400+ 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.

useEffect can mimic lifecycle methods by specifying dependencies and cleanup functions.

• useEffect can be used to mimic componentDidMount by specifying an empty dependency array.

• useEffect can be used to mimic componentDidUpdate by specifying a dependency array.

• useEffect can be used to mimic componentWillUnmount by returning a cleanup function.

• useEffect can be used to mimic shouldComponentUpdate by using memoization techniques.

• useEffect can be used to mimic componentDidCatch by ...read more

To join three different tables in SQL, you can use the JOIN keyword along with the appropriate join conditions.

• Use the JOIN keyword to combine tables based on a common column

• Specify the join conditions using the ON keyword

• You can join more than two tables by chaining multiple JOIN statements

• Different types of joins include INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN

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

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

Optimize React component rendering by restructuring state management to prevent sibling re-renders.

• Use local state in child components instead of lifting state up to the parent.

• Implement context API for shared state without causing sibling re-renders.

• Utilize refs to manage mutable state that doesn't trigger re-renders.

• Consider using a state management library (like Redux) to isolate state changes.

Redux offers centralized state management, time-travel debugging, middleware support, and better performance compared to Context API in React.

• Redux provides a centralized store for managing application state, making it easier to maintain and update state across components.

• Redux allows for time-travel debugging, enabling developers to easily track and debug state changes over time.

• Redux supports middleware, allowing for additional functionality such as logging, asynchronous ac...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

Accessing deeply nested arrays in JavaScript requires recursion or iterative methods to traverse the structure dynamically.

• Use recursion to traverse the nested structure. Example: function getNestedValue(obj) { return Array.isArray(obj) ? obj.map(getNestedValue) : obj; }

• Utilize a stack or queue to iteratively access elements. Example: let stack = [data]; while (stack.length) { let item = stack.pop(); // process item }

• Leverage libraries like Lodash for deep access. Example: _....read more

Perform mathematical operations on 100 using given functions.

• The given functions are add20, multiply30, divide10, and subtract10.

• Start with 100 and apply the functions in the given order.

• The result will be ((100 - 10) * 30) / 10 + 20 = 290.

Uncontrolled components are useful when you want to allow user input without controlling the value of the input field.

• Useful for simple forms where you don't need to validate or manipulate the input data.

• Can be used for search bars where the input value is not critical to the application state.

• Helpful for input fields that are part of a larger form and don't require immediate validation.

Local storage is considered more secure due to its same-origin policy and limited access by other websites.

• Local storage is more secure as it is limited to the same origin policy, meaning only the website that stored the data can access it.

• Local storage data is not sent with every HTTP request, reducing the risk of data interception.

• Session storage is also secure but data is lost when the browser is closed, making it less suitable for long-term storage.

• Cookies are less secure...read more

Closures capture variable scope, hoisting affects variable declarations, and call by reference vs. value defines how data is passed.

• Closures are functions that retain access to their lexical scope even when executed outside that scope.

• Example of closure: function outer() { let x = 10; return function inner() { return x; }; } const innerFunc = outer(); innerFunc(); // returns 10

• Hoisting is JavaScript's behavior of moving declarations to the top of their containing scope during...read more

Hooks in React allow functional components to manage state and side effects, enhancing component functionality.

• Hooks are functions that let you use state and other React features in functional components.

• useState: A hook that allows you to add state to functional components. Example: const [count, setCount] = useState(0);

• useReducer: A hook for managing complex state logic, similar to Redux. Example: const [state, dispatch] = useReducer(reducer, initialState);

• useState is simpl...read more

Web performance can be measured using tools like Lighthouse, which provides scores on various metrics affecting user experience.

• Lighthouse evaluates performance based on metrics like First Contentful Paint (FCP) and Time to Interactive (TTI).

• A score of 90+ is considered good, while below 50 indicates poor performance.

• Lighthouse also assesses accessibility, SEO, and best practices, providing a holistic view of web performance.

• For example, optimizing images can improve FCP, whi...read more

Optimizing React app performance involves techniques like code splitting, memoization, and efficient state management.

• Use React.memo to prevent unnecessary re-renders of functional components.

• Implement code splitting with React.lazy and Suspense to load components only when needed.

• Utilize useMemo and useCallback hooks to memoize expensive calculations and functions.

• Optimize images and assets to reduce load times.

• Leverage the React Profiler to identify performance bottlenecks....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

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);

Use 'let' for block-scoped variables, 'var' for function-scoped variables.

• Use 'let' when you want to declare a variable with block scope.

• Use 'var' when you want to declare a variable with function scope.

• Using 'let' helps prevent variable hoisting and can lead to cleaner code.

• Example: let x = 10; for (let i = 0; i < x; i++) { console.log(i); }

• Example: var y = 20; function test() { var z = 30; console.log(y + z); }

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

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