React JS Interview Questions and Answers
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React JS Interview Questions and Answers

React JS Interview Questions and Answers

In this article, we will look at the various React js interview questions and answers that will help you in getting a job as a React Front End Developer.

The most stressful part of qualifying for a job as a React js Front End Developer is getting started. You have to be assertive, and having the best answers is critical. Everyone can attest to the fact that having precise answers is vital to be chosen as the best candidate.

Interviewers will ask questions expecting correct responses. You have to understand React JS and understand the ways to use JavaScript. You cannot fumble around after getting posed with a question. Let’s dive right into it.

All the questions are based on real React JS Interviews

Getting insight into the React JS is vital because it will boost your confidence. Some of the questions asked during an interview include the definitions and essential items like differentiating between a DOM and the virtual one.

Getting confidence requires understanding the details so that you stand out among the interviewer. The following are some of the questions and the correct answers that conspire during an interview:

What is React?

React is a JavaScript library for Front End Development. It is made by Facebook in May 2013 and it’s in demand these days because of its popularity. React makes development easy and loads the view faster because of its outstanding features like Virtual DOM and one-way data flow.

The popularity of this front end library has surpassed other libraries and frameworks in the market. It uses XML syntax which is a JSX and can be retrieved through the props.

What are the advantages of using React?

Using react is excellent because it does not get outdated by other tools that are invented in the market. This makes it widely famous and competitive to be managed by Facebook and the developers. This gives the reason why Netflix, Apple, and PayPal use it.

  • Makes front end development easy and fast as it is capable to create reusable pieces of user interfaces.
  • React can develop fast loading web applications because of its outstanding approach “The Virtual DOM”.
  • React has a debugging tool (React Developer Tools) that makes the development process easy.
  • It uses an Oneway data flow that also makes it capable to load view faster.
  • React has an SEO friendly approach that makes a web application or website faster by reducing load time.

Reactjs increases the speed of most applications, making it easy to have high performance.

What are the disadvantages of using React?

Remember that everything that has a disadvantage will always have some drawbacks. What makes the tool exceptional is the ability to produce an excellent service despite the drawbacks that it possesses.

The following are some of the disadvantages that have to be highlighted so that you can be realistic when the interviewer poses this question. Don’t fumble around with the answer simply because you don’t want to talk about the negatives. Here are some downsides:

  • React is a library with poor documentation which causes an issue for newcomers.
  • Continuous updates and releases are very irritating.

What is JSX?

When you are asked this question, you should have a background that it’s a sub of JavaScript XML. It is incorporated in React to be used by JavaScript and HTML syntax. You can comprehend respond well if you understand HTML. JSX is just an extension of JavaScript but not a usual one.

Must Read – You must read this article for a better understanding of JSX – What is JSX in React JS?

What is Virtual DOM in React?

Virtual DOM is the representation of the real DOM. In other words, we can say it’s a lightweight copy of the actual DOM. React uses virtual DOM to differentiate and update the real DOM.

Interviewer may want to ask more questions on Virtual DOM.

How does Virtual DOM work in React?

React uses a reconciliation process, where it first creates a Virtual DOM and syncs it with the real DOM using the ReactDOM library.

This approach lets us set the state of the UI component. Whenever that state changes it runs the diffing algorithm to recognize the changes in virtual DOM. And then it updates the real DOM immediately.

Here is an illustration of the process to understand how does virtual dom work in React.

How does Virtual DOM work in React?

What is the difference between Functional Component and Class Component in React?

In React there are two ways to create a component. The first is the functional component and the second is the class component. The following are the difference between them:

Functional Component

  • Functional components are stateless components. But if we still want to use state, then after React 16.8 Hooks update, of course, the useState hook can do that.
  • A functional component is a basic JavaScript function.
  • We can also create a functional component using JavaScript’s arrow function that was introduced in ES6.
  • The syntax is different to create a functional component if compare with the class component.
  • They can accept data in props and can be used for rendering React elements.
  • Lifecycle methods of React can’t be used in functional components. But after React 16.8 Hooks update the useEffect hook makes it possible.

