React js interview questions and Answers

One should never update the state directly because of the following reasons:

• If you update it directly, calling the setState() afterward may just replace the update you made.
• When you directly update the state, it does not change this.state immediately. Instead, it creates a pending state transition, and accessing it after calling this method will only return the present value.
• You will lose control of the state across all components.

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To call a function inside a render in React, use the following code:

Props stand for Properties in React. They are read-only components that must be kept pure. These components are passed down from the parent to the child component throughout the app. A child component cannot send the prop back to the parent component. This can help in maintaining the unidirectional data flow and is used to render the dynamically generated data.

The latest release of React is 16.6.9 and here are its newest features:

• Measuring performance using
• Improved Asynchronous act() for testing
• No more crashing when calling findDOMNode() inside a tree
• Efficient handling of infinite loops caused due to the setState inside of useEffect has also been fixed

JSX stands for JavaScript XML. It is a file type used by React that utilizes the expressiveness of JavaScript along with HTML. This makes the HTML file easy to understand. This file makes applications robust and boosts their performance.

A Controlled Component is one that takes the current value through the props and notifies of any changes through callbacks such as onChange.

Example:

<input type="text" value={value} onChange={handleChange} />

An uncontrolled component is one storing its own state internally, and you can query the DOM via a ref to find its current value as and when needed. Uncontrolled components are a bit closer to traditional HTML.

Example:

<input type="text" defaultValue="foo" ref={inputRef} />

Note: React is a widely used open-source library used for building interactive user interfaces. It is a web platform licensed under MIT. This is one of the very popular react interview questions.

Basically Function Based Component are the component which are made by JavaScript Functions.It takes props as argument and return JSX.

Refs are a function in React having the following uses:

• Accessing DOM and React elements that you have created.
• Changing the value of a child component without using props and others.

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The Main Difference is of Syntax. The function-Based component used props as an argument and is also called stateless component and In class-based component, we extend all the property of predefined function and used according to our need.

Class Based Component use state and the state is used as props in child component.In Class Based component we can use different lifecycle method of component.

Arrow functions, also called ‘fat arrow‘ (=>), are brief syntax for writing the expression. These functions bind the context of the components because auto binding is not available by default in ES6. Arrow functions are useful if you are working with the higher-order functions.

It is compulsory for each React component to have a render(). If more than one element needs to be rendered, they must be grouped inside one tag such as <form>, <group>,<div>. This function goes back to the single react element which is the presentation of native DOM Component. This function must return the same result every time it is invoked.

States are at the heart of components. They are the source of data and must be kept simple. States determine components’ rendering and behavior. They are mutable and create dynamic and interactive components, and can be accessed via this.state().

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Component lifecycle is an important part of ReactJS. Component lifecycle falls into three categories - property updates, Initialization and Destruction. Lifecycle is a method of managing the state and properties of the components.

Events are the triggered reactions to actions like mouse hover, click, key press, etc.

Redux is used to handle data in a reliable manner. It performs task accurately and ensures that the data has been controlled. You can also apply filters if only a specific part of data is required.

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Flux is an illustration that helps to maintain a unidirectional data stream. It also controls construed data unique fragments and makes them interface without creating issues. The configuration of Flux is insipid and is not specific to applications.

The server needs to be regularly monitored for updates. The aim is to check for the presence of new comments. This process is considered as pooling. It checks for the updates almost every 5 seconds. Pooling helps keep an eye on the users and ensure there is no negative information on servers.

If your component has a state or lifecycle method, use Class component. Else, a Functional component.

Enclosing the multi-line JSX expression is the best option. Sometimes it becomes necessary to avoid multi-lines in order to perform the task reliably and for getting the expected results.

No. This is because the word Class is an occupied word in JavaScript. If you use the word Class JSX will get translated to JavaScript.

Note: Before getting confused with the plethora of react coding interview questions, you need to ensure the basics. This question will help you to understand the core basics of react and make you even more confident enough to jump onto the advanced questions.

Yes. This is a complex process, and the best way to do it is by using a spread operator.

React is a JavaScript Library which is used to show content to user and handle user interaction.

