ReactJs Interview Questions – A MUST Have

By | December 5, 2016
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ReactJs is undoubtedly an amazing open source JavaScript front end UI library for creating interactive, stateful & reusable UI components for web as well as mobile applications. In this ReactJs Interview Questions and Answers Tutorial, we have tried to cover almost all main topics related to React and further given detailed answers to all major topics.

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You can easily  compare features of ReactJs with other related technologies in one more detailed article here.

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ReactJs Interview Questions List

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Q1: What is ReactJS? What are the advantages of using ReactJS and how it’s different from other JavaScript Frameworks? What are its limitations?

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.

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.

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.

ReactJs Operations

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Q2: Please explain step by step approach on how to setup environment for ReactJS?

Next we will discuss how to set up environment for React.js successful development.

For this we will be needing NodeJS so if not installed, below its mentioned how to install nodejs?

  • Step 1: Download and Install NodeJs
    To Verify installation: Create a js file named Test.js on your machine (Either on Windows or Linux) and try the below mentioned code.
    Execute/run test.js file using Node.js interpreter to see the below result:
  • Step 2: Installing Global Packages
    We need to install several packages for setting up of environment.We will be needing some of the babel plugins.
  • Step 3: Create Root Folder
  • Step 4: Add Dependencies and Plugins
    Since we want to use React, we need to install it first. The –save command will add these packages to package.json file.
    We already mentioned that we will need some babel plugins so let’s install it too.
  • Step 5: Create App files for ReactJs
  • Step 6: Running the Server
    ReactJs Environment Setup

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Q3: What is ReactJS-JSX? What are the advantages of using JSX? Explain basic code snippet of JSX with the help of a practical example?

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/>.

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.

Our 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.

ReactJS JSX
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Q4: What are Components in ReactJS? Explain the Component Lifecycle with the help of a diagram explaining each component method in detail. Also, Give an example of both Stateless and Stateful components with source code?

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.

Component Lifecycle Methods

Component Lifecycle Methods

  • getDefaultProps and getInitialState, both methods are called only once while initially rendering of the component.
  • componentWillMount : This method is executed just before rendering on both client and server side.
  • ComponentDidMount: This method is executed after first rendering only on the client side. This is where AJAX requests and DOM or state updates usually occurs. This method is also used for integration with other JavaScript frameworks.
  • ComponentWillReceiveProps: This method is invoked as soon as the props are updated before another render is called.
  • ShouldComponentUpdate: This method  should return true or false value. This  determines whether if a component will be updated or not. Default it is set to true.
  • ComponentWillUpdate: This method  is called just before rendering.
  • ComponentDidUpdate: This method is called just after rendering.
  • ComponentWillUnmount: This method is called after the component is unmounted from the DOM.

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.

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.

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Q5: What are State and Props in ReactJS? What is the difference between the two? Give a proper example of using State and Props to elaborate the concept with complete source code?

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.

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).

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

Props State
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. 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.
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Q6: How to apply validation on Props in ReactJS? Apply validation in previously developed example in above question?

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
Execute the above example, when wrong prop type is being detected it generate error message in console, try using React.PropTypes.string in the code above and we will  see that our page still loads fine, but an error message  in console.Validation in ReactJs
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Q7: How to use Forms in ReactJS? Give an example of using Forms in ReactJS by developing a User Registration Form?

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.
ReactJs Basic Form
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Q8: How to use Events in ReactJS? Give an example of using events?

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.
Events in ReactJs
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Q9: How to set up routing in ReactJS. Explain with the help of step by step approach?

Routing is the  key features of web applications (and even other platforms) could not be left out in React. We can develop full featured single page applications (SPA) with React, one of the key feature is routing.

We use library called React-Router for routing in reactJS. React goodness is  to keep things as simple as possible and that is why the core library exactly does what React is about, components. Routing, DOM rendering and other logics are abstracted to a different library.

We need to install library by command to initialization router

After Installing library we need to write following below code to verify routing is working in desired manner.

ReactJs Routing
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Q10: What is Flux concept in ReactJS? Explain various flux elements including Action, Dispatcher, Store and View. Give a simple practical example of installing and using Flux in an application?

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.

Flux can be better explained by defining its individual components:

  • Actions – They are helper methods that facilitate passing data to the Dispatcher.
  • Dispatcher – It is Central hub of app, it receives actions and broadcasts payloads to registered callbacks.
  • Stores – It is said to be Containers for application state & logic that have callbacks registered to the dispatcher. Every store maintains particular state and it will update  when it is needed. It wakes up on a relevant dispatch to retrieve the requested data. It is accomplished by registering with the dispatcher  when constructed. They are  similar to  model in a traditional MVC (Model View Controller), but they manage the state of many objects —  it does not represent a single record of data like ORM models do.
  • Controller Views – React Components  grabs the state from Stores and pass it down through props to child components to view to render application.

In this Web Development Tutorial, first we have to install all the steps mentioned in Question 2 i.e. step by step installation of react.

then, we need to install flux:

Flux Overview – How it works?

Flux encourage the design in  of a uni-directional data flow.

All user interactions within a view call an action creator, which causes an action event to be emitted from a singleton dispatcher.

The dispatcher is a single-point-of-emission for all actions in a flux application. The action is sent from the dispatcher to stores, which update themselves in response to the action.

The flow doesn’t change significantly for additional stores or views. The dispatcher simply sends every action to all the stores in the application.

Each store is responsible for a domain of the application, and only update themselves in response to actions.

When a store updates, it emits a change event. In many React applications, special views (known sometimes as “controller-views”) are responsible for watching for this change event, reading the stores’ new data, and passing that data through properties to child views.

It completely avoids performance issues , complex bugs that can rise while trying to watch for specific property changes on models and  views only slightly.

Flux Architecture
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