Class Component

  • Class components are stateful components.
  • They can be created using ES6 class to extend the Component class in of React.
  • We can use Lifecycle methods of React in a class component.
  • In a class component, a render function is required to return a React element.
  • A class component can be used to handle the data that might change.

It is recommended to use functional components. Because the code becomes easier to read and understand. They don’t have any state or lifecycle methods, so they are easy to debug.

When should you use a Functional Component and when a Class Component?

Keeping in mind that if we do not need to use State in a component then we should use a functional component, which is also called Stateless Components.

In case we are going to create a Stateful Component. Then we should consider Class Component. It completely depends on the requirement.

What is the difference between Element and Component?

Element

A React element is just a plain object that describes a component instance or DOM nodes and properties like key, props, ref, and type. It contains information about the component like the component type, it’s properties like color and it’s child elements.

A React element is a way to tell React, what we want to display on the screen. And of course, they are immutable so we can’t call any methods on it.

Here is an example of how does a React element look like:

const element = React.createElement(
  'div',
  {id: 'myBtn'},
  'Login'
)

This above example will return below given object:

{
  type: 'div',
  props: {
    children: 'Login',
    id: 'myBtn'
  }
}

It will render the given HTML:

<div id='myBtn'>Login</div>

Component

A component refers to a piece, part, or section. It can be a class or a function. It’s a reusable piece of code that can return React elements to be rendered on the view.

A component can return other components also and to create a component we have to make sure that the name of a component should start with a capital letter. As already discussed there are two types of components, functional components and class components.

A functional component looks like this:

function Welcome(props) {
  return <h1>Hello, {props.name}</h1>;
}

A class component looks like this:

class Welcome extends React.Component {
  render() {
    return <h1>Hello, {this.props.name}</h1>;
  }
}

How do we update a rendered element in React?

We can’t update any rendered element because in React all the elements are immutable. Once we create an element, we can’t change its attributes. The only way to update a rendered element is to create a new element and render it.

Here is an example:

function tick() {
  const element = (
    <div>
      <h1>Hello, world!</h1>
      <h2>It is {new Date().toLocaleTimeString()}.</h2>
    </div>
  );
  ReactDOM.render(element, document.getElementById('root'));}

setInterval(tick, 1000);

In the above code setInterval() callback calls tick() function every second and as you can see there is ReactDOM.render() in  the tick() function that is rendering our element.

What are props in React?

In React, “props” stands for properties. They are like arguments passed in JavaScript functions, that can be passed into React components in a uni-directional flow. Props are read-only, so we can’t change them as per React single strict rule.

Props can be passed to a component by declaring attributes and value. Here is an example for you:

<MyComponent userName = "Dheeraj Verma" />

In the above example “MyComponent” is the component and “userName” is the prop that we are passing to our component “MyComponent”. And as you can understand the value of our prop is “Dheeraj Verma”. Now to access this prop we can write “this.props.userName;”.

What are Hooks in React?

React Hooks are the new feature introduced in React 16.8 version update. This new feature let us use state and other features of React from a functional component and of course without writing a class component.

Hooks can also help you to extract any stateful logic from a component so that it can be reused and Hooks can do that without changing your component hierarchy.

Here is an example of using Hooks:

import React, { useState } from 'react';

function Example() {
  // Declare a new state variable, which we'll call "count"  const [count, setCount] = useState(0);
  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

We can also create custom Hooks by creating a normal javascript function that starts with the word “use”. It can contain our stateful logic and can be reused with other components. As Hooks have some rules so React provides the linter plugin to enforce these rules automatically and find bugs in the use of these Hooks.

What is the use of State in React?

State is a built-it object in React where we can store some information that belongs to a component. It determines how will that component render and behave.

In other words, we can say that state is one of the best features in React that allows us to create dynamic and interactive components. When a value of the state changes, the component re-renders immediately.

