400+ Reactjs Developer Interview Questions and Answers

Implement a counter with increment and decrement buttons and an input field to set the value.

• Create a state variable to hold the counter value

• Use onClick event handlers for the buttons to increment and decrement the counter

• Use onChange event handler for the input field to update the counter value

• Set the initial value of the counter to 0

• Validate the input value to ensure it is a number

Call, apply, and bind are methods used to manipulate the context of a function. Currying is a technique to transform a function with multiple arguments into a sequence of functions with single arguments. Closure is a feature that allows a function to access variables from its outer scope. Promises are objects used for asynchronous programming. Hoisting is a behavior where variable and function declarations are moved to the top of their containing scope. Event loop is a mechan...read more

Implement a product list with flexbox layout and sorting functionality in React.

• Use React's useState to manage the product list and sorting order.

• Create a Product component to display individual product details.

• Utilize CSS Flexbox for responsive layout: e.g., 'display: flex; flex-wrap: wrap;'.

• Implement a sort function that updates the state based on selected criteria (e.g., price, name).

• Example sort function: 'const sortedProducts = [...products].sort((a, b) => a.price - b.pr...read more

Development dependencies are needed for development and testing, while regular dependencies are needed for the application to run.

• Development dependencies are used during the development process, such as testing frameworks or build tools.

• Regular dependencies are required for the application to function properly in production.

• Development dependencies are typically listed in the devDependencies section of package.json, while regular dependencies are listed in dependencies.

• Examp...read more

Questions related to ReactJS development including ES6 features, lifecycle, custom hooks, and deployment.

• ES6 features include let, const, arrow functions, and destructuring

• Function components are simpler and class components have state and lifecycle methods

• Sort method is used to sort arrays based on a given criteria

• Browser router is used for client-side routing in React applications

• Custom hooks are reusable functions that can be used across multiple components

• Webpack is a mod...read more

To make a page responsive, use Bootstrap layouts and alerts.

• Use Bootstrap's grid system to create responsive layouts

• Use media queries to adjust the layout based on screen size

• Use Bootstrap's responsive utility classes to hide/show elements on different devices

• Use Bootstrap's responsive navigation components for mobile-friendly menus

• Use Bootstrap's responsive images to ensure they scale properly

• Use Bootstrap's responsive embeds for videos and other media

• Use Bootstrap's respons...read more

Code for functional component to call API and show list from response

• Use useEffect hook to call API on component mount

• Use useState hook to store API response

• Map through response data to display list

• Handle loading and error states

Presentation components are class components that handle rendering, while functional components are simpler and handle stateless rendering.

• Presentation components are also known as container components.

• Functional components are also known as stateless components.

• Presentation components are used for complex UI logic and state management.

• Functional components are used for simple UI logic and stateless rendering.

• Functional components are easier to test and maintain.

• Example of pr...read more

Modularizing code in ReactJs involves breaking down the application into smaller components for better organization and reusability.

• Create separate components for different parts of the UI

• Use props to pass data and functions between components

• Use state to manage component-specific data

• Use React Router to handle routing and navigation

• Use CSS modules or styled-components for component-specific styling

• Use Redux or Context API for state management across components

• Perform unit te...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

Symbols ^ and ~ in package.json represent version ranges for dependencies.

• The symbol ^ represents a range that allows minor version updates.

• The symbol ~ represents a range that allows only patch updates.

• For example, ^1.2.3 allows any version from 1.2.3 to <2.0.0, while ~1.2.3 allows any version from 1.2.3 to <1.3.0.

A single page application is a web application that loads once and dynamically updates the content without refreshing the page.

• Loads once and dynamically updates content

• No page refreshes

• Uses JavaScript frameworks like React to handle routing and rendering

• Improves user experience by providing a seamless and responsive interface

A reusable React form component that dynamically generates fields and displays inputs in a table format.

• Use React's state to manage form data dynamically.

• Utilize a mapping function to render form fields based on an array of field definitions.

• Implement a submit handler to process the form data.

• Display the form inputs in a table format using HTML table elements.

• Example of field definitions: [{ label: 'Name', type: 'text' }, { label: 'Age', type: 'number' }]

A function to recursively flatten an object and return a string representation.

• The function should handle nested objects and numbers.

• Use recursion to traverse through the object properties.

• Concatenate the keys and values into a single string.

