Angular Interview Questions

What is Angular?

Angular is a popular open-source web application framework maintained by Google. It is written in TypeScript and is used for building dynamic, single-page web applications (SPAs) and progressive web apps (PWAs). Angular provides a comprehensive set of tools and features that make it easy for developers to create modern, interactive, and responsive web applications.

Key Features and characteristics of Angular include:

1. Component-Based Architecture: Angular follows a component-based architecture, where applications are built by composing reusable components. Each component represents a part of the user interface and encapsulates its behavior and presentation logic.
2. Templates and Data Binding: Angular uses declarative templates to define the UI. It supports two-way data binding, meaning changes in the UI are automatically reflected in the underlying data, and changes in data are automatically reflected in the UI.
3. Dependency Injection: Angular has a powerful dependency injection system that allows components to define their dependencies explicitly. This promotes code modularity, testability, and flexibility.
4. Directives: Angular provides built-in directives like ngFor, ngIf, ngSwitch, etc., which allow developers to extend the HTML syntax and add custom behavior to the elements.
5. Routing: Angular comes with a powerful routing module that enables developers to create multiple views and handle navigation between different pages or components without requiring a full page reload.
6. Forms: Angular provides rich support for building complex forms with features like form validation, form controls, form groups, and form arrays.
7. HTTP Client: Angular includes an HTTP client module that simplifies making HTTP requests to remote servers, interacting with APIs, and handling responses.
8. Cross-Platform: Angular applications can run on various platforms, including desktop browsers, mobile devices, and even desktop applications using technologies like Electron.
9. RxJS Integration: Angular integrates seamlessly with Reactive Extensions for JavaScript (RxJS), allowing developers to work with asynchronous data streams and events.
10. CLI (Command Line Interface): Angular provides a powerful command-line interface (CLI) that helps developers with project setup, scaffolding, testing, and building.

Angular is widely used for building large-scale web applications, including enterprise-level applications, e-commerce platforms, content management systems, and many other types of web applications. It is actively maintained and has a strong community, which contributes to its ongoing development and improvement.

It’s important to note that there are different versions of Angular, and the major releases include AngularJS (Angular 1.x) and Angular (Angular 2+). AngularJS was the first version of the framework, and Angular is the modern, more feature-rich successor to AngularJS.

How does an Angular application work?

An Angular application follows a specific lifecycle and architecture to provide a dynamic and interactive user experience. Understanding how an Angular application works involves knowing the key components, concepts, and processes involved in the application’s lifecycle. Here’s an overview of how an Angular application works:

1. Component-Based Architecture:

• An Angular application is built using a component-based architecture. Components are the building blocks of the application, representing different parts of the user interface.
• Each component consists of a TypeScript class that contains the component’s logic and data, along with an HTML template that defines the component’s UI.

2. Module System:

• Angular applications are organized into modules. Modules are containers that group related components, services, directives, and other building blocks together.
• The root module (often named AppModule) is the starting point of the application and defines the components that will be bootstrapped when the application starts.

3. Bootstrapping:

• When an Angular application starts, the Angular platform bootstraps the root module. Bootstrapping involves loading the root component and its associated templates and initializing the application.

4. Templates and Data Binding:

• Angular uses declarative templates (HTML with Angular-specific syntax) to define the UI of the application.
• Data binding enables synchronization between the component’s data (the model) and the UI. There are two-way data binding (data changes in the UI and model are synced) and one-way data binding (data flows from the model to the UI).

5. Dependency Injection:

• Angular has a built-in dependency injection system that allows components to declare their dependencies explicitly.
• Services, which are reusable application logic, are typically injected into components through dependency injection.

6. Lifecycle Hooks:

• Components have lifecycle hooks that provide a way to tap into specific moments in the component’s lifecycle, such as when the component is created, rendered, or destroyed.
• Developers can use lifecycle hooks to perform custom actions at specific stages of a component’s life.

7. Routing:

• Angular applications often have multiple views, and routing is used to navigate between these views without full page reloads.
• The Angular Router allows developers to define routes, associate them with components, and handle navigation.

