Top 23 Reactjs Interview Questions You Must Prepare 29.Nov.2023

In React’s virtual DOM, HTML Input element presents an interesting problem. With the others DOM environment, we can  render the input or textarea and thus allows the browser maintain its   state that is (its value). we can then get and set the value implicitly with the DOM API.

In HTML, form elements such as <input>, <textarea>, and <select> itself  maintain their own state and update its state  based on the input provided by user .In React, components’ mutable state is handled by the state property  and is only updated by setState().

  • HTML <input> and <textarea> components use the value attribute.
  • HTML <input> checkbox and radio components, checked attribute is used.
  • <option> (within <select>) components, selected attribute is used for select box.

Props:

Passes in from parent component.<PropsApp headerProperty = “Header from props…” contentProperty = “Content&nbsp;from props…”/>This properties are being read by  PropsApp component and sent to ReactDOM View.

State:

Created inside component by getInitialState.this.state reads the property of component and update its value it by this.setState() method and then returns to ReactDOM view.State is private within the component.

JSX is completely optional and its not mandatory, we don’t need to use it in order to use React, but it has several advantages  and a lot of nice features in JSX.

  • JSX is always faster as it performs optimization while compiling code to vanilla JavaScript.
  • JSX is also type-safe, means it is strictly typed  and most of the errors can be caught during compilation of the JSX code to JavaScript.
  • JSX always makes it easier and faster to write templates if we are familiar with HTML syntax.

No, It uses JSX which is simiar to HTM.

Limitations of ReactJS:

  • React is only for view layer of the app so we still need the help of other technologies to get a complete tooling set for development.
  • React is using inline templating and JSX. This can seem awkward to some developers.
  • The library of react  is too  large.
  • Learning curve  for ReactJS may be steep.

  • React uses virtual DOM which is JavaScript object. This will improve apps performance 
  • It can be used on client and server side
  • Component and Data patterns improve readability.
  • Can be used with other framework also.

Stateless and Stateful components

Stateless: When a component is “stateless”, it calculates state is calculated internally but it directly  never mutates it. With the same inputs, it will always produce the same output. It means it has no knowledge of the past, current or future state changes.

var React = require('react');
var Header = React.createClass({
    render: function() {
        return( <img src={this.props.imageSource} />   ); }
});
ReactDOM.render(<Header imageSource="myImage.png"/>, document.body);

Stateful : When a component is “stateful”, it is a central point that stores every information in memory about the app/component’s state, do has the ability to change it. It has knowledge of past, current and potential future state changes. Stateful component  change the state, using this.setState method.

var React = require('react');
var Header = React.createClass({
    getInitialState: function() {
        return { imageSource: "header.png" }; 
  },
    changeImage: function() {
        this.setState({imageSource: "changeheader.png"});
    },
    render: function() {
        return(
            <img src={this.state.imageSource} onClick={this.changeImage.bind(this)} />
        );
    }
});
module.exports = Header;

React is used to handle the view part of Mobile application and Web application.

  • Initialization
  • State/Property Updates
  • Destruction

  • JSX: JSX is JavaScript syntax extension.
  • Components : React is all about components.
  • One direction flow: React implements one way data flow which makes it easy to reason about your app

Flux is the architecture of an application that Facebook uses for developing client-side web applications. Facebook uses internally when working with React. It is not a framework or a library. This is simply a new technique that complements React and the concept of Unidirectional Data Flow.

Facebook dispatcher library is a sort of global pub/sub handler technique which broadcasts payloads to registered callbacks.

  • Version: 15.5 
  • Release on: April 7, 2017

import React from 'react';
class App extends React.Component {
   render() {
      return (
         <div>
            <Header/>
            <Content/>
         </div>
      );

   }
}

class Header extends React.Component {
   render() {
      return (
         <div>
            <h1>Header</h1>
         </div>
      );

Advantages of ReactJS:

  • React uses virtual DOM which is JavaScript object. This improves application performance as JavaScript virtual DOM is faster than the regular DOM.
  • React can be used on client and as well as server side too.
  • Using React increases readability and makes maintainability easier. Component, Data patterns improves readability and thus makes it easier for manitaing larger apps.
  • React can be used with any other framework (Backbone.js, Angular.js) as it is only a view layer.
  • React’s JSX makes it easier to read the code of our component. It’s really very easy to see the layout. How components are interacting, plugged and combined with each other in app.

React is an open source JavaScript front end UI library developed by Facebook  for creating interactive, stateful & reusable UI components for web and mobile app. It is used by Facebook, Instagram and many more web apps.

ReactJS is used for handling view layer for web and mobile applications. One of React’s unique major points is that  it perform not only on the client side, but also can be rendered on server side, and they can work together inter-operably.

JSX (JavaScript XML), lets us to build DOM nodes with HTML-like syntax. JSX is a preprocessor step which adds XML syntax to JavaScript.

Like XML, JSX tags have a tag name, attributes, and children JSX also has the same. If an attribute/property value is enclosed in quotes(“”), the value is said to be string. Otherwise, wrap the value in braces and the value is the enclosed JavaScript expression. We can represent JSX as <HelloWorld/>.

