# Blkbox FrontEnd Interview

- What is React JS?

- How virtual DOM work?

- What are the advantage and disadvantage of using React JS?

- React: What is and how you can take advantage of PureComponent

- What Is Load Balancing?

- Explain the importance of the HTML DOCTYPE?

- Explain what is lazy loading?

- What will be output for the followings:

typeof null
typeof NaN
typeof undefined

- Explain type conversion in JS

- State truthy and falsy value of Following

null
NaN
'undefined'
[]
{}
''

- What will be output and why

0 || ‘0’ || false

0 && ‘0’ && false 

- What will be output and Why

for(var i=0; i<6;i++) {
   // some code
}

console.log(i, ‘outer’)

for(let i=0; i<6;i++) {
  // some code
}

console.log(i, ‘outer’’)

- What will the output and why

let primitiveValue = 5;

function referenceTestPrimitive(number) {
   number++;
   return number;
}

const result = referenceTestPrimitive(primitiveValue);
console.log(primitiveValue === result)

let nonPrimitiveValue = {‘fName’: ‘Kushal’};

function ReferenceTestNonPrimitive(object) {
  object[‘lName’] = ‘Dave’
  return object;
}

const result = referenceTestNonPrimitive(nonPrimitiveValue);

console.log(nonPrimitiveValue === result)

- What is DOM ?

- What is `event.preventDefault()` somecase we use it in Form handling why we use it?

- Explain Event Delegation?

- Explain Closure

- What is callback function and HOF ?

- Implement `mapMethod` using for loop

e.g -

function addTwo(num) {
  return num + 2;
}

mapArray([1,2,3], addTwo) return // [3,4,5]

- What will be output and why

console.log(1)

setTimeout(() => {
 console.log(2)
}, 0)

setTimeout(() => {
 console.log(3)
}, 1)

console.log(4)

- What is promise in JS?

- What is JSX and which tool we used to convert JSX to React Element

- What is differnece between state and props?

- What is mean by props drilling and how to deal with it in react way

- What is redux and explain how it work ? when to use component state or redux state (How to identify one over other to use)

- Life Cycle method in react

- Explain the following


- What is hooks and different type of hooks you know

- What is HOC? Where it used?

- Why we need to provide a key in react when returning a multiple element using map method?

- What is controlled component and uncontrolled component

- What is memoization in react where it used

- Extra

Create a basic blog App using the react class component

API - `https://jsonplaceholder.typicode.com/posts`