The primary purpose of directives in AngularJS is to extend HTML with new attributes and elements. Directives are markers on a DOM element that tell AngularJS to attach a specific behavior to that DOM element or transform it. They play a crucial role in enhancing HTML functionality and enabling the creation of reusable components in AngularJS applications. Understanding directives is fundamental to building dynamic and interactive user interfaces.

Data binding in AngularJS controllers relies on the digest mechanism to update the view. The digest cycle is a core part of AngularJS's two-way data binding system. During the digest cycle, AngularJS checks for changes in the model and updates the view accordingly. Understanding the digest cycle is crucial for efficient data binding in AngularJS applications.

Using AngularJS expressions within HTML attributes, such as event handlers, class attributes, and form validation, can enhance user interaction and experience. These expressions allow developers to create dynamic and responsive user interfaces. For example, updating UI elements based on user actions, dynamically applying styles, and providing real-time feedback in forms contribute to a more engaging and user-friendly experience. This understanding is crucial for leveraging AngularJS expressions effectively in different scenarios.

In AngularJS, the business logic should ideally be placed in a Service. Services are responsible for encapsulating and organizing business logic, data manipulation, and other functionalities that are not directly related to the view or user interactions. By placing business logic in services, AngularJS promotes reusability, maintainability, and testability of code. Understanding the role of services is key to building scalable and modular AngularJS applications.

The AngularJS service commonly used for making HTTP requests to external APIs is the $http service. It abstracts the complexities of making AJAX requests and provides a straightforward API for sending GET, POST, and other types of HTTP requests. Developers use the $http service to interact with external APIs and handle data exchange seamlessly in AngularJS applications.

AngularJS handles DOM manipulations in the context of two-way data binding by automatically updating the DOM when the underlying data changes. It establishes a bidirectional connection between the model and the view, allowing changes in one to reflect in the other and vice versa. This is achieved through the use of watchers and a digest cycle that efficiently updates only the parts of the DOM affected by the data changes. Understanding this mechanism is crucial for building responsive and dynamic user interfaces in AngularJS.

Two-way data binding in AngularJS facilitates the implementation of reactive user interfaces by automatically updating the user interface (UI) in response to changes in the underlying data. When the model changes, the view is automatically updated, and vice versa. This eliminates the need for manual DOM manipulation and event handling, making it easier to create reactive and dynamic UIs. Understanding this aspect of two-way data binding is crucial for building modern and responsive user interfaces in AngularJS.

The primary method used in AngularJS for integrating with external APIs is the $http service. AngularJS provides the $http service as a convenient way to make HTTP requests, allowing developers to communicate with external APIs and retrieve or send data. Understanding how to use the $http service is crucial for building AngularJS applications that interact with external data sources.

To integrate with external APIs, AngularJS uses the $http service for sending and receiving HTTP requests. This service simplifies the process of making HTTP calls and handling responses in AngularJS applications. Developers can configure various aspects of the request, such as headers and parameters, using the $http service, making it a fundamental tool for API integration in AngularJS.

$scope.$apply() in AngularJS is used to trigger a $digest cycle on the current scope and its children. It is typically used when changes to the model occur outside of AngularJS (e.g., in event handlers). By using $scope.$apply(), developers can ensure that AngularJS is aware of the changes and updates the view accordingly, preventing issues with data binding.