Deloitte Angular JS Developer interview Questions

Table of Contents:

• Explain the concept of two-way data binding?
• What are services in AngularJS?
• How does AngularJS handle dependency injection?
• How do you handle memory leaks?
• How do you implement custom filters?

Introduction

AngularJS is a widely-used JavaScript framework for building dynamic, single-page web applications, making it a vital skill for modern web developers. Angular developers are expected to possess a solid understanding of core concepts like data binding, directives, dependency injection, and working with APIs. In a competitive landscape, developers must also be proficient in using Angular CLI, managing components, and optimizing application performance. These skills not only ensure a developer’s ability to create responsive and interactive applications but also make them valuable candidates for leading organizations looking for experts in cutting-edge technologies.

Deloitte, a global leader in consulting and professional services, frequently hires skilled AngularJS developers to work on innovative digital transformation projects. Known for its rigorous recruitment process, Deloitte seeks candidates who can demonstrate their technical expertise and problem-solving abilities. Preparing for an AngularJS developer interview at Deloitte involves understanding the key areas of the framework, as well as solving real-world coding challenges. By mastering these interview questions, candidates can significantly improve their chances of securing a position at Deloitte, showcasing their ability to contribute effectively to high-profile projects and collaborate with cross-functional teams.

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1. What is AngularJS, and how does it differ from other JavaScript frameworks?

AngularJS is an open-source JavaScript framework designed to create dynamic web applications. It allows developers to extend HTML vocabulary with directives, making it easier to build interactive and responsive user interfaces. What sets AngularJS apart from other JavaScript frameworks is its use of two-way data binding, which keeps the model and the view in sync automatically. Unlike traditional frameworks that follow a DOM manipulation approach, AngularJS emphasizes declarative programming, making it more intuitive for developers to build single-page applications.

2. Explain the concept of two-way data binding in AngularJS.

Two-way data binding is one of the key features of AngularJS, allowing synchronization between the model and the view. When changes occur in the model, the view updates automatically, and when the user interacts with the view, the model is updated. This seamless synchronization minimizes the amount of code needed for data handling, making AngularJS applications more efficient and easier to maintain. It eliminates the need for manually tracking changes, which is common in other frameworks, thus reducing the development time and improving user experience.

Read more about Data Binding in Angular

3. What are directives in AngularJS, and how are they used?

Directives in AngularJS are one of the framework’s most powerful features, enabling developers to extend HTML functionality. Directives allow you to create reusable components and manipulate the DOM in a clean, declarative way. Built-in directives such as ng-if, ng-repeat, and ng-model provide commonly used functionality, while custom directives give developers the flexibility to define their own behavior. For example, you can create a custom directive to encapsulate complex UI components or implement specific user interactions without cluttering your HTML.

In addition to simplifying the code, directives also promote modularity in AngularJS applications. By breaking down the user interface into smaller, manageable components, directives make it easier to maintain, test, and scale large applications. This separation of concerns is a key advantage when developing complex, data-driven web applications.

How to Set up the Development Environment for Angular Application?

4. Can you explain what a scope is in AngularJS?

In AngularJS, the scope is an object that acts as a bridge between the controller and the view. It is the context in which the model and the view interact. Scope objects are available within controllers and allow you to define properties and methods that can be used by the view for rendering data. Essentially, the scope helps maintain the state of the application, keeping track of the variables and functions that need to be shared between the controller and the view.

Scopes can be hierarchical, meaning child controllers inherit the scope of their parent controllers. This enables AngularJS to handle complex data structures and user interactions within nested components. The scope also plays a vital role in the digest cycle, where it watches for changes in the model and updates the view accordingly. Understanding how scopes work is crucial for building robust and maintainable AngularJS applications, as it allows developers to control how data flows through different parts of the app.

Boost your Angular expertise by learning interesting facts about State Management in Angular through this comprehensive guide.

