What is AngularJS and how does it differ from Angular?

AngularJS is a JavaScript MVC framework for building dynamic web apps. Angular (2+) is a complete rewrite using TypeScript with components instead of controllers, better performance, and mobile support.

What is $scope in AngularJS?

$scope is an object that binds controllers to views, enabling two-way data binding. It's the glue between your JavaScript code and HTML templates, automatically syncing model changes to the view.

What is the digest cycle in AngularJS?

The digest cycle is AngularJS's change detection mechanism. It loops through all watchers, checks for changes, and updates the DOM. It runs automatically after user events, HTTP responses, and $timeout calls.

What is two-way data binding in AngularJS?

Two-way data binding automatically synchronizes data between model ($scope) and view (HTML). Changes in the input field update the model, and model changes update the view, using ng-model directive.

What is the difference between ng-show/ng-hide and ng-if?

ng-show/ng-hide toggle CSS display property while keeping elements in DOM. ng-if actually adds/removes elements from DOM, destroying scope and watchers. Use ng-if for better performance with rarely shown content.

What is the difference between service, factory, and provider?

Service uses 'new' to create instance, factory returns any value from a function, provider is configurable during config phase. Factory is most flexible, provider is needed for app-wide configuration.

How does dependency injection work in AngularJS?

AngularJS automatically provides dependencies by name. Declare dependencies as function parameters, and the injector creates/provides them. Use $inject annotation for minification safety.

What are directives in AngularJS?

Directives extend HTML with custom attributes, elements, or classes. Built-in directives include ng-repeat, ng-model, ng-click. Custom directives encapsulate reusable DOM manipulation and behavior.

What is ng-repeat and how does it work?

ng-repeat iterates over collections to create DOM elements. It creates a child scope for each item with $index, $first, $last variables. Use track by for better performance with large lists.

What is the controller as syntax and why is it preferred?

Controller as syntax binds controller to view as a named object (e.g., 'vm'). It avoids $scope confusion, prevents prototype inheritance issues, and makes clear which controller owns each property.

What is $apply and when should you use it?

$apply triggers the digest cycle manually. Use it when updating scope from outside AngularJS context (setTimeout, jQuery events, third-party callbacks) to sync changes with the view.

What is $watch and what are its performance implications?

$watch registers a listener for scope changes. Each watcher runs during every digest cycle, so too many watchers slow performance. Use one-time binding (::) or track by to reduce watchers.

How does form validation work in AngularJS?

AngularJS provides built-in validation with ng-required, ng-minlength, ng-pattern directives. Forms expose $valid, $invalid, $pristine, $dirty states. CSS classes like ng-invalid enable visual feedback.

What is the difference between ng-route and ui-router?

ng-route is AngularJS's basic router supporting single views. ui-router supports nested views, multiple named views, and state-based routing. ui-router is more powerful for complex applications.

What is ngUpgrade and how does it help with migration?

ngUpgrade allows running AngularJS and Angular side by side in the same app. It enables incremental migration by upgrading AngularJS components to Angular or downgrading Angular components for use in AngularJS.

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