Ernst & Young React Developer Interview Questions, Process, and Tips

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. Javascript Question
What are callbacks?
Ans. 
A callback is a function that will be executed after another function gets executed.

In javascript, functions are treated as first-class citizens, they can be used as an argument of another function, can be
returned by another function and can be used as a property of an object.

Functions that are used as an argument to another function are called callback functions.


Example Code :

function divideByHalf(sum){
console.log(Mat...read more
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Q2. Javascript Question
Explain Hoisting in javascript.
Ans. 
Hoisting is a default behaviour of javascript where all the variable and function declarations are moved on top.
This means that irrespective of where the variables and functions are declared, they are moved on top of the scope.
The scope can be both local and global.


EXAMPLE 1 :

hoistedVariable = 10;
console.log(hoistedVariable); // outputs 10 even when the variable is declared after it is initialized
var hoistedVariable;


EX...read more
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Q3. Javascript Question
What are closures?
Ans. 
A closure is the combination of a function and the lexical environment within which that function was declared. i.e, It is
an inner function that has access to the outer or enclosing function’s variables. The closure has three scope chains

i) Own scope where variables defined between its curly brackets
ii) Outer function’s variables
iii) Global variables

Let's take an example of closure concept,

function Welcome(name){
var gr...read more
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Q4. Javascript Question
What is the difference between slice and splice?
Ans. 
Slice :
1) Doesn't modify the original array(immutable)
2) Returns the subset of original array
3) Used to pick the elements from array

Splice : 
1) Modifies the original array(mutable)
2) Returns the deleted elements as array
3) Used to insert or delete elements to/from array
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Q5. Javascript Question
What is a first class function?
Ans. 
In Javascript, functions are first class objects. First-class functions means when functions in that language are treated like any other variable.

For example, in such a language, a function can be passed as an argument to other functions, can be returned by another function and can be assigned as a value to a variable. For example, in the below example, handler functions assigned to a listener

const handler = () => c...read more
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Q6. Javascript Question
Sort an array of integers in JS.
Ans. 
1) The sort() method allows us to sort elements of an array in place. Besides returning the sorted array, the sort()
method changes the positions of the elements in the original array.

2) By default, the sort() method sorts the array elements in ascending order with the smallest value first and largest
value last.

3) The sort() method casts elements to strings and compares the strings lexicographically to determine the ord...read more
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Q7. Covid Vaccination
We are suffering from the Second wave of Covid-19. The Government is trying to increase its vaccination drives. Ninja wants to help the Government to plan an effective method to help incr...read more
Ans. 
Code : 

function binarySearch(arr,value,startPos,endPos){
if(startPos > endPos) return -1;

let middleIndex = Math.floor(startPos+endPos)/2;

if(arr[middleIndex] === value) return middleIndex;

elsif(arr[middleIndex > value]){
return binarySearch(arr,value,startPos,middleIndex-1);
}
else{
return binarySearch(arr,value,middleIndex+1,endPos);
}
}
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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. ReactJS Question
What are the differences between a class component and functional component?
Ans. 
Class Components :

1) Class-based Components uses ES6 class syntax. It can make use of the lifecycle methods.
2) Class components extend from React.Component.
3) In here you have to use this keyword to access the props and functions that you declare inside the class
components.


Functional Components :

1) Functional Components are simpler comparing to class-based functions.
2) Functional Components mainly focuses on the UI of ...read more
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Q2. ReactJS Question
What are stateless and stateful components?
Ans. 
Stateless Components : If the behaviour is independent of its state then it can be a stateless component. You can use
either a function or a class for creating stateless components. But unless you need to use a lifecycle hook in your
components, you should go for function components.


Stateful Components : If the behaviour of a component is dependent on the state of the component then it can be
termed as stateful component....read more
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Q3. ReactJS Question
What is diffing algorithm?
Ans. 
React needs to use algorithms to find out how to efficiently update the UI to match the most recent tree. The diffing
algorithms is generating the minimum number of operations to transform one tree into another. However, the
algorithms have a complexity in the order of O(n^3) where n is the number of elements in the tree.


In this case, for displaying 1000 elements would require in the order of one billion comparisons. Thi...read more
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Q4. ReactJS Question
What is reconciliation?
Ans. 
When a component's props or state change, React decides whether an actual DOM update is necessary by
comparing the newly returned element with the previously rendered one. When they are not equal, React will update
the DOM. This process is called reconciliation.
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Q5. ReactJS Question
What are props in React?
Ans. 
The props in React are the inputs to a component of React. They can be single-valued or objects having a set of values that will be passed to components of React during creation by using a naming convention that almost looks similar to HTML-tag attributes. We can say that props are the data passed from a parent component into a child component.

The main purpose of props is to provide different component functionalities ...read more
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Q6. Redux Question
What are the advantages of using Redux?
Ans. 
Some of the advantages of using Redux are as follows:

1) Redux provides extremely easy state transfer between the components.

2) The states are always predictable in Redux and its maintenance is relatively easy.

3) Debugging and testing code in Redux is simple through logging behaviour and status.

4) Redux provides great performance. It might occur to us that keeping the application's state global would result in bad perfo...read more
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Q7. Redux Question
Highlight the key differences between mapStateToProps() and mapDispatchToProps()?
Ans. 
mapStateToProps() : 
1) The mapStateToProps() method is used to render the stored data to the component.
2) The entirety of the results of the mapStateToProps() method is a plain object which is later merged into the component’s prop.
3) This method's use is to connect the redux state to the props of the react component.


mapDispatchToProps() : 
1) The mapDispatchToProps() method is used to render the action creato...read more
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Q8. Redux Question
What is Combine Reducer?
Ans. 
The combineReducers helper function turns an object whose values are different reducing functions into a single
reducing function you can pass to createStore . The resulting reducer calls every child reducer, and gathers their
results into a single state object.
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Q9. Redux Question
How Relay is different from Redux?
Ans. 
Relay is similar to Redux in that they both use a single store. The main difference is that relay only manages state
originated from the server, and all access to the state is used via GraphQL queries (for reading data) and mutations
(for changing data). Relay caches the data for you and optimizes data fetching for you, by fetching only changed data
and nothing more.
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Q10. Redux Question
How can we structure the top level directories in Redux?
Ans. 
Every Redux application has multiple top-level directories as given below :

1) Components: Components are used for “dumb” React components unfamiliar with Redux.

2) Containers: Containers are used for “smart” React components that are connected to Redux.

3) Actions: Actions are used for all the action creators, where the file name should be corresponding to the part of the
app.

4) Reducers: Reducers are used for all the red...read more
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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. Basic HR Question
Do you know anything about the company ?
Ans. 
General Tip : Before an interview for any company , have a breif insight about the company , what it does , when was
it founded and so on . All these info can be easily acquired from the Company Website itself .
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Q2. Basic HR Question
Why should we hire you?
Ans. 
Tip 1 : The cross questioning can go intense some time, think before you speak.

Tip 2 : Be open minded and answer whatever you are thinking, in these rounds I feel it is important to have opinion.

Tip 3 : Context of questions can be switched, pay attention to the details. It is okay to ask questions in these round,
like what are the projects currently the company is investing, which team you are mentoring. How all is the ...read more
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