---
# System prepended metadata

title: Blkbox FrontEnd Interview

---

# 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:

```jsx=
typeof null
typeof NaN
typeof undefined
```

- Explain type conversion in JS

- State truthy and falsy value of Following

```js
null
NaN
'undefined'
[]
{}
''
```

- What will be output and why

```js
0 || ‘0’ || false

0 && ‘0’ && false 
```

- What will be output and Why

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

console.log(i, ‘outer’)

```

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

console.log(i, ‘outer’’)
```

- What will the output and why

```js 
let primitiveValue = 5;

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

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

```js
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 -


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

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

- What will be output and why

```js
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

![image alt](https://i2.wp.com/programmingwithmosh.com/wp-content/uploads/2018/10/Screen-Shot-2018-10-31-at-1.44.28-PM.png?ssl=1)

- 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`