400+ React Js Frontend Developer Interview Questions and Answers

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

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

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

To optimize data fetching in React, techniques like pagination, lazy loading, and caching can significantly reduce load times.

• Pagination: Instead of fetching all 1000 records at once, implement pagination to load data in chunks, e.g., 100 records per request.

• Lazy Loading: Load data only when needed, such as when a user scrolls down or navigates to a specific section of the app.

• Caching: Use caching mechanisms like local storage or libraries like React Query to store previously...read more

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

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); }; }

Create an HTML table with data from a mock API and implement a search feature for filtering results.

• 1. Set up a React project using Create React App.

• 2. Use 'fetch' or 'axios' to retrieve data from the mock API.

• 3. Store the fetched data in a state variable using useState.

• 4. Map over the data to create table rows in JSX.

• 5. Implement a search input that updates a search term state.

• 6. Filter the displayed data based on the search term before rendering.

Authentication verifies the identity of a user, while authorization determines the user's access rights.

• Authentication confirms the user's identity through credentials like username and password.

• Authorization determines what actions the authenticated user is allowed to perform.

• Authentication precedes authorization in the security process.

• Example: Logging into a website (authentication) and then accessing specific pages based on user roles (authorization).

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

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

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

Local storage persists even after the browser is closed, while session storage is cleared when the browser is closed.

• Local storage has no expiration date, while session storage expires when the browser is closed.

• Local storage stores data with no limit, while session storage has a limit of around 5MB.

• Local storage data is available across all windows/tabs for that domain, while session storage data is only available within the same tab.

• Example: Local storage can be used for st...read more

Migrating a JavaScript Bootstrap project to React involves componentization and state management.

• Identify reusable components in your Bootstrap project, such as buttons, modals, and forms.

• Create React components for each identified UI element, e.g., <Button />, <Modal />, <Form />.

• Use React's state management (useState, useContext) to handle dynamic data instead of jQuery.

• Replace Bootstrap's JavaScript functionality with React's lifecycle methods or hooks.

• Utilize libraries li...read more

JSX is transformed into JavaScript through a process called transpilation, enabling browsers to understand and render it.

• JSX is a syntax extension for JavaScript, primarily used with React to describe UI components.

• Browsers do not understand JSX directly; it must be converted to standard JavaScript.

• Transpilation is the process of converting JSX into JavaScript using tools like Babel.

• For example, <div>Hello, World!</div> in JSX becomes React.createElement('div', null, 'Hello, ...read more

React offers a component-based architecture, virtual DOM, and strong community support, making it a popular choice for modern web development.

• Component-Based Architecture: React promotes reusable components, which enhances code maintainability and scalability. For example, a button component can be reused across different parts of an application.

• Virtual DOM: React uses a virtual DOM to optimize rendering, resulting in faster updates and improved performance. This means that o...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.

Redux is a predictable state container for JavaScript apps, enabling centralized state management and easier debugging.

• Centralized State: Redux maintains the application state in a single store, making it easier to manage and debug.

• Actions: State changes are triggered by actions, which are plain JavaScript objects describing what happened.

• Reducers: Pure functions called reducers specify how the state changes in response to actions.

• Middleware: Redux supports middleware for han...read more

Manage API data by storing it in state, using effects for fetching, and handling loading and error states effectively.

• Use React's useState to store the fetched data, e.g., const [data, setData] = useState([]);

• Implement useEffect to call the API when the component mounts, e.g., useEffect(() => { fetchData(); }, []);

• Handle loading states by using another state variable, e.g., const [loading, setLoading] = useState(true);

• Manage errors by using a state variable for error messages...read more

call, apply, and bind are methods to set the 'this' context in JavaScript functions, with different invocation styles.

• call: Invokes a function with a specified 'this' value and arguments provided individually. Example: func.call(thisArg, arg1, arg2);

• apply: Similar to call, but arguments are provided as an array. Example: func.apply(thisArg, [arg1, arg2]);

• bind: Returns a new function with a specified 'this' value, allowing for later invocation. Example: const boundFunc = func....read more

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}
  )}

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.

A Countdown Timer app built with React to track time remaining for a specified duration.

• Use React's useState to manage timer state.

• Implement useEffect to handle countdown logic.

• Provide input for users to set the countdown duration.

• Display the remaining time in a user-friendly format.

• Add start, pause, and reset functionality.

A simple React stopwatch with play, pause, and stop functionality using hooks.

• Use React's useState to manage time and state (running, paused).

• Use useEffect to handle the timer logic and update the time.

• Create buttons for play, pause, and stop functionalities.

• Example of useEffect: Update time every second when running.

• Use clearInterval to stop the timer when paused or stopped.