100+ React Developer Interview Questions and Answers

Reconciliation in ReactJS is the process of updating the DOM to match the virtual DOM after a component's state or props have changed.

• Reconciliation is the algorithm React uses to update the UI efficiently.

• It compares the virtual DOM with the actual DOM and only updates the parts that have changed.

• Reconciliation is a key feature that helps React achieve high performance.

• Example: When a user interacts with a React component, reconciliation ensures that only the necessary parts...read more

Stateless components are functional components that don't have state, while stateful components are class components that have state.

• Stateless components are simpler and easier to test.

• Stateful components can hold and modify state, and can trigger re-renders.

• Stateless components can receive data through props.

• Stateful components can have lifecycle methods.

• Example of stateless component: const Button = ({ onClick }) => Click me

• Example of stateful component: class Counter exten...read more

State and props are two important concepts in React. State represents the internal data of a component, while props are used to pass data from a parent component to a child component.

• State is mutable and can be changed within a component.

• Props are read-only and cannot be modified within a component.

• State is used to manage component-specific data, while props are used for inter-component communication.

• State is initialized and managed within a component, while props are passed ...read more

Service Workers are scripts that run in the background and can intercept network requests, cache or modify responses.

• Offline support for web applications

• Push notifications

• Background synchronization

• Reduced network usage and faster page loads

• Progressive Web Apps (PWA)

Lifecycle methods are methods that get called at various stages of a component's life.

• componentDidMount() - called after component is mounted

• shouldComponentUpdate() - determines if component should update

• componentDidUpdate() - called after component is updated

• componentWillUnmount() - called before component is unmounted

Redux helps manage application state in a predictable way.

• Centralized state management

• Predictable state changes with actions and reducers

• Time-travel debugging with Redux DevTools

• Ecosystem of middleware for additional functionality

Local Storage is persistent storage that remains even after the browser is closed, while Session Storage is temporary and is cleared when the browser is closed.

• Local Storage has no expiration date, while Session Storage is cleared when the session ends

• Local Storage can store larger amounts of data compared to Session Storage

• Local Storage is accessible across different browser tabs and windows, while Session Storage is limited to the current tab or window

• Local Storage can be a...read more

Combine Reducer is a function in Redux that combines multiple reducers into a single reducer function.

• Combines multiple reducers into a single reducer function

• Helps manage different pieces of state in Redux store

• Improves code organization and maintainability

• Example: combineReducers({ reducer1, reducer2 })

• Example: const rootReducer = combineReducers({ reducer1, reducer2 })

setState is a method used in React to update the state of a component.

• setState is asynchronous and batched for performance optimization.

• It merges the new state with the previous state.

• It schedules a re-render of the component and its children.

• Passing a function to setState ensures the previous state is used correctly.

• Example: this.setState({ count: this.state.count + 1 })

Optimizing performance in React involves using different rendering methods like Virtual DOM, memoization, and lazy loading.

• Use Virtual DOM to efficiently update the UI by only re-rendering components that have changed.

• Implement memoization techniques like useMemo and useCallback to prevent unnecessary re-renders of components.

• Utilize lazy loading to load components only when they are needed, reducing the initial load time of the application.

Closures in JavaScript are functions that have access to variables from their outer scope even after the outer function has finished executing.

• Closures allow functions to access variables from their parent function's scope

• They maintain a reference to the variables they need, even after the parent function has finished executing

• Closures are commonly used to create private variables and data encapsulation in JavaScript

• Example: function outerFunction() { let outerVar = 'I am out...read more

Promise is a callback function that returns a value in the future. Async-Await is a syntax that simplifies working with Promises.

• Promises are used to handle asynchronous operations and avoid callback hell.

• Async-Await is a syntax that allows writing asynchronous code that looks like synchronous code.

• Async-Await is built on top of Promises and uses the same underlying mechanism.

• Async-Await can only be used within an async function.

• Async-Await can handle errors using try-catch b...read more

Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their scope.

• Hoisting applies to both variable and function declarations.

• Variable declarations are hoisted but not their initializations.

• Function declarations are fully hoisted, including their definitions.

• Hoisting can lead to unexpected behavior if not understood properly.

map returns a new array with modified elements, forEach executes a function for each element in an array.

• map returns a new array, forEach does not

• map modifies the elements of the array, forEach does not

• map returns the same number of elements as the original array, forEach does not necessarily

• forEach is used for side effects, map is used for transformation

Custom hooks are important for reusability and abstraction of logic in React components.

• Custom hooks allow for the reuse of logic across multiple components without repeating code.

• They help in abstracting complex logic into reusable functions, making components cleaner and easier to maintain.

• Custom hooks can encapsulate side effects like API calls, local storage interactions, etc., making them easier to manage.

