
    export class MultipleAttributeSelectorComponent {

    }

component view code
<p>multiple-attribute-selector works!</p>

Key Points
Attribute selectors are useful when you want to apply a component to multiple elements based on a common attribute.
They are often used for directives that provide additional functionality to existing elements.
Attribute selectors are case-sensitive.

Note. For attribute values, Angular supports matching an exact attribute value with the equals (=) operator. Angular does not support other attribute value operators.
CSS Class Selector
 
Class selector
A CSS class selector is used to target elements that have a specific class attribute. The class attribute is defined within the opening tag of an HTML element and is preceded by a dot (.).
import { Component } from '@angular/core';

@Component({
  selector: '.app-css-class-selector',
  standalone: true,
  imports: [],
  templateUrl: './css-class-selector.component.html',
  styles: ``
})
export class CssClassSelectorComponent {

}

component view code
<p>css-class-selector works!</p>

:not pseudo-class
The: not pseudo-class in CSS allows you to target elements that do not match a specific selector. You can leverage this in Angular component selectors to create more targeted components.

Explanation

• The :not(.disabled) part of the selector ensures that the component only matches elements with the class "container" that do not also have the class "disabled".
• This provides a way to conditionally apply the component based on the presence or absence of other classes.

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

    @Component({
      selector: '.app-css-not-pseudo-selector:not(.disabled)',
      standalone: true,
      imports: [],
      templateUrl: './css-not-pseudo-selector.component.html',
      styles: ``
    })
    export class CssNotPseudoSelectorComponent {

    }

component view code
<p>css-not-pseudo-selector works!</p>

Targeting elements that are not direct children of a specific element.
@Component({
  selector: 'div:not(.parent) > p'
})
export class MyComponent {
}

Targeting elements that do not have a specific attribute.
@Component({
  selector: 'button:not([disabled])'
})
export class MyButtonComponent {
}

Key Points

• The :not pseudo-class can be combined with other selectors to create more complex targeting rules.
• It's a powerful tool for creating reusable components that can be applied conditionally based on the structure of your HTML.
• Be cautious when using :not with complex selectors, as it can sometimes lead to unexpected behavior if not used correctly.

By understanding and utilizing the : not pseudo-class, you can create more flexible and targeted components in your Angular applications.

Combining selectors
You can combine selectors in Angular using CSS-like syntax to create more specific targeting rules. Here are some examples.
You can combine with Element, class, id, pseudo-class, attributes, comma-separated selectors, and so on.

Combining by Element, Class, and ID
@Component({
  selector: 'div.container#my-container'
})
export class MyComponent {
}

This component will only match elements that are.

• A div element
• Have the class container
• Have the ID my-container

Combining with Pseudo-Classes
TypeScript
@Component({
  selector: 'button:not(.disabled):hover'
})
export class MyButtonComponent {
}

This component will match buttons that.

• Do not have the class disabled
• Are being hovered over

Combining with Attributes
TypeScript
@Component({
  selector: '[data-type="product"][is-active]'
})
export class ActiveProductComponent {
}

This component will match elements that.

• Have the attribute data type with the value "product"
• Have the attribute is-active

Combining Multiple Selectors

You can combine multiple selectors using commas.
TypeScript
@Component({
  selector: '.container, .card'
})
export class MyComponent {
}

This component will match elements that have either the class container or the class card.

Remember

• Specificity: The more specific your selector is, the higher its priority.
• Cascading Stylesheets (CSS): If multiple selectors match an element, the most specific selector takes precedence.
• HTML Structure: Ensure that your selectors match the structure of your HTML elements.

By effectively combining selectors, you can create targeted components that accurately match the elements you intend to interact with in your Angular application.

Below is the output of the Selectors explained above.
    App.component.ts

    import { Component } from '@angular/core';
    import { RouterOutlet } from '@angular/router';
    import { AttributeSelectorComponent } from './attribute-selector/attribute-selector.component';
    import { MultipleAttributeSelectorComponent } from './multiple-attribute-selector/multiple-attribute-selector.component';
    import { AttributeSelectorWithValueComponent } from './attribute-selector-with-value/attribute-selector-with-value.component';
    import { CssClassSelectorComponent } from './css-class-selector/css-class-selector.component';
    import { CssNotPseudoSelectorComponent } from './css-not-pseudo-selector/css-not-pseudo-selector.component';
    import { CombiningSelectorsComponent } from './combining-selectors/combining-selectors.component';

    @Component({
      selector: 'app-root',
      standalone: true,
      imports: [
        RouterOutlet,
        AttributeSelectorComponent,
        MultipleAttributeSelectorComponent,
        AttributeSelectorWithValueComponent,
        CssClassSelectorComponent,
        CssNotPseudoSelectorComponent,
        CombiningSelectorsComponent
      ],
      templateUrl: './app.component.html',
      styleUrl: './app.component.scss',
    })
    export class AppComponent {
      show = false;
      testId = 'main-cta';
      title = 'angular-dev-test';
    }

component view code
@if(show){
<button [attr.data-test-id]="testId">Show CTA</button>
} @else {
<button [attr.data-test-id]="testId">Normal CTA</button>
}

<div app-attribute-selector is-active>will be ignored</div>
<div app-attribute-selector-with-value="test">will be ignored</div>
<div app-multiple-attribute-selector="test" is-active>will be ignored</div>
<div class="app-css-class-selector">will be ignored</div>
<div class="app-css-not-pseudo-selector">will be ignored</div>
<button type="reset">Reset</button>

Page Output

The route guards provided by Angular are effective tools for controlling access control inside your application. They improve security and user experience by assisting in making sure that users can only access routes that they are permitted to view. Poorly designed route guards, however, might cause security flaws and performance snags as your program expands. In order to successfully balance performance and security, we'll look at advanced ways for improving Angular route guards in this post.

