Ernst & Young React Developer Interview Questions and Answers

Round duration - 60 Minutes
Round difficulty - Medium

This was a preety intense round revolving mainly around the core concepts of JavaScript . I was confident about my
skills in JavaScript as I already had some projects in JS and I also completed the Guided Path of JS in CodeStudio
which boosted my preparation and helped me crack these Frontend Interviews.

Q1. What are callbacks in JavaScript?
Ans. 
Callbacks in JavaScript are functions passed as arguments to other functions to be executed later.
• Callbacks are commonly used in event handling, asynchronous programming, and functional programming.

• They allow for functions to be executed after another function has finished its execution.

• Example: setTimeout(callbackFunction, 1000) will execute callbackFunction after 1 second.
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Q2. Can you explain hoisting in JavaScript?
Ans. 
Hoisting is a JavaScript mechanism where variable and function declarations are moved to the top of their containing scope.
• Variable declarations are hoisted to the top of their scope but not their assignments.

• Function declarations are fully hoisted, meaning they can be called before they are declared.

• Hoisting can lead to unexpected behavior if not understood properly.
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Q3. What are closures in JavaScript?
Ans. 
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

• Ex...read more
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Q4. What is the difference between slice and splice in JavaScript?
Ans. 
slice() returns a shallow copy of a portion of an array without modifying the original array, while splice() changes the contents of an array by removing or replacing existing elements.
• slice() does not modify the original array, while splice() does

• slice() returns a new array, while splice() returns the removed elements

• slice() takes start and end index as arguments, while splice() takes start index, number of elements t...read more
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Q5. What is a first-class function in JavaScript?
Ans. 
A first-class function in JavaScript is a function that can be treated like any other variable.
• Can be passed as an argument to other functions

• Can be returned from other functions

• Can be assigned to variables

• Can be stored in data structures
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Q6. How would you sort an array of integers in JavaScript?
Ans. 
Use the built-in sort() method to sort an array of integers in JavaScript.
• Use the sort() method with a compare function to sort the array in ascending order.

• For descending order, modify the compare function to return b - a instead of a - b.

• Example: const numbers = [4, 2, 5, 1, 3]; numbers.sort((a, b) => a - b);
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Q7. 
Covid Vaccination Distribution Problem
As the Government ramps up vaccination drives to combat the second wave of Covid-19, you are tasked with helping plan an effective vaccination schedule. Your goal is...read more
Ans. 
Maximize the number of vaccines administered on a specific day while adhering to certain rules.
• Given n days, maxVaccines available, and a specific dayNumber, distribute vaccines to maximize on dayNumber

• Administer positive number of vaccines each day with a difference of 1 between consecutive days

• Ensure sum of vaccines distributed does not exceed maxVaccines

• Output the maximum number of vaccines administered on dayNumber...read more
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Round duration - 60 Minutes
Round difficulty - Medium

In this round, I was asked questions from React and Redux. Since I already had prior experience in working with React
and Redux , I answered most of the questions correctly and was preety much confident about passing this round.

Q1. What are the differences between a class component and a functional component in ReactJS?
Ans. 
Class components are ES6 classes that extend from React.Component and have access to state and lifecycle methods, while functional components are simple functions that take props as arguments and return JSX.
• Class components are defined using ES6 classes and extend from React.Component

• Functional components are simple functions that take props as arguments and return JSX

• Class components have access to state and lifecycle...read more
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Q2. What are the differences between stateless and stateful components in ReactJS?
Ans. 
Stateless components do not have internal state, while stateful components have internal state.
• Stateless components are functional components that do not have internal state.

• Stateful components are class components that have internal state.

• Stateless components are simpler and easier to test.

• Stateful components are more complex and can hold and update internal state.

• Example: Stateless component - const Button = () =>...read more
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Q3. What is the diffing algorithm in ReactJS?
Ans. 
React uses a diffing algorithm called Virtual DOM to efficiently update the actual DOM based on changes in state or props.
• Virtual DOM is a lightweight copy of the actual DOM.

• React compares the Virtual DOM with the previous Virtual DOM to identify the minimal number of changes needed to update the actual DOM.

• This process is known as reconciliation and helps in optimizing performance by reducing unnecessary re-renders.

• Ex...read more
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Q4. What is reconciliation in ReactJS?
Ans. 
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 compo...read more
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Q5. What are props in React?
Ans. 
Props in React 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.

• Props are passed down the component tree.

• Props can be any type of data, such as strings, numbers, objects, or functions.

• Example: <ChildComponent name='John' age={25} />
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Q6. What are the advantages of using Redux?
Ans. 
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
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Q7. What are the key differences between mapStateToProps() and mapDispatchToProps() in Redux?
Ans. 
mapStateToProps() is used to access the Redux state in a component, while mapDispatchToProps() is used to dispatch actions to update the state.
• mapStateToProps() is used to access the Redux state and return data as props for a component.

• mapDispatchToProps() is used to dispatch actions to update the Redux state.

• mapStateToProps() is a function that takes the current state as an argument and returns an object with props th...read more
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Q8. What is the Combine Reducer in Redux?
Ans. 
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 })
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Q9. How is Relay different from Redux?
Ans. 
Relay is a GraphQL client specifically designed for React, while Redux is a state management library for any JavaScript application.
• Relay is tightly integrated with GraphQL, making it easier to fetch and manage data from a GraphQL server.

• Redux is a more general-purpose state management library that can be used with any backend technology.

• Relay uses a declarative approach to data fetching, where components declare their...read more
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Q10. How can we structure the top-level directories in Redux?
Ans. 
Top-level directories in Redux should be structured based on functionality and feature modules.
• Separate directories for actions, reducers, and components

• Group related functionality together in separate directories

• Use feature modules to encapsulate related actions, reducers, and components

• Example: 'actions', 'reducers', 'components', 'utils', 'constants'
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Round duration - 30 Minutes
Round difficulty - Easy

This was a Technical Cum HR round where I was first asked some basic principles around Frontend Web Development
and then we discussed about my expectations from the company , learnings and growth in the forthcomig years. I would
suggest be honest and try to communicate your thoughts properly in these type of rounds to maximise your chances of
getting selected.

Q1. What do you know about the company?
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Q2. Why should we hire you?
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