300+ React Js Frontend Developer Interview Questions and Answers

Abstract class can have implementation while interface cannot. A class can implement multiple interfaces but only extend one abstract class.

• Abstract class can have constructors while interface cannot.

• Abstract class can have non-abstract methods while interface can only have abstract methods.

• A class can implement multiple interfaces but only extend one abstract class.

• Abstract class can have instance variables while interface cannot.

• Abstract class is used for code reusability w...read more

Primary key uniquely identifies a record in a table, while unique key ensures that all values in a column are distinct.

• Primary key is used to enforce entity integrity, while unique key enforces domain integrity.

• A table can have only one primary key, but multiple unique keys.

• Primary key can't have null values, while unique key can have null values.

• Primary key is automatically indexed, while unique key may or may not be indexed.

Arrow functions are a concise way to write functions in JavaScript.

• Arrow functions have a shorter syntax compared to regular functions.

• They do not have their own 'this' value, instead, they inherit 'this' from the surrounding context.

• Arrow functions are always anonymous and cannot be used as constructors.

• They are commonly used in React components for event handlers and callback functions.

Interfaces in Java provide a way to achieve abstraction and multiple inheritance.

• Interfaces define a set of methods that a class must implement.

• They allow for loose coupling between classes.

• Interfaces can be used to achieve polymorphism.

• Java does not support multiple inheritance, but interfaces provide a way to achieve it.

• Interfaces are used extensively in Java frameworks like Spring and Hibernate.

Overloading is having multiple methods with the same name but different parameters. Overriding is implementing a method in a subclass that already exists in the parent class.

• Overloading is compile-time polymorphism while overriding is runtime polymorphism.

• Overloading is used to provide different ways of calling the same method with different parameters.

• Overriding is used to provide a specific implementation of a method in a subclass that is already defined in the parent class...read more

Yes, exception handling is required in a big project.

• Exception handling helps in identifying and resolving errors during runtime.

• It improves the stability and reliability of the application.

• Exceptions can occur due to various reasons like network issues, server errors, or user input errors.

• Handling exceptions gracefully prevents the application from crashing and provides a better user experience.

• It allows for proper error logging and debugging, making it easier to identify an...read more

Access specifiers in Java are keywords that determine the visibility of a class, method, or variable.

• There are four access specifiers in Java: public, private, protected, and default.

• Public: accessible from anywhere in the program.

• Private: accessible only within the same class.

• Protected: accessible within the same class, subclasses, and same package.

• Default: accessible within the same package only.

Encapsulation is a mechanism of wrapping data and code acting on the data together as a single unit.

• Encapsulation is used to hide the implementation details of an object from the outside world.

• It helps in achieving data abstraction and data hiding.

• In Java, encapsulation is achieved by declaring the variables of a class as private and providing public getter and setter methods to access and modify the data.

• Example: A bank account class with private variables like account numbe...read more

Finding index of elements in an array in JavaScript

• To find the index of the 3rd element, use arr.indexOf('three')

• To find the index of the 5th element without using indexOf, use arr[4]

• Arrays in JavaScript are zero-indexed, so the 5th element is at index 4

JWT stands for JSON Web Token.

• JWT is an open standard for securely transmitting information between parties as a JSON object.

• It is commonly used for authentication and authorization purposes in web applications.

• JWT consists of three parts: header, payload, and signature.

• The header contains the algorithm used to sign the token.

• The payload contains the claims or statements about the user.

• The signature is used to verify the authenticity of the token.

• Example: eyJhbGciOiJIUzI1NiIs...read more

To download a large CSV file (>1000mb) and show the data while downloading, use streaming and pagination techniques.

• Implement streaming to download the file in chunks instead of all at once.

• Use pagination to display the downloaded data in parts as it is being downloaded.

• Consider using libraries like PapaParse for parsing CSV data efficiently.

• Show a progress bar or indicator to inform the user about the download status.

Responsive design using media queries and flexible layouts to adapt frontend to different devices.

• Use media queries in CSS to apply different styles based on screen size

• Utilize responsive frameworks like Bootstrap or Foundation for pre-built responsive components

• Implement flexible layouts using percentage-based widths and max-width properties

• Optimize images for different screen resolutions using srcset and sizes attributes

• Test the frontend on various devices and screen sizes ...read more

Typescript offers advantages like static typing, improved code quality, better tooling support, and easier refactoring.

• Static typing helps catch errors at compile time

• Improved code quality due to type checking

• Better tooling support with features like code navigation and auto-completion

• Easier refactoring with the help of type annotations

• Faster development with enhanced IDE support

You can pause a process in JavaScript using the setTimeout function.

• Use setTimeout function to delay the execution of a code block.

• Specify the time in milliseconds for the delay.