Your browsers does not understand JSX code natively, we need to convert it to JavaScript first which can be understand by our browsers. We have aplugin which handles including Babel 5’s in-browser ES6 and JSX transformer called browser.js.

Babel will understand and recognize JSX code in <script type=”text/babel”></script> tags and transform/convert it to normal JavaScript code.

In case of production we will need to pre-compile our JSX code into JS before deploying to production environment so that our app renders faster.

<!DOCTYPE html>
<html lang="en">
  <head><title>My First React JSX Example</title></head>
  <body>
    <div id="hello-world"></div>
    <script src="https://fb.me/react-15.0.0.js"></script>
    <script src="https://fb.me/react-dom-15.0.0.js"></script>
   <script src="https://cdnjs.cloudflare.com/ajax/libs/babel-core/5.8.34/browser.min.js"></script>
     <script type="text/babel">
      var HelloWorld = React.createClass({
        render: function() {
          return ( <p>Hello, World</p> )
        }
      });
      ReactDOM.render( <HelloWorld/>, document.getElementById('hello-world')); 
    </script>
  </body>
</html>

State is the place where the data comes from. We must follow approach  to make our state as simple as possible and minimize number of stateful components.

For example, ten components that need data from the state, we should create one container component that will keep the state for all of them.

The state starts with a default value and when a Component mounts and then suffers from mutations in time (basically generated from user events).

A Component manages its own state internally, but—besides setting an initial state—has no business fiddling with the stateof its children. You could say the state is private.

import React from 'react';
import ReactDOM from 'react-dom';
 var StepCounter = React.createClass({
 getInitialState: function() {
         return {counter: this.props.initialCount};
},
  handleClick: function() {
  this.setState({counter: this.state. counter + 1});
 },
  render: function() {
  return <div onClick={this.handleClick}>{this.state.counter }</div>;
 }
});
ReactDOM.render(< StepCounter initialCount={7}/>, document.getElementById('content'));

Props: They are immutable, this is why container component should define state that can be updated and changed. It is used to pass data down from our view-controller(our top level component).

 When we need immutable data in our component we can just add props to reactDOM.render() function.

import React from 'react';
import ReactDOM from 'react-dom';
class PropsApp extends React.Component {

   render() {
      return (
         <div>
            <h1>{this.props.headerProperty}</h1>
            <h2>{this.props.contentProperty}</h2>
         </div>
      );
   }
 } 
ReactDOM.render(<PropsApp headerProperty = "Header from props..." contentProperty = "Content
   from props..."/>, document.getElementById('app'));

}

import React from 'react';
import ReactDOM from 'react-dom';
 var StepCounter = React.createClass({
                    getInitialState: function() { return {counter: this.props.initialCounter }; },
                    handleClick: function() {      
                    this.setState({counter: this.state.counter + 1});  },
                    render: function() {
                    return <div onClick={this.handleClick}> OnClick Event, Click Here: {this.state.counter }</div>;
            }
});
 ReactDOM.render(< StepCounter initialCounter={7}/>, document.getElementById('content'));

When the application is running in development mode, React will automatically check  for all props that we set on components to make sure they must right correct and right data type.

For instance, if we say a component has a Message prop which is a string and is required, React will automatically check and warn  if it gets invalid string or number or boolean objects. For performance reasons this check is only done on dev environments  and on production it is disabled so that rendering of objects is done in fast manner .

Warning messages are generated   easily  using a set of predefined options such as:

  • React.PropTypes.string
  • React.PropTypes.number
  • React.PropTypes.func
  • React.PropTypes.node
  • React.PropTypes.bool

React encourages the idea of reusable components. They are widgets or other parts of a layout (a form, a button, or anything that can be marked up using HTML) that you can reuse multiple times in your web application.

ReactJS enables us to create components by invoking the React.createClass() method  features a render() method which is responsible for displaying the HTML code.

When designing interfaces, we have to break down the individual design elements (buttons, form fields, layout components, etc.) into reusable components with well-defined interfaces. That way, the next time we need to build some UI, we can write much less code. This means faster development time, fewer bugs, and fewer bytes down the wire.

React identifies every events so that it must  have common and consistent behavior  across all the browsers. Normally, in normal JavaScript or other frameworks, the onchange event is triggered after we have typed something into a Textfield and then “exited out of it”. In  ReactJS we cannot do it in this way.

The explanation is typical and  non-trivial:

*”<input type=”text” value=”dataValue”> renders an input textbox initialized with the value, “dataValue”.

When the user changes the input in text field, the node’s value property will update and change. However, node.getAttribute(‘value’) will still return the value used at initialization time that is dataValue.

Form Events:

  • onChange: onChange event  watches input changes and update state accordingly.
  • onInput: It is triggered on input data
  • onSubmit: It is triggered on submit button.

Mouse Events:

  • onClick: OnClick of any components event is triggered on.
  • onDoubleClick: onDoubleClick of any components event is triggered on.
  • onMouseMove: onMouseMove of any components, panel event is triggered on.
  • onMouseOver: onMouseOver of any components, panel, divs event is triggered on.

Touch Events:

  • onTouchCancel: This event is for canceling an events.
  • onTouchEnd: Time Duration attached to touch of a screen.
  • onTouchMove: Move during touch device .
  • onTouchStart: On touching a device event is generated.