5. What is the role of $scope in AngularJS controllers?

The $scope in AngularJS acts as a key player in how the controller communicates with the view. It is an object that contains the data and functions that the view can use to display information and respond to user input. In a typical AngularJS controller, $scope serves as a shared space where variables and functions are defined, and it is responsible for binding the model to the view. For example, if you define a variable on the $scope object, it becomes accessible in the HTML view, and any changes made by the user are reflected back in the model.

Moreover, $scope helps in organizing the logic of your AngularJS application by keeping the data and methods localized to specific controllers. This reduces complexity and allows for better code organization, especially in large-scale applications. The $scope object also has built-in methods that allow developers to control the flow of data, such as $watch to observe changes in the model or $digest to manually trigger a digest cycle. Mastering $scope is essential for any AngularJS developer looking to build scalable and efficient web applications.

This blog offers a deep dive into Pipes in Angular—find out what you need to know.

6. How do you implement routing in AngularJS applications?

In AngularJS, routing allows us to build single-page applications (SPAs) by loading different views based on the URL without refreshing the page. We use the $routeProvider service to configure routes, specifying the path and the corresponding view template and controller. For example, we can define routes for home, about, and contact pages, each having its own template and controller. This approach enhances user experience by making navigation faster and more seamless since only the relevant portions of the page get updated.

How to Set up the Development Environment for Angular Application?

7. What are services in AngularJS, and how are they created?

Services in AngularJS are singleton objects that allow us to share data and functionality across different parts of an application. They are used to encapsulate logic that can be reused in various controllers or components, such as data retrieval, authentication, or logging services. We can create services using AngularJS’ built-in methods such as factory(), service(), and provider(). For example, a service might fetch data from an API and make it available to multiple controllers without duplicating code.

Services promote modularity and maintainability in large applications. By isolating business logic into reusable services, developers can avoid repeating code across controllers. Moreover, services are injected via dependency injection, making the application easier to test. Since services are instantiated only once during the application lifecycle, they are efficient and perfect for managing application-wide data or configuration.

Read more about Angular Material and UI Components

8. What is the difference between ng-if, ng-show, and ng-hide directives?

The ng-if, ng-show, and ng-hide directives are all used to conditionally display elements in AngularJS, but they differ in how they handle the DOM. The ng-if directive completely removes or adds elements from the DOM based on the condition provided. If the condition is false, the element is removed from the DOM entirely. On the other hand, ng-show and ng-hide work by toggling the visibility of an element using CSS. They don’t remove the element from the DOM; they just change its display property based on the condition.

This difference can affect the performance and behavior of your AngularJS application. Since ng-if removes elements from the DOM, it is useful when we want to free up resources or avoid unnecessary event listeners on hidden elements. However, if the condition frequently changes, ng-show or ng-hide might be more efficient because they don’t involve DOM manipulation every time the condition changes. Deciding between these directives depends on the specific use case and performance considerations in your application.

9. How does AngularJS handle dependency injection?

Dependency injection (DI) is a design pattern that AngularJS implements to manage components and services in a decoupled way. AngularJS uses DI to inject services, factories, and other dependencies into controllers, directives, or other components without explicitly creating them. This makes the code more modular, testable, and easier to maintain. For instance, if a controller needs a $http service to fetch data, AngularJS automatically provides the $http service as an argument to the controller without the need to instantiate it manually.

Dependency injection also enhances testability, allowing developers to mock services during unit tests easily. It decouples the logic from the instantiation of dependencies, making the application architecture more scalable. AngularJS handles DI at runtime, and developers can specify dependencies in various ways, such as using the array notation or $inject property. Understanding DI is crucial for creating modular and reusable code, as it helps developers build flexible and maintainable applications.

10. How do you handle forms and form validation in AngularJS?

Forms in AngularJS are handled using the ng-model directive, which binds input fields to model variables, allowing us to capture user input dynamically. AngularJS also provides built-in form validation mechanisms. For example, we can mark form controls as required, check for valid email formats, or enforce minimum and maximum lengths for input fields. The framework automatically updates the validation state of each input field and the form as a whole, making it easier to handle validation feedback in real-time.