• Example input: {l: {x: 1}, o: '+', p: 2} should return 'x:1 + 2'.

• Ensure to handle both objects and numbers appropriately.

Find largest element, sum of all elements, and count of each element in array without sorting.

• To find the largest element, iterate through the array and keep track of the maximum value.

• To find the sum of all elements, iterate through the array and add each element to a running total.

• To find the count of each element, use a hashmap to store the count of each element as you iterate through the array.

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

• Hoisting applies to both variable and function declarations.

• Variable declarations are hoisted but not their initializations.

• Function declarations are fully hoisted, allowing them to be called before they are declared.

• Hoisting does not apply to variables declared with let or const.

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

Form validation using HTML and validating phone number with country code

• Use HTML5 form validation attributes like 'required', 'pattern', 'minlength', etc.

• For phone number validation, use 'pattern' attribute with regex for specific country code format

• Example: <input type='tel' pattern='[0-9]{3}-[0-9]{3}-[0-9]{4}' required>

Data can be passed from children to parents in React components by using callback functions.

• Use callback functions to pass data from child components to parent components

• Parent component passes a function as a prop to child component

• Child component calls the function with the data as an argument to pass data to parent component

Four people with different crossing times need to cross a bridge using one torch in the least time possible.

• A and B cross first (1 min), A returns with the torch (2 min).

• C and D cross together (10 min), B returns with the torch (1 min).

• A and B cross again (2 min).

• Total time = 1 + 2 + 10 + 1 + 2 = 16 minutes.

Count pairs in an array that sum up to a specified number using efficient algorithms.

• Use a hash map to store numbers and their counts for O(n) time complexity.

• For each number, check if the complement (target - number) exists in the map.

• Example: For array [1, 2, 3, 4] and target 5, pairs are (1,4) and (2,3).

• Handle duplicates carefully to avoid overcounting pairs.

A React.js code example to add circles dynamically on button click, either from left to right or right to left.

• Use React's useState to manage the array of circles.

• Create a button that triggers a function to add a new circle.

• Use CSS for styling the circles and positioning them.

• Implement a conditional to determine the direction of addition.

The output for the given code snippets will be: 1) 20 2) ReferenceError: Cannot access 'b' before initialization 3) ReferenceError: b is not defined

• In the first snippet, variable 'a' is declared using 'var' after it is assigned a value, so it logs 20 without any issues.

• In the second snippet, variable 'b' is declared using 'let' after it is assigned a value, so it throws a ReferenceError as 'b' is accessed before initialization.

• In the third snippet, variable 'b' is declared us...read more

Create a context menu in React that opens panels on the right when children are present.

• Use a state variable to manage the visibility of the context menu.

• Implement a right-click event listener to trigger the context menu.

• Render the context menu conditionally based on the presence of children.

• Use CSS for styling the context menu and panels for a better user experience.

• Example: <ContextMenu>{children}</ContextMenu> to render children.

var is function scoped, let and const are block scoped.

• var can be redeclared and updated within its scope

• let can be updated but not redeclared within its scope

• const cannot be updated or redeclared once declared

• let and const are not hoisted like var

• const must be initialized during declaration

SASS is a preprocessor scripting language that is interpreted into CSS, offering more features and flexibility.

• SASS is a preprocessor for CSS, allowing for variables, nesting, and mixins to be used in stylesheets.

• SASS code needs to be compiled into CSS before being used in a web project.

• SASS helps in writing cleaner and more organized CSS code, making it easier to maintain and update styles.

• CSS is the styling language used for web development, while SASS is a tool that extend...read more

Component lifecycle in functional components involves useEffect hook for side effects.

• Functional components use useEffect hook to handle side effects like fetching data, subscribing to events, etc.

• useEffect hook can be used to mimic componentDidMount, componentDidUpdate, and componentWillUnmount lifecycle methods.

• useEffect hook takes a callback function as its first argument and an optional array of dependencies as its second argument.

• The callback function inside useEffect ru...read more

Answering how to fetch data from API and display in UI list format for Reactjs Developer interview.

• Use fetch or axios to fetch data from API

• Store the data in state or Redux store

• Map through the data and display in UI list format

• Handle loading and error states

• Example: fetch('https://api.example.com/data').then(response => response.json()).then(data => setData(data))

• Example: {data.map(item =>

• {item.name}
)}