AngularJS is an open-source JavaScript framework developed by Google to build Single Page Applications (SPAs) with dynamic user interfaces. It facilitates the development of SPAs by using features such as two-way data binding, dependency injection, and client-side routing. These features allow the application to load a single HTML page and dynamically update content as the user interacts with it, without refreshing the entire page.

AngularJS enhances user experience through fast rendering and a seamless, app-like experience. The MVC (Model-View-Controller) architecture helps structure the code efficiently, and the use of $routeProvider supports deep linking and client-side navigation, enabling SPA behavior.

Two-way data binding in AngularJS synchronizes the model and the view, meaning changes in the user interface are reflected in the application model and vice versa. This is achieved using ng-model, which binds HTML controls to application data. When a user updates a form input, the $scope model updates automatically, and any changes in the model are instantly reflected in the view.

This bidirectional synchronization simplifies DOM manipulation, eliminates boilerplate code, and improves developer productivity. Two-way data binding is a key feature that distinguishes AngularJS from other JavaScript frameworks and is essential for developing responsive web applications efficiently.

In AngularJS, the $scope object acts as a bridge between the controller and the view. It is an execution context for expressions and binds data to the HTML, enabling two-way data binding. Developers can define variables and functions on the $scope, which are then accessible in the view through AngularJS directives like ng-model, ng-bind, and ng-repeat.

Each controller in AngularJS has its own $scope, making it a powerful mechanism for maintaining application state and managing user interactions. Furthermore, $scope supports event propagation with methods like $emit, $broadcast, and $on, enabling inter-component communication.

Directives in AngularJS are markers on DOM elements that tell the framework to attach specified behavior. Core directives like ng-if, ng-show, and ng-repeat control rendering, visibility, and iteration. Custom directives extend this capability, allowing developers to create reusable components and encapsulate functionality.

By using the directive() method, one can define templates, isolated scopes, and custom logic. Custom directives enhance modularity, improve code reuse, and facilitate the creation of a component-based architecture in AngularJS applications. This abstraction is particularly powerful when developing complex, dynamic web applications that require organized and maintainable code.

In AngularJS, $watch, $digest, and $apply are part of the digest cycle, which is the mechanism for updating the view when the model changes. $watch registers a watch expression that listens for changes in a $scope property.

$digest iterates through all watch lists and updates the DOM if any changes are detected. $apply is used to execute external code (like event handlers or AJAX callbacks) and then triggers a $digest cycle. These functions are crucial for maintaining synchronization between the model and view. Improper use can lead to performance issues, so understanding their roles is essential for advanced AngularJS development.

Dependency Injection (DI) in AngularJS is a design pattern that allows components to declare their dependencies without creating them directly. AngularJS provides a built-in injector subsystem that identifies required services using function parameter names and supplies them when the component is instantiated.

Common services like $http, $location, or custom services can be injected into controllers, directives, and other components. DI promotes modular design, testability, and separation of concerns. By removing the responsibility of instantiating dependencies from components, AngularJS enables easier maintenance and flexible unit testing, which is critical for building scalable web applications.

ng-repeat is a directive in AngularJS used to iterate over collections (like arrays or objects) and render a set of DOM elements for each item.

It binds each item in the collection to the HTML template and tracks changes using track by expressions to optimize performance. By assigning a unique identifier to each repeated item, ng-repeat minimizes DOM manipulations when items are added or removed. This behavior improves rendering speed in data-heavy applications. Additionally, ng-repeat supports nested structures and complex binding, making it indispensable for building dynamic, data-driven user interfaces in AngularJS.

In AngularJS, both services and factories are used for creating reusable business logic, but they differ in their instantiation and structure. A service is instantiated with the new keyword and is typically used to define a class-like object.

In contrast, a factory is a function that returns an object and provides more flexibility, such as dynamic return values and private variables. Services are best for simple, stateless logic, while factories are ideal when internal state or conditional logic is required. Both enable code reuse, modularity, and easy dependency injection, improving application architecture.