In addition to built-in validators, AngularJS allows us to create custom validation functions using the ng-model-options directive or by adding custom validators to input elements. These custom validators can address specific business logic that the standard validators might not cover. By combining AngularJS’ built-in and custom validators, we can create highly interactive forms that provide instant feedback to users, improving both usability and data integrity.

Explore this blog to uncover key facts about Data Binding in Angular, crucial for any developer

11. Can you explain the digest cycle in AngularJS and how it works?

The digest cycle in AngularJS is a key concept responsible for keeping the model and view in sync. It involves checking all the watch expressions in the $scope and updating the DOM when changes occur in the model. Every time an event like a user interaction or an HTTP request completes, AngularJS triggers a digest cycle. This ensures that any changes in the model are reflected in the view, and vice versa, without the need for manual updates. The cycle repeats until there are no more changes to propagate, ensuring the UI remains consistent with the application state.

12. How do you optimize performance in large-scale AngularJS applications?

Optimizing performance in large AngularJS applications requires a combination of techniques. One approach is to minimize the number of watchers by reducing the number of data-bound elements. Since the digest cycle checks each watch expression, having too many watchers can slow down the application. Using one-time bindings (e.g., ::) for static content helps reduce the number of active watchers, improving performance. Another optimization strategy is to limit the use of heavy directives, like ng-repeat, in large data sets. For better performance, consider using pagination or lazy loading to render data incrementally.

Another effective technique is caching frequently used data or services to avoid redundant API calls. For example, using $cacheFactory to store data locally can improve load times when data needs to be reused across the application. Additionally, delegating heavy computations to Web Workers or breaking down complex operations into smaller tasks ensures the application remains responsive. These strategies help maintain optimal performance in large-scale applications while ensuring scalability.

13. Explain the concept of $digest vs. $apply in AngularJS.

In AngularJS, $digest and $apply are methods that manage the update of bindings between the model and view, but they serve different purposes. $digest triggers a manual digest cycle on the current scope and its children, updating the bindings only in that limited context. It’s often used when we want to update specific parts of the UI without triggering a complete application-wide digest cycle. For instance, in custom directives or service callbacks, we might call $digest to update a section of the view without affecting the entire scope hierarchy.

$apply, on the other hand, triggers a global digest cycle, updating all the bindings in the application. It is often used to ensure changes from external code (like jQuery or a setTimeout function) are reflected in the AngularJS scope. While $apply is more powerful, it is also more expensive in terms of performance. Here’s an example to show the difference:

$scope.name = 'John';
$scope.updateName = function() {
  $scope.name = 'Doe';
  $scope.$digest();  // Only triggers digest cycle for the current scope.
};

This approach ensures that only the current scope gets updated, keeping the application efficient.

14. What are custom directives, and how do you create them in AngularJS?

Custom directives in AngularJS allow us to encapsulate complex logic or UI components and reuse them across the application. We can define custom behavior by creating a directive using the directive method on an AngularJS module. A custom directive typically has a template, a scope, and behavior defined in a linking function. For instance, if we want to create a reusable UI component like a custom button, we can write a directive like this:

app.directive('customButton', function() {
  return {
    restrict: 'E',
    template: '<button>Click me</button>',
    link: function(scope, element, attrs) {
      element.on('click', function() {
        alert('Button clicked!');
      });
    }
  };
});

In this example, we’ve created a simple custom button directive that can be reused as <custom-button></custom-button>. This encapsulation makes the code modular and easier to maintain.

Custom directives are powerful because they allow us to define new HTML elements or attributes that suit the specific needs of our application. Directives can have isolated scopes, which means we can pass data to the directive using attributes, making it more flexible and reusable. We can also create complex behavior with custom logic in the directive’s linking function, enabling us to manage the DOM directly. Custom directives are a great way to keep your code DRY (Don’t Repeat Yourself) while building reusable components.