• Example: setTimeout(() => { console.log('Paused for 2 seconds'); }, 2000);

Class components are ES6 classes that extend from React.Component and have their own state, while functional components are simple functions that take props as arguments.

• Class components have lifecycle methods like componentDidMount, componentDidUpdate, etc., while functional components do not.

• Functional components are simpler and easier to read/write compared to class components.

• Functional components are preferred for simple UI components, while class components are used for...read more

To check for duplicate strings in an array, use a Set data structure. To remove duplicates, convert the array to a Set and then back to an array.

• Create a Set from the array to automatically remove duplicates

• Check if the size of the Set is equal to the size of the original array to determine if there are duplicates

• Convert the Set back to an array to get the array without duplicates

Explanation of CSS specificity hierarchy

• Inline styles have the highest specificity

• IDs have higher specificity than classes and elements

• Multiple selectors with the same specificity are resolved by the order in which they appear in the stylesheet

• The universal selector (*) has the lowest specificity

ReactJS has a component life cycle that defines how a component behaves and updates. Hooks are functions that allow state and other React features to be used in functional components.

• ReactJS has three main phases in its component life cycle: Mounting, Updating, and Unmounting.

• Hooks are functions that allow state and other React features to be used in functional components. There are two types of hooks: State hooks and Effect hooks.

• State hooks allow functional components to ha...read more

Create an Admin component with a form that submits data to the parent component.

• Create a child component called Admin

• Add a form to the Admin component

• Handle form submission in the Admin component

• Pass the submitted data to the parent component using props

• Render the submitted data in the parent component

Some popular hacker attacks include XSS (Cross-Site Scripting) and SQL Injection.

• XSS (Cross-Site Scripting) - attackers inject malicious scripts into web pages viewed by other users

• SQL Injection - attackers insert malicious SQL code into input fields to manipulate database queries

Props are inputs passed to React components. There are two types: ownProps and childrenProps.

• ownProps are passed directly to the component from its parent

• childrenProps are passed to the component through its children

• ownProps can be accessed using this.props in the component

• childrenProps can be accessed using this.props.children in the component

React uses a virtual DOM and a reconciliation algorithm to determine when to re-render components.

• React compares the current virtual DOM with the previous virtual DOM to identify changes.

• If there are differences, React updates the actual DOM to reflect the changes.

• React's reconciliation algorithm efficiently updates only the necessary parts of the DOM.

A closure is a function that has access to its own scope, as well as the scope in which it was defined.

• A closure allows a function to access variables from its outer function even after the outer function has finished executing.

• Closures are created whenever a function is defined within another function.

• Example: function outerFunction() { let outerVar = 'I am outer'; return function innerFunction() { console.log(outerVar); }; }

The Reducer hook is used to manage state in React by allowing you to update and control the state of a component.

• It is used to handle complex state logic in a more organized and scalable way.

• It takes in a reducer function and an initial state, and returns the current state and a dispatch function.

• The reducer function specifies how the state should be updated based on the action dispatched.

• Actions are objects that describe what changes should be made to the state.

• The dispatch ...read more

Debouncing in JavaScript is a technique used to limit the number of times a function is called in a short period of time.

• Debouncing is used to improve performance and prevent unnecessary function calls.

• It delays the execution of a function until after a certain amount of time has passed since the last invocation.

• It is commonly used in scenarios like search bars, scroll events, and input validation.

• Debouncing can be implemented using setTimeout and clearTimeout functions.

• Examp...read more

OOPs stands for Object-Oriented Programming. It is a programming paradigm based on the concept of objects.

• OOPs focuses on creating objects that contain both data and methods to manipulate that data.

• The four main principles of OOPs are encapsulation, inheritance, polymorphism, and abstraction.

• Examples of OOPs languages include Java, C++, and Python.

• The structure of OOPs involves classes, objects, inheritance, polymorphism, and encapsulation.

Lazy loading is a technique used to defer loading non-essential resources until they are needed.

• Lazy loading helps improve performance by only loading resources when they are required.

• In React, lazy loading can be implemented using React.lazy() and Suspense components.

• Example: const MyComponent = React.lazy(() => import('./MyComponent'));

• Example: Loading...}>

React routing is a way to handle navigation in a React application by defining routes and rendering components based on the URL.

• React routing allows for declarative routing, where routes are defined using JSX elements.

• It enables dynamic routing based on the URL, allowing for different components to be rendered based on the route.

• React Router is a popular library for handling routing in React applications.

• Unlike conventional routing methods, React routing is more efficient as ...read more

React hooks are functions that let you use state and other React features without writing a class.

• useState: allows functional components to have local state

• useEffect: performs side effects in function components

• useContext: allows you to subscribe to React context without introducing nesting

• useReducer: an alternative to useState for more complex state logic

• useMemo: memoizes the result of a function

• useRef: returns a mutable ref object whose .current property is initialized to t...read more

useMemo is used to memoize a value, while useCallback is used to memoize a function.