The $http service in AngularJS is a built-in abstraction for making AJAX calls to remote servers. It supports all HTTP methods like GET, POST, PUT, and DELETE and returns promises, allowing asynchronous handling of responses. The service simplifies error handling, configuration, and data transformation using .then() and .catch() constructs. It supports features like interceptors, which can be used for global request/response transformation or authentication.

This makes $http essential for RESTful API communication and client-server interaction in AngularJS, enabling developers to build real-time, dynamic web applications efficiently.

The Single Responsibility Principle (SRP) is a software design guideline stating that a module or component should have only one reason to change. In AngularJS, SRP is implemented by separating concerns into different components like controllers, services, directives, and filters. Controllers manage the scope and logic, services handle business rules, directives control DOM manipulation, and filters transform output.

This separation enhances maintainability, scalability, and testability. By ensuring that each part of the application handles a distinct task, developers can debug, extend, and refactor code more efficiently, a key practice in building robust AngularJS applications.

Filters in AngularJS are functions used to format the value of expressions for display purposes. Built-in filters like uppercase, currency, date, and filter allow developers to modify how data is presented in the view without altering the model. Filters can be applied in templates using the | (pipe) character and can be chained for complex transformations.

Custom filters can also be defined to suit application-specific formatting needs. Filters contribute to data binding clarity and improve user experience by providing a concise and readable way to display dynamic content in AngularJS-based web applications.

AngularJS provides built-in support for form validation using directives like ng-required, ng-minlength, ng-pattern, and validation states such as $valid, $invalid, $touched, and $dirty.

These features enable developers to implement real-time feedback and enforce input constraints directly in the view. AngularJS maintains the form’s validation status on the $scope through the formName object, which can be used to conditionally enable or disable form submission. Best practices include modularizing validation logic, avoiding excessive watchers, and displaying user-friendly error messages. This approach enhances user interface interactivity and ensures data integrity in AngularJS applications.

The digest cycle in AngularJS is the process by which the framework checks for changes in the model and updates the view accordingly. When triggered, it loops through all watchers and compares current and previous values. If a change is detected, the corresponding view is updated.

This cycle is initiated by AngularJS events or manually through $apply. While efficient for moderate data, performance can degrade if the application has many watchers or nested scopes. Developers must optimize their code by minimizing watchers, using one-time bindings, and limiting scope complexity to maintain smooth application performance.

Routing in AngularJS enables the development of Single Page Applications (SPAs) by loading different views dynamically without refreshing the page. The ngRoute module facilitates routing by mapping URLs to templates and controllers using the $routeProvider. This enables deep linking, where each view has a unique URL, enhancing user navigation and bookmarkability.

Developers define routes with .when() and .otherwise() methods, specifying paths, templates, and associated controllers. By using route parameters and resolves, AngularJS manages asynchronous data loading and navigation control effectively. Routing is crucial for creating seamless user experiences in AngularJS SPAs.

The $location service in AngularJS provides an interface for reading and manipulating the browser's URL. It allows developers to access parts of the URL, such as the path, search parameters, and hash. The $location service is essential for implementing routing, deep linking, and programmatic navigation in AngularJS Single Page Applications.

It works in sync with the $routeProvider and the HTML5 history API, providing cleaner URLs without hash fragments when configured. Developers can use methods like .path(), .search(), and .hash() to manage and respond to URL changes effectively.

Directives in AngularJS are markers on DOM elements that extend HTML functionality by attaching specific behavior or transforming the DOM. Built-in directives like ng-model, ng-if, and ng-repeat provide core capabilities.

Custom directives enable developers to encapsulate reusable UI components or behaviors, promoting code reusability and separation of concerns. A custom directive can include templates, event listeners, and isolated scope to create highly modular widgets. This feature is crucial for building large-scale AngularJS applications, where maintainability and clean component-based architecture are essential for long-term scalability and performance.

In AngularJS, data synchronization between the model and view is managed by the $watch, $digest, and $apply mechanisms. $watch registers listeners that monitor scope expressions. When changes occur, the $digest loop evaluates these watches and updates the view. The $apply method triggers the digest cycle manually, often used when integrating AngularJS with external libraries or custom events.

