Core React Js
Interview Question
1. What is React?
React is an **open-source frontend JavaScript library** which is used for building user interfaces especially for single page applications. It is used for handling view layer for web and mobile apps. React was created by [Jordan Walke](https://github.com/jordwalke), a software engineer working for Facebook. React was first deployed on Facebook's News Feed in 2011 and on Instagram in 2012.
2. What are the major features of React?
The major features of React are:
* It uses **VirtualDOM** instead RealDOM considering that RealDOM manipulations are expensive.
* Supports **server-side rendering**.
* Follows **Unidirectional** data flow or data binding.
* Uses **reusable/composable** UI components to develop the view.
3. What is JSX?
*JSX* is a XML-like syntax extension to ECMAScript (the acronym stands for *JavaScript XML*). Basically it just provides syntactic sugar for the `React.createElement()` function, giving us expressiveness of JavaScript along with HTML like template syntax.
In the example below text inside `<h1>` tag return as JavaScript function to the render function.
```jsx harmony
class App extends React.Component {
render() {
return(
<div>
<h1>{'Welcome to React world!'}</h1>
</div>
)
}
}
```
4. What is the difference between Element and Component?
An *Element* is a plain object describing what you want to appear on the screen in terms of the DOM nodes or other components. *Elements* can contain other *Elements* in their props. Creating a React element is cheap. Once an element is created, it is never mutated.
The object representation of React Element would be as follows:
```javascript
const element = React.createElement(
'div',
{id: 'login-btn'},
'Login'
)
```
The above `React.createElement()` function returns an object:
```
{
type: 'div',
props: {
children: 'Login',
id: 'login-btn'
}
}
```
And finally it renders to the DOM using `ReactDOM.render()`:
```html
<div id='login-btn'>Login</div>
```
Whereas a **component** can be declared in several different ways. It can be a class with a `render()` method. Alternatively, in simple cases, it can be defined as a function. In either case, it takes props as an input, and returns a JSX tree as the output:
```javascript
const Button = ({ onLogin }) =>
<div id={'login-btn'} onClick={onLogin}>Login</div>
```
Then JSX gets transpiled to a `React.createElement()` function tree:
```javascript
const Button = ({ onLogin }) => React.createElement(
'div',
{ id: 'login-btn', onClick: onLogin },
'Login'
)
```
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