15. How do you handle memory leaks in AngularJS applications?

Memory leaks in AngularJS applications can occur when objects, event listeners, or DOM elements are not properly cleaned up. One common cause of memory leaks is not removing $watch listeners when a scope is destroyed. Since $watch listeners track model changes, leaving them active can cause unnecessary memory usage and slow down the application. To prevent this, I make sure to remove any listeners by calling deregister functions when a scope is no longer needed.

Another important area to address is event listeners that aren’t removed after use. If we add event listeners directly to the DOM in a directive’s linking function, we need to clean them up when the scope is destroyed. This can be done by unbinding the events in the $destroy event handler:

app.directive('customEvent', function() {
  return function(scope, element) {
    var clickHandler = function() {
      alert('Element clicked');
    };
    element.on('click', clickHandler);
    
    scope.$on('$destroy', function() {
      element.off('click', clickHandler); // Unbind event listener
    });
  };
});

This ensures that the application doesn’t retain unnecessary references or objects after a directive is no longer in use, preventing memory leaks and improving performance.

Read this awesome blog on Introduction to Angular to enhance your understanding and skills.

16. Imagine you have an AngularJS app with multiple nested components. How would you manage communication between them?

Managing communication between multiple nested components in AngularJS can be achieved using different strategies depending on the data flow. Typically, parent-child communication is managed using $scope inheritance, where a parent scope provides data to child components. When it comes to sibling components or deeply nested components, services act as a more scalable solution. By creating a shared service, I can store shared data and facilitate communication between components that don’t directly interact through a parent-child relationship. This makes the architecture cleaner and easier to maintain, especially in large applications with complex component hierarchies.

When sibling components need to communicate, broadcasting events using $rootScope.$broadcast or $scope.$emit can be effective, though it’s important to use these sparingly, as excessive broadcasting can make debugging difficult. Using a service, however, keeps the architecture more modular. Here’s an example of how a service can facilitate communication:

app.service('SharedService', function() {
  var sharedData = {};
  
  this.getData = function() {
    return sharedData;
  };
  
  this.setData = function(data) {
    sharedData = data;
  };
});

app.controller('ComponentA', function($scope, SharedService) {
  $scope.updateData = function(data) {
    SharedService.setData(data);
  };
});

app.controller('ComponentB', function($scope, SharedService) {
  $scope.$watch(function() {
    return SharedService.getData();
  }, function(newVal) {
    $scope.sharedData = newVal;
  });
});

In this example, Component A updates the shared data via the service, and Component B listens for changes using $watch. This approach ensures data consistency across components.

17. You’re tasked with improving the performance of an existing AngularJS application. What steps would you take?

When tasked with improving an AngularJS application’s performance, my first step is to analyze the number of watchers, as each watcher contributes to the digest cycle. Reducing the number of watchers can significantly enhance performance. I would use one-time bindings for static data with :: syntax to limit the scope of watchers. Next, I would minimize the use of heavy directives like ng-repeat by implementing pagination or virtual scrolling for large datasets, which prevents unnecessary DOM rendering.

In addition to reducing watchers, I’d focus on optimizing API calls and caching data. By implementing services that cache frequently used data, I can reduce redundant API calls and improve the application’s response time. Moreover, I’d review any DOM manipulations in the code, ensuring that direct DOM manipulation is minimized and handled efficiently via AngularJS directives. Lastly, optimizing images and scripts, as well as lazy loading components, can drastically improve the overall performance of the application.