Here is an example:

Class MyClass extends React.Component 
{ 
    constructor(props) 
    { 
        super(props); 
        this.state = { brand : "maruti" }; 
    } 
}

In the above example, we have set the initial state of the brand property. Now we can change the state using the given example.

this.setState({brand: "Audi"});

What is Strict Mode in React?

According to React documentation, StrictMode is a very good tool introduced in version 16.3 that helps us in identifying potential problems in an application. StrictMode checks work in development mode only. It will display all the errors and warnings in the console.

It will do the following checkes:

  • Identifying components with unsafe lifecycles
  • Warning about legacy string ref API usage
  • Warning about deprecated findDOMNode usage
  • Detecting unexpected side effects
  • Detecting legacy context API

Here is an example of using StrictMode in React:

ReactDOM.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>,
  document.getElementById('root'),
)

We can put any component inside “<React.StrictMode>” to enable it and let it work for the component inside it.

What is the use of React Router in React?

React Router is used to conditionally render certain components to display the UI depending on the route used in the URL in a single page application. React Router uses a component-based technique for routing.

Here is an example:

import React from 'react';
import { Home } from './view/Home';
import { Contact } from './view/Contact';
import { NavBar } from './components/NavBar';
import { Route, Switch, Redirect } from 'react-router-dom';

export const Routes = () => {
  return (
    <div>
      <NavBar />
      <Switch>
        <Route exact path="/Home" component={Home} />
        <Route exact path="/">
          <Redirect to="/Home" />
        </Route>
        <Route exact path="/Contact" component={Contact} />
      </Switch>
    </div>
  );
};

Must Read – You must read this article for a better understanding of using React Router – Routing in React JS using react-router-dom

What is the difference controlled and uncontrolled components in React?

Controlled Component

A controlled component is where a form element’s value is handled by a React component. With a controlled component first we declare the initial value of the element using state within the class constructor.

Then we use event handlers, for example, we can add onChange attribute to our element that allows us to pass in a function that will change the value of the element whenever a new value is typed using the setState() method in that function.

Here is an example:

class ApplicationForm extends React.Component {
  constructor(props) {
    super(props);
    this.state = {userName: ''};
    this.handleChange = this.handleChange.bind(this);
  }

  handleChange(event) {    this.setState({userName: event.target.value});  }

  render() {
    return (
      <form>
        <label>
          Name:
          <input type="text" value={this.state. userName } onChange={this.handleChange} />        </label>
        </form>
    );
  }
}

Uncontrolled Component

An uncontrolled component is where the form element’s value is handled by the DOM itself. We can create an uncontrolled component using ref to get the form element’s value directly from the DOM.

Uncontrolled components can be used when you have to validate a form without updating the input value. Otherwise, React recommends controlled components in most of the cases.

For example:

class ApplicationForm extends React.Component {
  constructor(props) {
    super(props);
    this.handleSubmit = this.handleSubmit.bind(this);
    this.input = React.createRef();  }

  handleSubmit(event) {
    alert('A name was submitted: ' + this.input.current.value);    event.preventDefault();
  }

  render() {
    return (
      <form onSubmit={this.handleSubmit}>
        <label>
          Name:
          <input type="text" ref={this.input} />        </label>
        <input type="submit" value="Submit" />
      </form>
    );
  }
}

Don’t forget to prepare yourself for JavaScript Interview Questions. Because React is incomplete without JavaScript. So the interviewer can ask you questions about JavaScript also.

Must Read – You must read this article to prepare yourself for the questions related to JavaScript – Top 85 JavaScript Interview Questions & Answers

Conclusion

Understanding the questions about React JS is essential so that you qualify for the job. You should comprehend the process of debugging and the formula that makes the JavaScript easy. This article has expounded on some of the questions and answers that make an interview successful. You have to understand and have a reason why React is your choice. Don’t forget to leave a comment after your successful interview and share your experience with me.

Author

I am a Full Stack Web Developer. I have more than 8 years of experience in the web design and development industry.

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