Component is a building block of any React app which is made by either JavaScript Function of Class. Ex. Header Component, Footer Component etc.

There are two types of Components.

• Function Based Component
• Class Based Component

Class Based Component is the JavaScript Class. In these Components we extends the JavaScript predefined class into our component.

A higher-order component or HOC in React is basically an advanced technique used for reusing component logic. HOCs are not a part of the React API. They are a pattern emerging from the React's compositional nature.

The term pure function in React means that the return value of the function is entirely determined by its inputs and that the execution of the function produces no side effects.

The ReactDOMServer is an object which enables you to render components as per a static markup. Generally, it is used on a Node Server.

The App.proptypes is used for validating props in React components.

Here is the syntax:

class App extends React.Component {
    render() {}
}
Component.propTypes = { /*Definition */};

There are 3 ways to create a component in React:

• Using a depreciated variable function
• Using a Class
• Using a Stateless Functional Component

1. Using a depreciated variable function

In this, you need to declare a variable in your Javascript file/tag and use the React.createClass() function to create a component using this code.

var MyComponent = React.createClass({
   render() {
      return <div>
      <h1>Hello World!</h1>
      <p>This is my first React Component.</p>
      </div>
   }
})

Now, use the div element to create a unique ID for the specific page and then link the above script onto the HTML page.

<div id=”react-component”></div>

Use ReactDOM.render() function to take 2 arguments, the react component variable and the targeted HTML page.

ReactDOM.render(<MyComponent />, document.getElementById('react-component'))

Finally, save the HTML page to display the result like this one below:

2. Using a Class

Javascript has lately started supporting classes. Any React developer can easily create a class using the class extends (Inherits). Following is the code

3. Using a Stateless Functional Component

It might sound a bit complex, but this basically means that a normal function will be used in place of a variable to return a react component.

Create a const called MyComp and set it as equal to a () function. Use the arrow function, => to declare the logic of the function as follows:

const MyComponent = () => {
   return <div>
   <h1>Hello!</h1>
   <p>This is my first React Component.</p>
   </div>
}

There are two ways to link one page to another to React:

Using React Router: Use the feature from the react-router to link the first page to the second one.

<Link to ='/href' ></Link>

Reloading/Refreshing a page using Javascript:
Use Vanilla JS to call the reload method for the specific page using the following code:

window.location.reload(false);

The best way to redirect a page in React.js depends on what is your use case. Let us get a bit in detail:

If you want to protect your route from unauthorized access, use the component and try this:

In React, an object this.refs which is found as an instance inside the component can be used to access DOM elements in Reactjs.

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React Hooks were brought in by the React team to efficiently handle state management and the side-effects in function components. React Hooks is a way of making the coding effortless. They help by enabling us to write React applications by only using React components.

To render a component with another component in Reactjs, let us take an example:

Let us suppose there are 2 components as follows:

React Router is the in-built routing library for React. React Router helps to keep your UI in sync with the respective URL. It works on a simple API having powerful features like lazy loading, locational transition, and dynamic route matching built right in.

Now, there are two ways to render this object as an array,

First:

render() {
    const data =[{"name":"test1"},{"name":"test2"}];
    const listItems = data.map((d) => <li key={d.name}>{d.name}</li>);

    return (
      <div>
      {listItems }
      </div>
    );
  }

Or, you could directly write the map function in the return

render() {
    const data =[{"name":"test1"},{"name":"test2"}];
    return (
      <div>
      {data.map(function(d, idx){
         return (<li key={idx}>{d.name}</li>)
       })}
      </div>
    );
  }

Note: React is a widely used open-source library used for building interactive user interfaces. It is a web platform licensed under MIT. This is one of the very popular react interview questions.

You can avoid Prop Drilling in the following ways:

• Using a Context API
• Using Render Props

Key Benefits of ReactJS Development

• Allows Developers to Reuse the Components
• It is Much Easier to Learn
• The Benefit of Having JavaScript Library
• SEO Friendly
• It ensures faster rendering
• The Increase in Community Base
• The Support of Handy Tools
• Scope for Testing the Codes etc

It is a cross-browser wrapper that is around the native event of the same browser. This is the most important question in react interview questions.