8. Services:

• Services are used to encapsulate application logic that is not directly related to the user interface. They are often used to fetch data from APIs, perform calculations, handle state management, and more.
• Services are typically injected into components using dependency injection.

9. Observables and RxJS:

• Angular leverages Reactive Extensions for JavaScript (RxJS) to work with asynchronous data streams, events, and observables.
• Observables are used to handle events, perform HTTP requests, and manage asynchronous operations.

10. Change Detection:

• Angular’s change detection mechanism ensures that the UI remains in sync with the component’s data. When data changes in the component, Angular updates the UI accordingly.

The Angular application works by combining these key concepts and mechanisms to create a dynamic and responsive user interface. As users interact with the application, components, services, and other parts of the application work together to handle user input, fetch data, update the UI, and provide a seamless user experience.

How are Angular expressions different from JavaScript expressions?

Angular expressions and JavaScript expressions share similarities, but there are some important differences between the two due to Angular’s specific use case within its templates. Here are the key differences:

1. Context:

• Angular Expressions: Angular expressions are evaluated within the context of Angular templates. They are used primarily in the view (HTML template) to bind data and display dynamic content to the user. Angular expressions can access properties and methods from the component’s class, but they have some limitations, such as no access to global JavaScript objects like window or document.
• JavaScript Expressions: JavaScript expressions, on the other hand, can be used anywhere in the JavaScript codebase, including external JavaScript files and within script tags in HTML. They have access to the global JavaScript objects and can perform any valid JavaScript operation.

2. Security:

• Angular Expressions: For security reasons, Angular expressions are designed to be “sandboxed,” meaning they cannot execute arbitrary JavaScript code or access sensitive data. They are intentionally limited to only accessing the properties and methods of the component’s class.
• JavaScript Expressions: JavaScript expressions have full access to the JavaScript runtime, which can be a security concern if untrusted code is executed.

3. Error Handling:

• Angular Expressions: When an Angular expression encounters an error during evaluation (e.g., accessing a non-existing property), Angular gracefully handles the error and does not interrupt the entire application. Instead, it shows a placeholder error message or leaves the content empty.
• JavaScript Expressions: If a JavaScript expression encounters an error, it can potentially break the execution of the entire JavaScript application, leading to unexpected behavior or crashes.

4. Usage in Templates:

• Angular Expressions: Angular expressions are used directly within double curly braces {{ }} or square brackets [] in Angular templates. For example, {{ myProperty }} binds the value of myProperty from the component class to the template.
• JavaScript Expressions: JavaScript expressions are used in regular script tags or inline event handlers. For example, <button onclick="myFunction()">Click me</button> would call the myFunction when the button is clicked.

5. Operators:

• Angular Expressions: Angular expressions support a limited set of operators and do not include more complex JavaScript operators like bitwise operators.
• JavaScript Expressions: JavaScript expressions support the full range of JavaScript operators and functionalities.

In summary, Angular expressions are specialized for use within Angular templates and are more limited in scope and security compared to regular JavaScript expressions. They provide a safe way to bind data and display dynamic content within the view, while JavaScript expressions have broader capabilities and are used throughout the JavaScript codebase.

What are templates in Angular?

In Angular, templates are an essential part of the component-based architecture and are used to define the user interface (UI) of an Angular application. A template defines how the application’s data and logic should be presented to the user. It provides the structure and layout for rendering dynamic content and allows developers to build interactive and responsive user interfaces.

Angular templates are typically written using HTML combined with Angular-specific syntax. The Angular-specific syntax includes interpolation, directives, and other template expressions that allow for data binding and dynamic content rendering. Templates are associated with components and are an integral part of the component’s definition.

