ReactJS Interview Questions You Should Know March 15, 2019

ReactJS Interview Questions You Should Know

Are you expecting an interview soon?

If you’re a React.js developer, you’ll want to review the following questions you might be asked.

React.js is an open source javascript UI (User Interface) library that is essentially a community of developers. Maintained by Facebook and Instagram, it’s designed to evaluate your application state and allow you to write code for status input and return virtual DOM.

Since interactive UI is a necessity for most brands, standing out among the developer community to be hired can translate to significant benefits. The following React.js interview questions will ensure you’re thoroughly prepared for even the most complex questions.

1. What is ReactJS?

React.js is a javascript-based UI Library developed at Facebook to be interactive and create reusable UI components. It is among the more popular javascript frameworks that are meant to handle the presentation aspect of both web and mobile apps.

 

2. What are some advantages of ReactJS?

Advantages:

  • React.js is exceptionally efficient; it creates its own virtual DOM in which your components live. This technique allows you to have both flexibility and accessible increases in performance. In addition to this, React.js calculates the changes that need to be made in DOM, which surpasses the need to pay for expensive DOM operations.
  • React.js makes writing Javascript far easier. Through the use of a special syntax called JSX, HTML can be mixed with Javascript, and users are able to quickly drop a bit of HTML in the render function without having to concatenate strings.
  • React.js is incredible for SEO; you can run React.js on the server, and the virtual DOM will automatically be rendered to the browser as a regular web page. This gives React.js a significant advantage over the competition; most other Javascript frameworks are not search-engine friendly and generally have trouble with Javascript-heavy applications.

3. What are some disadvantages?

Disadvantages:

  • React handles the UI only because it’s a library, not a framework. Other parts of the application require the use of other assorted libraries.
  • There is no specific template for structuring the application, so the user would have to figure it out on their own.
  • The flexibility of ReactJS could easily permit poor choices by the developer, especially a developer new to React.

 

4. What are the components in ReactJS?

React Components allow you to split the UI into isolated, reusable pieces. Conceptually, components are comparable to JavaScript functions–they accept arbitrary inputs and return React elements that describe what should appear onscreen.

Included below are components written in ES6 class to display a welcome message.

 

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

const element = ;                                                                             
ReactDOM.render(                                                                              
element,                                                                                      
document.getElementById('root')                                                               
);                                                                                            

 

5. What is JSX?

JSX is similar to XML/HTML in syntax, and it’s employed by React which extends ECMAScript to use XML/HTML-like text with the JavaScript/React code. This syntax is intended for preprocessors to use and transform the HTML-like text in JavaScript files into standard JavaScript objects for the engine to interpret.

Essentially, JSX enables the user to write succinct HTML/XML-like structures in the same file as the JavaScript code. Then Babel transforms these expressions into actual JavaScript code.

 

6. Tell me about the life cycle of ReactJS components.

Each component adheres to several “lifecycle methods” that you can override to run code at specific times in the process. Prefixed will methods are called right before something happens, and prefixed with methods are called right after something happens.

Mounting

The following methods are called in an instance where a component is being created and inserted into the DOM.

  • Constructor()
  • componentWillMount()
  • Render()
  • componentDidMount()

Updating

An update could be prompted by changes to props or state, and these methods are called when a component is being –

Rendered
  • componentWillReceiveProps()
  • shouldComponentUpdate()
  • componentWillUpdate()
  • render()
  • componentDidUpdate()

Unmounting

This method is called when a component needs to be removed from the DOM:

  • componentWillUnmount()

Other APIs

In addition, each component provides some other APIs.

  • setState()
  • forceUpdate()

Class Properties

  • defaultProps
  • displayName

Instance Properties

  • props
  • state

 

7. What are some features of ReactJS?

React is notably one of the most popular Javascript UI frameworks, so features are not difficult to come by. Listed below is just a handful of the many features React has to help programmers create visually pleasing applications:

  • Adaptability
  • Flux Library
  • The usefulness of JSX
  • Destructuring assignments
  • Server-side communication

 

8. How do you handle events in ReactJS?

React JS events are similar, but only differ in the way they are written, or the syntax.  For instance, the name of the event is React.js and it’s always in camelCase. As for JSX, you pass a function as the event handler, rather than a string.