• Example custom hook for using local storage: const useLocalStorage...read more

State in Redux is changed by dispatching actions to reducers, which then update the state immutably.

• State changes in Redux by dispatching actions to reducers

• Reducers receive the current state and an action, then return a new state

• State is updated immutably in Redux

useMemo is used for memoizing a value, while useCallback is used for memoizing a function.

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

• useCallback is used to memoize a function and recompute it only when its dependencies change.

• Example: useMemo can be used to memoize the result of a complex computation, while useCallback can be used to memoize a callback function passed to a child component.

Closures are functions that have access to variables in their outer scope, even after the outer function has returned.

• Closures are created when a function is defined inside another function.

• The inner function has access to the outer function's variables and parameters.

• Closures can be used to create private variables and methods.

• Closures can also be used to create functions with pre-set arguments.

Context API is a feature in React that allows data to be passed down the component tree without manually passing props at each level.

• Context API provides a way to share data between components without the need for prop drilling.

• It consists of two main components: Provider and Consumer.

• The Provider component allows data to be provided and accessed by any component within its subtree.

• The Consumer component allows components to consume the data provided by the Provider.

• Context A...read more

Redux is a state management library for JavaScript applications, providing a predictable and centralized workflow.

• Redux follows a unidirectional data flow pattern.

• The application state is stored in a single JavaScript object called the store.

• Actions are dispatched to describe state changes.

• Reducers are pure functions that specify how the state should change based on the dispatched actions.

• Selectors are used to extract specific data from the store.

• Middleware can be used to add...read more

React JS is a JavaScript library for building user interfaces.

• React JS is a declarative, efficient, and flexible JavaScript library.

• It allows developers to build reusable UI components.

• React uses a virtual DOM to efficiently update and render components.

• Props are used to pass data from a parent component to a child component.

• Props are read-only and cannot be modified by the child component.

• useRef hook is used to create a mutable reference that persists across component render...read more

map creates a new array with the results of calling a provided function on every element, while forEach executes a provided function once for each array element.

• map returns a new array with the same length as the original array, while forEach does not return anything.

• map does not mutate the original array, while forEach can mutate the original array.

• map is more suitable for transforming data and creating a new array, while forEach is used for iterating over elements and perfo...read more

Redux is a predictable state container for JavaScript apps.

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

• State changes are made by dispatching actions, which are plain JavaScript objects.

• Reducers specify how the state changes in response to actions.

• Components can subscribe to the Redux store to access the state and re-render when it changes.

To create and optimize a React application, focus on efficient component structure, state management, code splitting, lazy loading, and performance monitoring.

• Use functional components and hooks for better performance.

• Implement state management with tools like Redux or Context API.

• Split code into smaller chunks and lazy load components for faster initial load times.

• Optimize performance by minimizing re-renders and using tools like React.memo and useMemo.

• Monitor performance us...read more

Hoisting is the JavaScript mechanism where variable and function declarations are moved to the top of their containing scope. HOC stands for Higher Order Component, a pattern used in React for code reusability. Debouncing is a technique used to limit the rate at which a function is executed.

• Hoisting moves variable and function declarations to the top of their scope during the compilation phase.

• HOC is a function that takes a component and returns a new component with additiona...read more

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

• useState is used to manage state in functional components

• It returns an array with two elements - the current state value and a function to update the state

• useEffect is used to manage side effects like fetching data or updating the DOM

• It takes a function as its argument and runs it after every render

• useEffect can also take a second argument which is an array of dependencies to control ...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 an 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.

useEffect is a hook in React that allows performing side effects in functional components.

• It is used to manage component lifecycle in functional components.

• It can be used to fetch data from an API, update the DOM, or set up event listeners.

• It takes two arguments: a function that performs the side effect and an array of dependencies.

• The function is executed after every render, and the dependencies array determines when the effect should be re-run.

• If the dependencies array is e...read more

To create slice and combine reducers in React, use the createSlice and combineReducers functions from Redux toolkit.

• Use createSlice function to define a slice of state with reducers and actions.

• Example: const counterSlice = createSlice({ name: 'counter', initialState: 0, reducers: { increment: state => state + 1, decrement: state => state - 1 } })

• Use combineReducers function to combine multiple slices into a single root reducer.

• Example: const rootReducer = combineReducers({ c...read more

Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their scope.

• Variables declared with var are hoisted to the top of their scope

• Function declarations are also hoisted to the top of their scope

• Function expressions are not hoisted

• Hoisting can lead to unexpected behavior if not understood properly

Semantic tags in HTML are special tags that provide meaning to the content they enclose.

• Semantic tags help search engines and screen readers understand the structure of a webpage.