To improve the performance of an existing AngularJS application, consider the following steps:

1. Enable Ahead-of-Time (AOT) Compilation: Use AOT to pre-compile Angular templates during the build process, reducing the load on the browser and improving startup performance.
2. Optimize Digest Cycle: Reduce the number of watchers in AngularJS by limiting two-way data binding, using track by in ng-repeat, and manually triggering $digest when necessary to reduce the load on the digest cycle.
3. Lazy Loading of Modules: Implement lazy loading for modules, routes, and components, so only the necessary parts of the application are loaded initially. This reduces the initial load time.
4. Use One-Time Binding: Where possible, replace regular Angular bindings ({{variable}}) with one-time bindings ({{::variable}}). This reduces unnecessary watches, improving performance.
5. Minimize Use of ng-repeat: For large data sets, consider using pagination, virtual scrolling, or libraries like ngInfiniteScroll to reduce the number of DOM elements rendered at once.
6. Limit DOM Manipulations: Avoid frequent DOM manipulations in Angular controllers. Use Angular’s built-in directives and minimize direct DOM access with document or jQuery.

18. If an AngularJS application has multiple asynchronous calls, how would you handle error management efficiently?

Handling multiple asynchronous calls in AngularJS can be tricky, especially when dealing with potential errors. I typically use AngularJS’s $q service for managing promises, which allows me to chain asynchronous operations and handle errors in a centralized way. By using $q.all(), I can wait for multiple promises to resolve and handle errors consistently in one place, rather than managing errors individually for each async call. This keeps the code clean and helps in maintaining control over the flow of asynchronous operations.

For example, if several HTTP requests are being made, I would wrap them in promises and use $q.all() to handle the results collectively. Here’s a small example:

$q.all([Service1.getData(), Service2.getData()]).then(function(responses) {
  // Handle successful responses
}, function(error) {
  // Handle any error from the requests
  console.error("Error occurred: ", error);
});

By centralizing error handling, this method ensures that even if one of the requests fails, I can manage it effectively without breaking the entire flow of the application. This approach improves error management and keeps the application robust.

19. In a scenario where a third-party API is inconsistent, how would you implement error handling and fallback mechanisms in AngularJS?

When dealing with inconsistent third-party APIs in AngularJS, it’s essential to have robust error-handling and fallback mechanisms. My first step is to wrap all HTTP calls in promises using the $http service. AngularJS’s promise-based structure allows me to easily handle errors using .catch() or .then() methods. For example, if a call to the third-party API fails, I can trigger a fallback operation, such as calling an alternative API or displaying a cached version of the data to ensure a seamless user experience.

Implementing retries with exponential backoff is another technique I use to handle API inconsistencies. This method attempts to call the API again after a delay, increasing the delay time after each failure. For example:

function makeApiCall(attempt) {
  $http.get('/third-party-api').then(function(response) {
    // Success
  }).catch(function(error) {
    if (attempt < 3) {
      $timeout(function() {
        makeApiCall(attempt + 1);
      }, Math.pow(2, attempt) * 1000);
    } else {
      // Fallback logic
      console.error("API failed after 3 attempts");
    }
  });
}
makeApiCall(1);

In this example, if the API call fails, the function retries up to 3 times, with an increasing delay after each failure. This fallback strategy ensures that the user is not immediately affected by inconsistent third-party APIs.

20. You have to migrate an old AngularJS project to a newer version of Angular. What would your migration strategy look like?

Migrating an AngularJS project to a newer version of Angular (Angular 2+), also known as “ngUpgrade,” is a multi-step process that requires careful planning. I would begin by identifying the most critical parts of the application that need migration and whether any features are heavily reliant on AngularJS-specific components. The migration process involves using the @angular/upgrade package, which allows AngularJS and Angular components to coexist during the transition. This hybrid approach enables me to gradually refactor the AngularJS code while keeping the application functional.

The key part of my strategy would involve migrating components and services incrementally. I would start with stateless components, as they are easier to refactor. Services can also be shared between AngularJS and Angular using the downgrade and upgrade services from the ngUpgrade library. Here’s an example of upgrading a service:

// AngularJS service
app.service('legacyService', function() {
  this.getData = function() {
    return 'data from AngularJS service';
  };
});

// Downgrade service for Angular
angular.module('app').factory('legacyService', downgradeInjectable(LegacyService));

By gradually upgrading components and services and testing the application at each stage, I ensure a smooth migration without disrupting the user experience. This strategy helps to modernize the codebase while maintaining a functional application throughout the migration process.

