If you want to get through the toughest of interviews, start practising the ReactJS interview questions and answers listed here. These are questions that will help you prepare for your ReactJS Interview for React js development services providing company.

Q1. How React works? How Virtual-DOM works in React?
React creates a virtual DOM. When state changes in a component it firstly runs a “diffing” algorithm, which identifies what has changed in the virtual DOM. The second step is reconciliation, where it updates the DOM with the results of diff.
The HTML DOM is always tree-structured — which is allowed by the structure of HTML document. The DOM trees are huge nowadays because of large apps. Since we are more and more pushed towards dynamic web apps (Single Page Applications — SPAs), we need to modify the DOM tree incessantly and a lot. And this is a real performance and development pain.
The Virtual DOM is an abstraction of the HTML DOM. It is lightweight and detached from the browser-specific implementation details. It is not invented by React but it uses it and provides it for free. ReactElements lives in the virtual DOM. They make the basic nodes here. Once we defined the elements, ReactElements can be render into the "real" DOM.
Whenever a ReactComponent is changing the state, diff algorithm in React runs and identifies what has changed. And then it updates the DOM with the results of diff. The point is - it’s done faster than it would be in the regular DOM.

Q2. What is JSX?
JSX is a syntax extension to JavaScript and comes with the full power of JavaScript. JSX produces React “elements”. You can embed any JavaScript expression in JSX by wrapping it in curly braces. After compilation, JSX expressions become regular JavaScript objects. This means that you can use JSX inside of if statements and for loops, assign it to variables, accept it as arguments, and return it from functions. Even though React does not require JSX, it is the recommended way of describing our UI in React app.
For example, below is the syntax for a basic element in React with JSX and its equivalent without it.

Equivalent of the above using React.createElement

Q3. What is React.createClass?
React.createClass allows us to generate component "classes." But with ES6, React allows us to implement component classes that use ES6 JavaScript classes. The end result is the same -- we have a component class. But the style is different. And one is using a "custom" JavaScript class system (createClass) while the other is using a "native" JavaScript class system.
When using React’s createClass() method, we pass in an object as an argument. So we can write a component using createClass that looks like this:

Using an ES6 class to write the same component is a little different. Instead of using a method from the react library, we extend an ES6 class that the library defines, Component.

constructor() is a special function in a JavaScript class. JavaScript invokes constructor() whenever an object is created via a class.

Q4. What is ReactDOM and what is the difference between ReactDOM and React?
Prior to v0.14, all ReactDOM functionality was part of React. But later, React and ReactDOM were split into two different libraries.
As the name implies, ReactDOM is the glue between React and the DOM. Often, we will only use it for one single thing: mounting with ReactDOM. Another useful feature of ReactDOM is ReactDOM.findDOMNode() which we can use to gain direct access to a DOM element.
For everything else, there’s React. We use React to define and create our elements, for lifecycle hooks, etc. i.e. the guts of a React application.

Q5. What are the differences between a class component and functional component?
Class components allows us to use additional features such as local state and lifecycle hooks. Also, to enable our component to have direct access to our store and thus holds state.
When our component just receives props and renders them to the page, this is a ‘stateless component’, for which a pure function can be used. These are also called dumb components or presentational components.
From the previous question, we can say that our Booklist component is functional components and are stateless.

On the other hand, the BookListContainer component is a class component.

Q6. What is the difference between state and props?
The state is a data structure that starts with a default value when a Component mounts. It may be mutated across time, mostly as a result of user events.
Props (short for properties) are a Component’s configuration. Props are how components talk to each other. They are received from above component and immutable as far as the Component receiving them is concerned. A Component cannot change its props, but it is responsible for putting together the props of its child Components. Props do not have to just be data — callback functions may be passed in as props.
There is also the case that we can have default props so that props are set even if a parent component doesn’t pass props down.

Props and State do similar things but are used in different ways. The majority of our components will probably be stateless. Props are used to pass data from parent to child or by the component itself. They are immutable and thus will not be changed. State is used for mutable data, or data that will change. This is particularly useful for user input.

Q7. What are controlled components?
In HTML, form elements such as ,