This triad ensures two-way data binding remains consistent. However, excessive watches or nested scopes can lead to performance issues. Understanding their interaction is vital for debugging and optimizing AngularJS application performance.

Scope inheritance in AngularJS is hierarchical, meaning child controllers inherit the scope of their parent controllers. This prototypal inheritance allows shared access to parent properties unless explicitly shadowed by the child. It simplifies data sharing in nested views but can lead to unintended behavior if not managed properly, especially when modifying primitive values in child scopes.

Developers must use controllerAs syntax or create service-based data models to avoid scope pollution and improve code clarity. Managing scope inheritance effectively is key to building predictable and maintainable AngularJS applications.

Two-way data binding in AngularJS creates a dynamic link between the model and the view, ensuring that changes in the model reflect in the view and vice versa. It is powered by $watch and the digest cycle, which monitor scope variables and synchronize them with the DOM.

This automatic propagation simplifies UI logic but introduces performance concerns in complex apps with numerous bindings. Developers should optimize by reducing watchers, using one-time bindings (::), and managing digest triggers wisely. Two-way binding is central to AngularJS's real-time UI responsiveness.

Transclusion in AngularJS allows a directive to include and render content that is defined outside the directive’s own template. This is useful for creating flexible and reusable components. By setting transclude: true in the directive definition, developers can pass custom HTML that integrates with the directive's context.

Transclusion supports content projection, maintaining the scope of the original content while combining it with the directive’s behavior. It's a powerful feature for building complex UI components that maintain separation of structure and functionality, a hallmark of scalable AngularJS development.

AngularJS provides the $exceptionHandler service for centralized error handling. By decorating or overriding this service, developers can implement custom logic to log errors to a remote server, display user-friendly messages, or trigger alerts. This is crucial for maintaining application stability, especially in production.

Additionally, try-catch blocks can be used in controller logic for synchronous operations, and promise-based services like $http support .catch() for asynchronous error handling. Proper error management enhances debugging, improves user experience, and facilitates proactive issue resolution in AngularJS-based applications.

Lazy loading in AngularJS refers to the practice of loading resources like controllers, services, or templates only when they are needed, rather than at initial application load. This technique reduces initial payload size and speeds up startup time.

While AngularJS does not provide built-in lazy loading, libraries like ocLazyLoad or custom implementations using require.js can facilitate this behavior. Lazy loading is especially useful in large Single Page Applications (SPAs) where modularizing and deferring components improves application performance, bandwidth usage, and user experience.

Securing AngularJS applications involves defending against Cross-Site Scripting (XSS) and Cross-Site Request Forgery (CSRF) attacks. AngularJS automatically escapes HTML content, mitigating XSS when using expressions. However, developers must avoid unsafe use of ng-bind-html without sanitization via $sanitize.

For CSRF, AngularJS supports token-based protection through the $http service, which reads a cookie and adds a matching custom header. Server-side validation of tokens ensures request integrity. Using strict content policies, avoiding eval(), and applying HTTPS are additional best practices to secure web apps built with AngularJS.

ng-if, ng-show, and ng-hide are conditional directives in AngularJS used to control DOM visibility. ng-if physically removes and recreates the DOM element based on the condition, offering performance advantages when the content is complex. ng-show and ng-hide merely toggle the visibility using CSS (display: none), keeping the element in the DOM.

While ng-show/ng-hide provide faster toggling for static content, ng-if is preferred for optimizing memory and avoiding watchers on unused elements. Choosing the appropriate directive affects rendering efficiency and application responsiveness.

AngularJS supports integration with jQuery or jQLite for DOM manipulation, but it recommends minimizing direct DOM access to preserve the MVC architecture. If jQuery is loaded before AngularJS, the framework will use the full jQuery library; otherwise, it defaults to the lightweight jQLite.

DOM manipulations should occur in directives rather than controllers to maintain separation of concerns. Developers should use Angular’s APIs for tasks like event handling or AJAX requests to avoid conflicts and ensure synchronization with the digest cycle, enhancing application consistency and stability.