1. Understanding Angular Route Guards
Angular offers several types of route guards, each serving a specific purpose.

• CanActivate: Determines if a route can be activated.
• CanDeactivate: Determines if the user can navigate away from the current route.
• CanActivateChild: Checks if a child route can be activated.
• CanLoad: Determines if a lazy-loaded module should be loaded.
• Resolve: Fetches data before a route is activated.

2. Route guards and lazy loading
By loading modules only when necessary, lazy loading is a crucial strategy for enhancing the efficiency of large Angular applications. Nevertheless, these advantages can be circumvented by using lazy-loaded modules in conjunction with route guards incorrectly.

Best Practices

• Use CanLoad Instead of CanActivate for Lazy-Loaded Modules: The CanLoad guard prevents the entire module from loading if the guard returns false. This is more efficient than CanActivate, which loads the module but prevents activation if conditions aren’t met.
• Preload Critical Modules: Use Angular's built-in preloading strategies, such as PreloadAllModules, to preload essential modules, ensuring quick access without sacrificing performance.

canLoad(route: Route): boolean {
  return this.authService.isLoggedIn();
}

3. Reducing Overhead in CanActivate Guards
CanActivate guards are commonly used to restrict access to routes based on user authentication or roles. However, performing complex logic or redundant checks in these guards can slow down navigation.

Best Practices

• Cache User Permissions: Instead of fetching permissions from the server on each route change, cache them locally using services like NgRx or simple in-memory storage. Update the cache only when necessary (e.g., on login or role change).
• Optimize Guard Logic: Ensure that the logic within your CanActivate guards is as efficient as possible. Avoid making unnecessary HTTP requests or performing complex calculations synchronously.
• Asynchronous Guard Execution: Use asynchronous operations in guards only when necessary. When using Observable or Promise-based guards, ensure that they resolve quickly to avoid delays in route transitions.

canActivate(route: ActivatedRouteSnapshot): boolean {
  const requiredRole = route.data['role'];
  return this.authService.hasRole(requiredRole);
}

4. Enhancing Security with Role-Based Access Control (RBAC)Implementing RBAC within your route guards is a robust way to enforce security policies across your application. However, a poorly designed RBAC system can lead to security loopholes.Best Practices

• Centralize Role Management: Manage user roles and permissions in a centralized service. This makes it easier to update and audit roles across your application.
• Dynamic Role Assignment: Assign roles dynamically based on user data rather than hardcoding roles within your application. This allows for more flexible and maintainable access control.
• Audit Route Guard Usage: Regularly audit your route guards to ensure that they are correctly implemented and up-to-date with your security policies. This helps prevent unauthorized access due to outdated or misconfigured guards.

canActivate(route: ActivatedRouteSnapshot): boolean {
  const requiredRoles = route.data['roles'] as Array<string>;
  return this.authService.hasAnyRole(requiredRoles);
}

5. Combining Multiple Guards Efficiently
In some cases, you may need to combine multiple guards for a single route. For example, you might want to check if a user is authenticated and if they have a specific role.

Best Practices
Use Composite Guards: Instead of chaining multiple guards, create composite guards that encapsulate all necessary checks. This reduces the overhead of multiple guard evaluations.

canActivate(route: ActivatedRouteSnapshot): boolean {
  return this.authService.isLoggedIn() && this.authService.hasRole('admin');
}

Order Guards for Efficiency: If you must use multiple guards, order them from least to most computationally expensive. This ensures that simple checks are performed first, potentially bypassing more complex checks if they fail.
const routes: Routes = [
  {
    path: 'admin',
    canActivate: [AuthGuard, RoleGuard],
    component: AdminComponent
  }
];

6. Debugging and Testing Route Guards
Even with optimizations, route guards can introduce issues if not thoroughly tested and debugged.

Best Practices

• Unit Testing Guards: Write unit tests for your route guards to ensure they behave as expected under various conditions. Use Angular's TestBed to create isolated tests for each guard.
• Logging and Monitoring: Implement logging within your guards to monitor their execution in production. This can help identify performance bottlenecks or security issues that may arise.
• Profiling Route Guards: Use Angular’s built-in profiling tools or browser developer tools to measure the performance impact of your route guards. Optimize any guards that are identified as slow or resource-intensive.

it('should allow the authenticated user to access route', () => {
  authService.isLoggedIn.and.returnValue(true);
  expect(guard.canActivate()).toBe(true);
});

Conclusion
Optimizing Angular route guards for performance and security requires a deep understanding of Angular’s routing system and careful consideration of best practices. By implementing lazy loading strategies, reducing overhead in guard logic, enhancing security with RBAC, and efficiently combining multiple guards, you can ensure that your Angular application is both fast and secure.