20+ 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

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

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.

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

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.

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

• useState is used to add state to functional components in React.

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

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

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

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

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 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.

Events in React are actions that occur as a result of user interaction or system events.

• Events are used to trigger functions or actions in response to user input.

• Examples of events include onClick, onChange, onSubmit, etc.

• Event handlers are functions that are executed when an event is triggered.

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