Here are some key features and characteristics of Angular templates:

1. Data Binding:
   Templates support data binding, which is the process of connecting the data from the component’s class (the model) with the UI. Angular provides two-way data binding, where changes in the UI automatically update the underlying data, and changes in the data automatically update the UI.
2. Interpolation:
   Interpolation is a way to embed expressions within the template to display dynamic content. It is denoted by double curly braces {{ }} and allows you to display the value of variables, properties, or expressions from the component class directly in the template.
3. Directives:
   Directives are special markers in the template that allow developers to extend the HTML syntax with custom behavior. Angular provides built-in directives like ngFor, ngIf, ngSwitch, etc., which enable you to add logic, conditionals, and loops to the template.
4. Event Binding:
   Event binding allows you to respond to user interactions (e.g., clicks, keypresses) by binding methods from the component class to the UI elements. When the event is triggered, the associated method is executed.
5. Template Expressions:
   Template expressions are used to evaluate and display dynamic content based on the state of the component’s data. Expressions can contain JavaScript-like syntax and are evaluated within the context of the component.
6. Template Refs:
   Template refs are used to obtain references to specific elements or components within the template. They enable you to interact directly with elements in the template, manipulate their properties, or call their methods.

An example of a simple Angular component with a template:

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

@Component({
  selector: 'app-greeting',
  template: `
    <h1>Hello, {{ name }}!</h1>
    <button (click)="greet()">Greet</button>
  `
})
export class GreetingComponent {
  name: string = 'John';

  greet() {
    alert('Hello, ' + this.name + '!');
  }
}

In this example, the GreetingComponent has a template that contains an interpolation {{ name }} to display the value of the name property, and an event binding (click)="greet()" to trigger the greet() method when the button is clicked.

Templates play a crucial role in Angular applications as they define the user interface and enable data binding and dynamic content rendering, helping developers build engaging and interactive applications.

What are directives in Angular?

In Angular, directives are a powerful feature that extends the HTML syntax and allows developers to add custom behavior and functionalities to elements, components, or even the entire DOM (Document Object Model). Directives are a key part of building dynamic and interactive web applications in Angular.

There are three types of directives in Angular:

1. Component Directives:

• Component directives are the most common type of directive in Angular.
• They are used to create reusable, self-contained UI components that encapsulate their logic and presentation.
• Component directives have their own templates, styles, and data, and they can be easily reused and composed to build complex user interfaces.

2. Attribute Directives:

• Attribute directives are used to change the behavior or appearance of elements.
• They are applied to elements as attributes and modify the element’s behavior or appearance based on the directive’s logic.
• Attribute directives are often used for tasks like adding CSS classes, handling events, or performing custom actions.

3. Structural Directives:

• Structural directives are used to add or remove elements from the DOM based on conditions or loops.
• They are applied to elements as attributes and modify the structure of the DOM, adding or removing elements dynamically.
• Structural directives are commonly used for tasks like conditionally rendering elements (ngIf, ngSwitch) or repeating elements (ngFor).

Examples of each type of directive:

1. Component Directive:

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

@Component({
  selector: 'app-custom-button',
  template: `
    <button (click)="onClick()">{{ label }}</button>
  `
})
export class CustomButtonComponent {
  label: string = 'Click Me';

  onClick() {
    console.log('Button clicked!');
  }
}

2. Attribute Directive:

import { Directive, ElementRef, HostListener } from '@angular/core';

@Directive({
  selector: '[appHighlight]'
})
export class HighlightDirective {
  constructor(private el: ElementRef) {}

  @HostListener('mouseenter') onMouseEnter() {
    this.highlight('yellow');
  }

  @HostListener('mouseleave') onMouseLeave() {
    this.highlight(null);
  }

  private highlight(color: string) {
    this.el.nativeElement.style.backgroundColor = color;
  }
}

In this example, the HighlightDirective is an attribute directive that changes the background color of an element when the mouse enters and leaves it.

3. Structural Directive:

<div *ngIf="showMessage">Hello, Angular!</div>

In this example, the *ngIf is a structural directive that conditionally renders the div element based on the value of the showMessage variable in the component.

Directives provide a way to extend and customize the behavior of elements and components, making them a fundamental building block for creating dynamic and interactive Angular applications.