• Examples of semantic tags include <header>, <footer>, <nav>, <article>, <section>, <aside>, <main>, <figure>, <figcaption>.

• Using semantic tags improves SEO and accessibility of a website.

Yes, I am available for complete 1 month.

• I am committed to completing any project I start

• I have no prior commitments that would interfere with this timeline

• I am willing to work extra hours if necessary to meet deadlines

Flexbox is a one-dimensional layout method for laying out items in rows or columns, while CSS Grid Layout is a two-dimensional layout system for grid-based layouts.

• Flexbox is best suited for laying out items in a single direction, like in navigation bars or card layouts.

• CSS Grid Layout is ideal for creating complex layouts with rows and columns, like in a grid of images or a magazine-style layout.

Performing operations on an array of objects using JavaScript.

• Use array methods like map, filter, reduce for operations on object array.

• Access object properties using dot notation or bracket notation.

• Iterate through the array using loops like for loop or forEach method.

• Example: Calculate total sum of 'price' property in an array of products.

Virtual DOM is a lightweight copy of the actual DOM that React uses to update the UI efficiently.

• Virtual DOM is a JavaScript object that represents the actual DOM.

• It allows React to update only the necessary parts of the UI, improving performance.

• Changes made to the Virtual DOM are compared with the previous version to determine the minimum number of changes needed to update the actual DOM.

• React uses a diffing algorithm to compare the Virtual DOM and actual DOM.

• React updates ...read more

Components are reusable UI elements while pages are unique views of an application.

• Components can be used multiple times throughout an application while pages are typically only used once.

• Components are often smaller and more focused on a specific task while pages are larger and more complex.

• Components can be composed together to create more complex UI while pages are standalone views.

• Examples of components include buttons, forms, and navigation menus while examples of pages ...read more

Use recursion to find the deepest object in a nested object structure.

• Use recursion to iterate through each key in the object

• Keep track of the depth level as you traverse the object

• Compare the depth levels of each nested object to find the deepest one

useEffect is a hook in React that allows side effects to be performed in function components.

• Used to perform side effects in function components

• Executes after render

• Can be used to fetch data, subscribe to events, update the DOM, etc.

• Takes a function as the first argument and an optional array of dependencies as the second argument

React guard is a higher-order component used to restrict access to certain routes in a React application.

• React guard is used for implementing authentication and authorization in React applications.

• It can be used to prevent unauthorized users from accessing certain routes.

• React guard can redirect users to a login page if they are not authenticated.

• It can also be used to restrict access based on user roles or permissions.

Axios interceptors are used to globally handle HTTP requests and responses in an application.

• Interceptors can be used to add headers, handle errors, or modify requests before they are sent.

• Example: Adding a token to all outgoing requests for authentication.

• Example: Logging all incoming responses for debugging purposes.

Using LIMIT and OFFSET in MySQL query to retrieve a specific subset of results.

• Use LIMIT to specify the maximum number of rows to return

• Use OFFSET to specify the number of rows to skip before starting to return rows

• Example: SELECT * FROM table_name LIMIT 10 OFFSET 5;

Program to draw * patterns

• Use nested loops to iterate through rows and columns

• Use conditional statements to determine when to print *

• Example: for a 5x5 square, the pattern would be a grid of *

SDLC lifecycle stands for Software Development Life Cycle, which is a process used by software development teams to design, develop, and test high-quality software.

• SDLC lifecycle consists of several phases including planning, analysis, design, implementation, testing, and maintenance.

• Each phase has its own set of activities and deliverables to ensure the successful completion of the software project.

• Examples of SDLC models include Waterfall, Agile, and DevOps, each with its o...read more

Event looping is the process in which a programming language continuously checks for and executes events in a non-blocking manner.

• Event looping allows for asynchronous operations to be handled efficiently.

• It involves a queue of events waiting to be processed, with the event loop continuously checking for new events to execute.

• Commonly used in JavaScript to handle user interactions, network requests, and timers.

Responsive design is creating web applications that adapt to different screen sizes and devices.

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

• Utilize flexible grids and layouts to ensure content adjusts accordingly

• Test the design on various devices and screen sizes to ensure responsiveness

To centralize an element in React, use CSS properties like display: flex, justify-content: center, align-items: center.

• Use CSS properties like display: flex, justify-content: center, align-items: center to center the element horizontally and vertically.

• Alternatively, you can use margin: auto; for both top and bottom, left and right to center the element.

• For absolute positioning, set top, bottom, left, right to 0 and margin to auto.

React Router is a library that allows for declarative routing in React applications.

• React Router is used for client-side routing in single-page applications.

• It allows for declarative routing using components and props.

• Routes can be defined using the component and nested using and .

• React Router also provides access to the current location and history objects using the useHistory and useLocation hooks.