• useMemo is used to memoize a computed value and recompute only when its dependencies change.

• useCallback is used to memoize a callback function and re-create it only when its dependencies change.

• useMemo is useful for optimizing performance by avoiding unnecessary re-computations.

• useCallback is useful for optimizing performance by avoiding unnecessary re-creations of callback functions.

Clone a Todo Application UI using ReactJS, Sass

• Create a new ReactJS project using create-react-app

• Design the UI using Sass and implement it in ReactJS

• Use state and props to manage the todo list

• Add functionality to add, delete and mark tasks as complete

• Implement local storage to persist data

• Test the application thoroughly

Arrow functions are more concise and have lexical scoping, while normal functions have their own 'this' value.

• Arrow functions do not have their own 'this' value, they inherit it from the parent scope.

• Arrow functions are more concise and have implicit return.

• Normal functions are more flexible and can be used as constructors.

• Arrow functions cannot be used as constructors.

• Arrow functions do not have 'arguments' object.

No, it is not advisable to use indexes as keys in React components.

• Using indexes as keys can lead to performance issues and incorrect behavior when reordering or deleting items in a list.

• It is recommended to use unique IDs as keys to ensure proper component rendering and reconciliation.

• Example:

  {items.map((item, index) =>
  • {item}
  )}

Callbacks, event delegation, React reconciliation, and React lifecycle methods are key concepts in React development.

• Callback functions are functions passed as arguments to be executed later, commonly used in event handling and asynchronous operations.

• Event delegation is a technique where a single event listener is attached to a parent element to manage events for multiple child elements.

• React reconciliation is the process of updating the DOM to match the virtual DOM represen...read more

Architectural patterns are reusable solutions to common problems in software architecture.

• MVC (Model-View-Controller) - Separates an application into three main components: the model, the view, and the controller.

• MVVM (Model-View-ViewModel) - Similar to MVC but with a ViewModel layer that manages the state and behavior of the view.

• Flux - Unidirectional data flow pattern commonly used with React applications.

• Microservices - Architectural style that structures an application as...read more

Yes, I have configured various parts of CI/CD pipelines.

• I have set up automated builds using tools like Jenkins or GitLab CI.

• I have configured automated testing processes within the pipeline.

• I have integrated deployment scripts to automatically deploy code to different environments.

• I have set up notifications for build status and deployment success/failure.

• I have worked on optimizing and improving the efficiency of CI/CD pipelines.

Difference between React.memo vs use memo vs useCallback

• React.memo is a higher-order component that memoizes a functional component

• useMemo is a hook that memoizes a value

• useCallback is a hook that memoizes a function

• React.memo and useMemo are used for performance optimization

• useCallback is used to prevent unnecessary re-renders

useState and useEffect are hooks in React used for managing state and side effects respectively.

• useState is a hook used to add state to functional components in React

• useEffect is a hook used to perform side effects in functional components

• Hooks in React allow functional components to have state and lifecycle methods similar to class components

Server-Side Rendering (SSR) is the process of rendering web pages on the server and sending the HTML to the client.

• SSR improves website performance and SEO by reducing load times and providing search engines with fully rendered pages.

• React provides a library called ReactDOMServer that allows developers to render React components on the server.

• SSR requires a server-side framework or library, such as Next.js or Gatsby, to handle the rendering and routing of pages.

• SSR can be com...read more

DOM stands for Document Object Model. It is a programming interface for HTML and XML documents.

• DOM represents the structure of a document as a tree of objects.

• We can use JavaScript to access and manipulate the DOM to change HTML tags.

• We can use DOM methods like getElementById, getElementsByClassName, etc. to select HTML elements.

• We can then modify the selected elements by changing their properties or attributes using JavaScript.

• For example, to change the text of a paragraph e...read more

Netflix uses a content delivery network (CDN) to efficiently load videos based on user location and network conditions.

• Netflix uses a distributed network of servers to store and deliver video content closer to users for faster loading times.

• Content is cached on servers located in various regions to reduce latency and improve streaming quality.

• Netflix employs adaptive bitrate streaming to adjust video quality based on available bandwidth, ensuring smooth playback even on slowe...read more

Redux is a predictable state container for JavaScript apps.

• Redux is a state management library for JavaScript applications.

• It helps in managing the state of the application in a predictable way.

• Redux stores the entire state of the application in a single immutable object.

• Actions are dispatched to update the state, and reducers specify how the state changes in response to actions.

• Redux is commonly used with React to manage the state of complex applications.

• Example: In a shoppi...read more

Filter students with average marks > 85

• Calculate average marks for each student

• Filter students with average marks > 85

• Return names of filtered students