 

9. Define flux in JavaScript, and how is it different from redux?

Flux is a new kind of architecture that isn’t a framework or a library, but rather a pattern. It’s an application architecture made for creating data layers. Designed at Facebook, it uses unidirectional data flow in order to complement React’s composable view components. Flux contains application state and logic while Redux acts as a container for JavaScript apps.

 

10. What are refs in React? When should you use it?

Refs provide a way to access DOM nodes or React elements created in the render component. They’re used to store the reference of element or component returned by the component render() configuration function, but Refs are typically avoided. However, the official React documentation gives three occasions in which its use is acceptable:

  • Managing focus, text selection, or media playback
  • Integrating with third-party DOM libraries
  • Triggering imperative animations

 

11. What are stateless components?

In React, stateless components are components that have their internal state calculates, but never actually mutates it. Stateless components can be created by using plain functions or Es6 arrow function. See the example below for a stateless component in react:

 

12. Explain the difference between State and props.

React controls the data flow in the components using state and props. Short for properties, the component’s props are used to customize the component when it’s being created. State looks very similar to props and even works comparably. However, the difference in its functionality is that it can be changed over time in response to certain events, while props are to be considered immutable.

State is used to create interactions that bring components to life, and distinguish how the component renders.

 

13. Define Synthetic events.

SyntheticEvent is React’s own event system that provides cross-browser support. Synthetic events work like the event system of browsers, but the same code will work with all browsers.

 

14. Describe the difference between DOM and Virtual DOM.

Document Object Model, referred to as DOM or HTML DOM, is a programming interface for valid HTML and well-formed XML documents. Nodes within DOM are commonly attributed to elements of HTML. DOM is a standard for which HTML elements can be changed, added, deleted or retrieved.

Virtual DOM is best described as the abstraction of HTML DOM itself. The virtual DOM  is considered a lightweight copy with very little difference to the original. It was not invented by React but is freely distributed for use.

 

15. What are controlled and uncontrolled components?

Controlled components are easier to insert forms in, as form data is handled by React components. In HTML, form elements like input, text area, and select maintain their own state, and they’re updated based on what the user submits.

Uncontrolled components, on the other hand, form data is handled by the DOM rather than the actual components.

 

16. What’s the difference between functional and class components?

One of the most obvious differences between functional and class is the syntax. Functional components are simply JavaScript functions that accept props as an argument and returns the React element. Essentially, these components allow for easy use of JavaScript functions.

Class components differ because they allow additional features like local state, variables, and lifecycle hooks. Often considered stateless, Functional components are used simply for presenting static data.

 

17. What is a higher order component?

A higher order component (HOC) is a function that reuses the component logic. When utilized, they loop over and apply a function to the elements in that array. While HOCs are not a part of the React API, they do originate from React’s compositional nature, and they allow you to reuse your code rather than rewriting it in multiple places that do the same thing.

 

18. How is React different from AngularJS?

While they both are very powerful approaches to front-end web development, but they differ in origin and benefit. One main difference is that Angular is a complete framework for JavaScript, while React is considered a library. Angular uses a bi-directional data flow in contrast to React’s uni-directional flow, and the framework updates the original DOM instead of only the virtual DOM that React does.

 

19. Define Arrow functions.

Arrow functions are primarily used to prevent bugs in React operations. They make it easier to predict the behavior of any bugs when passed as callbacks, but they don’t redefine the value within the function body.

 

20.  What is the purpose of render() function?

The render() function is used to update the user interface. React elements’ attributes cannot be changed once they are created. So, ReactDOM.render()  controls the container node. If there is any DOM element already there, it would be replaced first when called.