21. How do you implement custom filters in AngularJS?

Custom filters in AngularJS allow us to format data before displaying it in the view. To create a custom filter, we define it using the .filter() method on a module. Filters can be applied in templates using the pipe (|) symbol. Here’s an example of a custom filter that converts text to uppercase:

app.filter('uppercaseFilter', function() {
  return function(input) {
    if (!input) return '';
    return input.toUpperCase();
  };
});

In the view, you can use this filter as follows:

<p>{{ 'hello world' | uppercaseFilter }}</p>

This will display “HELLO WORLD” in the view, showcasing the custom filter in action.

22. How do you debounce an API call in AngularJS?

Debouncing API calls helps reduce the frequency of requests by ensuring that a function is only called after a certain delay, preventing multiple rapid calls. In AngularJS, this can be implemented using $timeout. For example, to debounce a search input field, we can use the following code:

app.controller('SearchController', function($scope, $timeout) {
  var timeout;
  
  $scope.search = function(query) {
    if (timeout) $timeout.cancel(timeout);
    
    timeout = $timeout(function() {
      // API call here
      console.log("Searching for:", query);
    }, 500);  // 500ms debounce time
  };
});

In this example, every time the user types, the previous API call is canceled if it hasn’t completed within 500 milliseconds, reducing unnecessary server load.

23. How do you share data between controllers in AngularJS?

Sharing data between controllers in AngularJS is typically done using services. Services are singleton objects, which means they are shared across the entire application, making them ideal for managing shared state. Here’s an example of a shared service for storing user information:

app.service('UserService', function() {
  var user = {};

  this.setUser = function(data) {
    user = data;
  };

  this.getUser = function() {
    return user;
  };
});

app.controller('ControllerA', function($scope, UserService) {
  $scope.saveUser = function(data) {
    UserService.setUser(data);
  };
});

app.controller('ControllerB', function($scope, UserService) {
  $scope.user = UserService.getUser();
});

Controller A sets the user data, and Controller B retrieves the data, ensuring seamless communication between them.

Prepare to crack your next tough Angular interview by mastering these Components and Modules concepts.

24. How do you create and use a custom directive with isolated scope in AngularJS?

Creating a custom directive with isolated scope allows the directive to have its own data without inheriting from the parent scope. This is useful when creating reusable components. Here’s how to create a custom directive with isolated scope:

app.directive('userCard', function() {
  return {
    restrict: 'E',
    scope: {
      user: '='
    },
    template: '<div>Name: {{user.name}}</div><div>Email: {{user.email}}</div>'
  };
});

This directive can be used in the HTML as follows:

<user-card user="userInfo"></user-card>

Here, userInfo is passed from the parent scope to the directive’s isolated scope, making the directive a reusable component for displaying user information.

25. How do you handle form validation in AngularJS using custom directives?

Custom form validation in AngularJS can be handled by creating a directive that works with the ngModel controller. Here’s an example of a custom directive that checks if the input is a valid email:

app.directive('validEmail', function() {
  var EMAIL_REGEX = /^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}$/;

  return {
    require: 'ngModel',
    link: function(scope, element, attrs, ctrl) {
      ctrl.$validators.validEmail = function(modelValue, viewValue) {
        return EMAIL_REGEX.test(viewValue);
      };
    }
  };
});

In the view, use the directive like this:

<form name="emailForm">
  <input type="text" name="email" ng-model="user.email" valid-email>
  <span ng-show="emailForm.email.$error.validEmail">Invalid email!</span>
</form>

This custom directive integrates with AngularJS form validation, showing an error message if the email input doesn’t match the regular expression.