30+ Reactjs Developer Interview Questions and Answers for Freshers

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

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.

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

States are mutable data managed within a component, while props are immutable data passed from parent to child components.

• States are managed within a component and can be changed using setState method

• Props are passed from parent to child components and are immutable

• States are used for internal component data management, while props are used for passing data between components

• Example: A counter component may have a state for the count value, while receiving a prop for the incr...read more

Synchronous programming in JavaScript executes code sequentially, blocking further execution until the current task completes.

• 1. Function Calls: Functions execute in the order they are called. Example: `functionA(); functionB();`

• 2. Loops: Iterative structures like `for` and `while` run synchronously. Example: `for (let i = 0; i < 5; i++) { console.log(i); }`

• 3. Conditional Statements: `if` and `switch` statements execute synchronously based on conditions. Example: `if (x > 10)...read more

The development cycle of a student registration app involves planning, designing, developing, testing, and deploying the app.

• Plan the app's features and functionality

• Design the user interface and user experience

• Develop the app using Reactjs and other technologies

• Test the app for bugs and errors

• Deploy the app to a server or hosting platform

• Continuously maintain and update the app as needed

This code demonstrates a simple JavaScript function that adds two numbers and returns the result.

• Define a function using the 'function' keyword: function add(a, b) { return a + b; }

• Call the function with arguments: let sum = add(5, 10);

• Use console.log to display the result: console.log(sum);

• Functions can also be assigned to variables: const add = function(a, b) { return a + b; };

Position properties in CSS for element placement.

• Absolute: Element is positioned relative to its closest positioned ancestor.

• Fixed: Element is positioned relative to the viewport.

• Relative: Element is positioned relative to its normal position.

React is a JavaScript library for building user interfaces, known for its virtual DOM and efficient rendering.

• React uses a virtual DOM to minimize actual DOM manipulation, improving performance.

• It employs a reconciliation algorithm to efficiently update the UI when data changes.

• React Fiber, a reimplementation of the core algorithm, enables incremental rendering for better user experience.

• React's component-based architecture allows for reusable and modular code, enhancing deve...read more

Demonstrating how to fetch data from an API using React.js with hooks.

• Use the useEffect hook to fetch data when the component mounts.

• Utilize the fetch API or axios for making HTTP requests.

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

• Handle loading and error states to improve user experience.

• Example: const [data, setData] = useState([]); useEffect(() => { fetch('API_URL').then(response => response.json()).then(data => setData(data)); }, []);

JSX is a syntax extension for JavaScript that allows writing HTML-like code within JavaScript. Babel is a tool used to transpile JSX code into plain JavaScript.

• JSX stands for JavaScript XML and is used to write HTML-like code in React components

• Babel is a JavaScript compiler that converts JSX code into plain JavaScript for browser compatibility

• JSX allows developers to write UI components in a more declarative and readable way

• Example:

  Hello, World!
  in JSX gets transpiled to Re...read more

DocType in HTML specifies the version of HTML being used in the document.

• DocType declaration is not an HTML tag.

• It is used to inform the web browser about the version of HTML being used.

• It is placed at the very beginning of an HTML document before the <html> tag.

• Example: <!DOCTYPE html> for HTML5.

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

• Hooks are introduced in React 16.8.

• They allow you to use state and other React features in functional components.

• Some commonly used hooks are useState, useEffect, useContext, and useReducer.

• Hooks help in reusing logic across components and making code more readable.

• Example: useState hook is used to add state to functional components.

Reverse a string in JavaScript

• Use the split() method to convert the string into an array

• Use the reverse() method to reverse the array

• Use the join() method to convert the array back to a string

Convert a number into words

• Break the number into groups of three digits

• Convert each group into words (e.g. '102' becomes 'one hundred two')

• Combine the words with appropriate thousand/million/billion separators

Reconciliation is the process of updating the UI to match the current state of the application.

• Reconciliation is the algorithm used by React to update the DOM efficiently.

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

• Reconciliation is a key feature of React that helps in improving performance by minimizing DOM updates.

• Examples of reconciliation include adding, updating, or removing elements in the UI based on changes in the appli...read more

States in React are mutable data that control the behavior of a component and determine what is rendered on the screen.

• States are used to store and manage data within a component.

• They are mutable and can be updated using the setState() method.

• Changes in state trigger re-rendering of the component.

• States are initialized in the constructor of a class component or using the useState hook in functional components.

Print pyramid pattern using nested loops

• Use nested loops to print each row of the pyramid

• Increment spaces and decrement stars in each row

• Example: for a pyramid with 5 rows -

• *

• ***

• *****

• *******

• *********

Props are read-only properties that are passed from a parent component to a child component in React.

• Props allow data to be passed between components in a unidirectional flow.

• Props are immutable and cannot be modified by the child component.

• Props can be used to customize the behavior or appearance of a component.

• Props are accessed using the 'this.props' syntax in class components or as function arguments in functional components.

• Example:

Closure is a feature in JavaScript that allows a function to access variables from its outer scope even after the function has finished executing.

• Closure is created when a function is defined inside another function.

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

• The inner function forms a closure with the outer function, preserving the state of the outer function's variables.

• Closures are commonly used to create private variables and functi...read more

Real DOM is the actual representation of the UI, while Virtual DOM is a lightweight copy used by React for efficient updates.

• Real DOM updates are slow and involve direct manipulation of the UI, which can lead to performance issues.

• Virtual DOM is a JavaScript object that represents the Real DOM, allowing React to optimize updates.

• When a component's state changes, React updates the Virtual DOM first, then compares it with the Real DOM (diffing).

• Only the changed elements are upd...read more

useEffect is a React hook that manages side effects in functional components, enabling lifecycle-like behavior.

• useEffect runs after every render by default, allowing you to perform side effects like data fetching.

• You can control when useEffect runs by passing a dependency array as the second argument.

• Example: useEffect(() => { fetchData(); }, [dependency]); runs fetchData only when 'dependency' changes.

• Cleanup can be handled by returning a function from useEffect, which runs ...read more

State in React is a JavaScript object that stores data relevant to a component.

• State is mutable and can be updated using the setState() method

• State is local and specific to a component

• Changes to state trigger re-rendering of the component

React is a JavaScript library for building user interfaces.

• React is used for creating interactive and dynamic user interfaces.

• It allows for building reusable UI components.

• React uses a virtual DOM for efficient rendering.

• React can be used for single-page applications, mobile apps, and more.

useState is a hook in React that allows functional components to have state.

• useState is a built-in hook in React.

• It allows functional components to have state.

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

• The initial state can be passed as an argument to useState.

• The state can be of any data type, such as a string, number, boolean, or object.

• Example: const [count, setCount] = useState(0);

The candidate can start immediately or as soon as required.

• Candidate can start immediately if needed

• Availability can be discussed during the interview process

• Flexibility in start date is appreciated

The lifecycle of a component refers to the different stages a component goes through from creation to deletion.

• Mounting: When a component is created and inserted into the DOM.

• Updating: When a component's state or props change.

• Unmounting: When a component is removed from the DOM.

• Examples of lifecycle methods include componentDidMount, componentDidUpdate, and componentWillUnmount.

• Lifecycle methods can be used to perform actions at specific stages of a component's lifecycle.