React Interview Questions and Answers for 2021

Top 6 React Interview Questions and Answers for 2021

Mar.14.2021

React.js interview questions may seem quite troublesome if you have not taken your time preparing for the BIG day. Slowly but steadily, React is becoming one of the most influential Javascript frameworks in the marketplace. Therefore the demand for React js developers is increasing daily. 

So, if you desire to find a job as a front-end developer or find a front-end developer for your project, get prepared with these React Interview Questions beforehand and achieve your objectives! 

 

Generic React Interview questions

 

What is React?  

React is a front-end JavaScript library developed by Facebook in 2011. Its principal purpose is to grow compound and interactive web and a  mobile User Interface. It is based on a component approach and helps to expand reusable UI components. 

The main features of React js are 

Virtual DOM

Server-side rendering 

Unidirectional data flow

React JS makes it altogether easy to write User Interface test cases. React conveniently incorporates other frameworks such as Angular, Meteor. 

It is essential to keep in mind that React is just a library rather than a fully developed framework. The library is wealthy, but don't worry, you have plenty of time to do your research.



What is JSX?

JSX stands for JavaScript XML. JavaScript XML is a file used by Reach employing the full potential of JavaScript with HTML-like syntax. The HTML file is accessible to understand, have a look below. 

 

render(){

    return(     

          

<div>

             

<h1> Hello World from Edureka!!</h1>

 

         </div>

 

    );

}

Browsers cannot read a JavaScript XML since they only read JavaScript objects. Therefore, JSX files get converted into a JavaScript object by JavaScript transformers such as Babel for browsers to read it. 

 

After such general questions, the interview will turn into a more technical discussion.

 

 

Is there any difference between Real DOM and Virtual DOM?

 

Yes, there certainly is. Real Dom is slow and creates a new DOM every time an element gets updated. Virtual DOM is faster and updates JSX if an element gets updated. Real Dom can directly update HTML, while Virtual Dom cannot, however Virtual DOM manipulation is much easier compared to Real Dom expensive manipulation. 

So, we can say that Virtual Dom is originally the copy of the Real DOM. It is a light JavaScript Object, a note tree that lists the elements, the attributes and content as Objects. The Virtual DOM works in three easy steps. 

Whenever any fundamental data changes, the entire User Interface gets re-rendered in Virtual DOM representation.

The difference between the previous representation and the new one is calculated. 

After the calculatioReact.js interview questions may seem quite troublesome if you have not taken your time preparing for the BIG day. Slowly but steadily React is becoming one of the most influential Javascript frameworks in the marketplace, hence the demand for React Certificate is increasing daily. 

So, if you desire to find a job as a front-end developer, prepare with these React Interview questions beforehand and achieve your dream!

 

What is Props? 

Props stand for properties in React. They are purely fixed read-only components that are always passed down from the parent to the child components and never back. A child component can never send a prop back to the parent. It guarantees the unidirectional data flow of React.

 

What is a State?

 

States are the core feature of React components. They are a source of the data and are mutable, unlike props. In a word, states directly determine components rendering and behavior. 

 

There is an underlying difference between Props and States. Props acquire initial value from parent components, while states do not. Meanwhile, the parent component cannot change its value in States, but it can in Props. The same goes for the child component as well. Both States and Props set the default insight component. The insight component is unchangeable in Props, while it can undergo mutation in States.

 

Describe the React js lifecycle

React defines components as classes or functions. Components defined as classes provide more features. You need to extend React.Component, to define a component as class. Render is the only method that must be defined in a React.Component subclass. All the other methods are optional. Have a look!

 

class App extends React.Component {

render() {

return (

<div className="App">

<h2>Hellow World!</h2>

</div>

);

}

}

Each component has several “Lifecycle Methods” that can override to run codes throughout the entire process. 

 

Lifecycle methods are divided into four classes: 

Mounting

Updating

Unmounting

Error handling

 

Mounting includes constructor(). Constructors are used for two main reasons:

Starting a local state by assigning an object to this.state.

Binding event handler methods to an instance.

 

static getDerivedStateFromProps() is called right before the render method. This method exists in rare usage cases when the state depends on whether props will change over time. This method is fired on every render, in contrast to UNSAFE_componentWillReceiveProps, which only fires when the parent causes a re-render and not as a result of a local setState.

 

render()

The render method is the only required method in a class component. When it gets called, it examines this.prop and this.state and returns one of the following types. The render function should not change the component state. It should return the same function every time it is invoked.  

The render function does not interact with the browser, but if you need it to interact, simply perform your work in componentDidMount() or the other lifecycle methods instead.

 

componentDidMount()

ComponentDidMount() is supplicated after a component is mounted. This method is a good place to set up any subscriptions. However, in that case, you should unsubscribe in componentWillUnmount(). SetState() can immediately be called in componentDidMount(), which will trigger another rendering. However, it will happen before the screen is updated by the browser. Therefore, the users won't see that the render has been called twice. 

 

shouldComponentUpdate()

ShouldComponentUpdate() is used to let React know if the state or props current change has affected the component’s return or not. The default behavior must re-render on every state change.

When shouldComponentUpdate() returns false, componentDidUpdate(), UNSAFE_componentWillUpdate, render() are not called. However, React may later treat shouldComponentUpdate() as a hint rather than a directive and returning false may still result in a re-rendering of the component.

 

shouldComponentUpdate()

ShouldComponentUpdate() is used to let React know if a component’s return is not affected by the state or props current change. The default behavior must re-render on every state change. 

When shouldComponentUpdate() returns false, componentDidUpdate(), UNSAFE_componentWillUpdate, render() are not called. However, React may later treat shouldComponentUpdate() as a hint rather than a directive and returning false may still result in a re-rendering of the component.

 

getSnapshotBeforeUpdate()

getSnapshotBeforeUpdate() enables your component to acquire some information from the DOM before it changes, passing any value returned by this lifecycle as a parameter to componentDidUpdate().

componentDidUpdate()

componentDidUpdate() is called immediately after updating occurs. This method is not called for the initial render.  ComponentDidUpdate() can be used for operating the DOM when the component has been updated. You can also do network requests as long as you compare the current props to previous props. 

Unmounting

This method is invoked when a component is being removed from the DOM:

componentWillUnmount()

componentWillUnmount() is called immediately before a component is unmounted and destroyed. 

Emergency cleanups such as invalidating timers, canceling network requests, cleaning componentDidMount() subscriptions should be performed straight away. 

SetState() should not be called in componentWillUnmount() because the component will never be re-rendered. Once a component instance is unmounted, it will never be mounted again.

Error Handling

Error Handling Methods are invoked when there is an error during the lifecycle methods, in the construction of any child component, or during rendering. 

static getDerivedStateFromError()

This lifecycle is called when a descendant component throws an error. It receives the error that was thrown as a parameter and should return a value to update state.

GetDerivedStateFromError() is called during the “render” phase, so side-effects are not permitted. To have side-effects permitted use componentDidCatch(). 

With the componentDidCatch(), the lifecycle is supplicated after a descendant component throws an error. It receives two parameters:

error- an error appears

info - An object with a componentStack key containing information about which component threw the error.

ComponentDidCatch() is called during the “commit” phase, so side-effects are permitted. It should be used for things like logging errors.

All in all, you will have to focus on both basic interview questions and more technical ones such as React component questions, React Redux, and Router Interview questions to pass the stage. Don’t stress much, take your time, and go for it. 

Good luck!!