Mastek Reactjs Developer Interview Questions and Answers

Q1. What are hooks in reactjs?
Ans. 
Hooks are a new feature in React 16.8 that allow you to use state and other React features without writing a class.
• Hooks are functions that let you use state and other React features in functional components.

• They allow you to reuse stateful logic without changing your component hierarchy.

• Some built-in hooks include useState, useEffect, useContext, etc.

• Hooks provide a more direct API to the React concepts you already kn...read more
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Q2. What is pure component?
Ans. 
Pure component is a class component that does not re-render if the input props and state remain the same.
• Pure components implement shouldComponentUpdate method with shallow prop and state comparison.

• They are optimized for performance as they prevent unnecessary re-renders.

• Example: class MyComponent extends React.PureComponent { // component code here }
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Q3. What is modules in javascript?
Ans. 
Modules in JavaScript are reusable pieces of code that can be exported from one file and imported into another.
• Modules help in organizing code into separate files for better maintainability.

• Modules can be imported using 'import' keyword and exported using 'export' keyword.

• Modules can be used to encapsulate code and prevent global namespace pollution.

• CommonJS and ES6 modules are two popular module systems in JavaScript.
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Q4. What is the difference between rem and em in css? How to hide something in CSS?
Ans. 
rem and em are both units in CSS for defining font sizes, with rem being relative to the root element and em being relative to the parent element.
• rem stands for 'root em' and is relative to the font size of the root element (usually the tag)

• em stands for 'element em' and is relative to the font size of the parent element

• To hide something in CSS, you can use the display property with a value of 'none' or the visibility...read more
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Q1. Difference between promise and async await ?
Ans. 
Promises are objects representing the eventual completion or failure of an asynchronous operation, while async/await is a syntactic sugar built on top of promises to write asynchronous code in a synchronous manner.
• Promises are used to handle asynchronous operations and provide a way to chain multiple asynchronous operations together.

• Async/await allows you to write asynchronous code that looks synchronous, making it eas...read more
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Q2. Event loop based a console output questions?
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Q3. UseMemo vs useCallback
Ans. 
useMemo is used for memoizing expensive calculations, while useCallback is used for memoizing functions.
• useMemo is used to memoize expensive calculations and only recompute the value when the dependencies change.

• useCallback is used to memoize functions and only re-create the function when the dependencies change.

• Example: useMemo can be used to memoize the result of a complex computation, while useCallback can be used t...read more
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Q1. Difference beetween thow and new thow
Ans. 
throw is used to throw an exception, new throw is used to throw an exception with a new object
• throw is used to throw an exception in JavaScript

• new throw is used to throw an exception with a new object

• Example: throw new Error('Something went wrong')
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Q2. How to add validation
Ans. 
Validation in React can be added using form validation libraries, custom validation functions, or built-in HTML5 validation attributes.
• Use form validation libraries like Formik or Yup for easy validation setup

• Create custom validation functions to validate input fields based on specific requirements

• Utilize built-in HTML5 validation attributes like required, pattern, and min/max for basic validation

• Display error messages...read more
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Q1. What is Event loop
Ans. 
Event loop is a mechanism in JavaScript that allows for asynchronous operations to be executed in a non-blocking way.
• Event loop is responsible for handling asynchronous operations in JavaScript.

• It allows for non-blocking execution of code by moving asynchronous tasks to the event queue.

• Event loop continuously checks the call stack and the event queue, moving tasks from the queue to the stack when the stack is empty.

• Exa...read more
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Q2. What are closures
Ans. 
Closures are functions that have access to their own scope, as well as the scope in which they were defined.
• Closures allow functions to access variables from their outer scope even after the outer function has finished executing.

• They are commonly used to create private variables in JavaScript.

• Closures are created whenever a function is defined within another function.
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