Best React Interview Questions And Answers

• React is a JavaScript front-end library developed by Facebook in 2011.
• It follows a component-based approach that helps in creating reusable UI components.
• It is used to develop complex and interactive web and mobile interfaces.
• Even though it only opened in 2015, it is backed by one of the largest communities.

The main features of React are listed below:

1. It uses virtual DOM instead of real DOM.
2. It can use server side rendering.
3. It follows a unidirectional data flow or one-way data binding.

Some of the main benefits of React are:

1. Improves application performance.
2. It can be conveniently used both client-side and server-side.
3. Thanks to JSX, code readability is increased.
4. React integrates easily with other frameworks like Meteor, Angular, etc.
5. Using React, writing UI test cases has become extremely easy

React's limitations are listed below:

1. React is just a library, not a complete framework
2. His library is very large and takes time to understand
3. It can be a little tricky for beginner programmers to understand
4. Coding gets tricky as it uses inline templates and JSX.

JSX is short for JavaScript XML. This is a type of file used by React that uses JavaScript expressiveness along with template-like HTML syntax. This makes the HTML file easier to understand. This file makes applications more reliable and improves their performance. Below is a JSX example:

render(){
   return(
      <div><h1> Hello World from Ideamotive!</h1>
</div>
   );
}

The virtual DOM is a lightweight JavaScript object that is originally a copy of the real DOM. It is a node tree that lists elements, their attributes and content as objects and their properties. React's render function creates a node tree from React components. It then updates this tree in response to mutations in the data model caused by various actions performed by the user or the system.

This virtual DOM works in three easy steps.

1. Whenever any underlying data changes, the entire UI is re-rendered in the virtual DOM view. 
2. The difference between the previous DOM representation and the new one is then calculated.
3. Once the calculations are done, the real DOM will only be updated with what has really changed.

Browsers can only read JavaScript objects, but JSX is not a regular JavaScript object. So in order for the browser to read JSX, we first need to convert the JSX file to a JavaScript object using JSX converters like Babel and then pass it to the browser.

Syntax has changed from ES5 to ES6 in following aspects:

1. require vs import
2. export vs exports
3. component and function
4. props
5. state

Components are the building blocks of a React application's user interface. These components divide the entire user interface into small, independent and reusable pieces. It then renders each of these components independently of each other without affecting the rest of the user interface.

Every React component must have render (). It returns a single React element, which is a native DOM component representation. If more than one HTML element needs to be displayed, then they must be grouped together within a single enclosing tag such as <form>, <group>, <div>, etc. This function must be clean, i.e. it should return the same result. every time it is called.

Props is short for Properties in React. These are read-only components and must remain clean, i.e. immutable. They are always passed from parent to child components throughout the application. A child component can never send a prop back to a parent component. This helps maintain unidirectional data flow and is commonly used to visualize dynamically generated data.

States are the heart of React components. States are the source of data and should be kept as simple as possible. Basically, states are objects that determine the rendering and behavior of components. They are mutable, unlike props, and create dynamic and interactive components. They are accessed through this.state ().

Arrow functions are a shorter syntax for writing a function expression. They are also called bold arrow (=>) functions. These functions allow you to bind the component context correctly, as in ES6, automatic binding is not available by default. Arrow functions are mostly useful when dealing with higher-order functions.

The life cycle of a React component has three stages:

1. Initial Rendering Phase: This is the phase when the component is about to start its life and get to the DOM.
2. Refresh phase: After a component is added to the DOM, it can potentially be refreshed and re-rendered only when a property or state changes. This only happens at this stage.
3. Unmounting phase: This is the last phase of a component's life cycle, in which the component is destroyed and removed from the DOM.

Some of the most important lifecycle methods are:

• componentDidMount () - executed on the client-side only after the first render.
• shouldComponentUpdate () - Returns true or false depending on certain conditions. If you want your component to update, return true, otherwise return false. Returns false by default.
• componentDidUpdate () - called immediately after rendering.
• componentWillUnmount () - Called after the component has been unmounted from the DOM. It is used to clean up memory space.

In React, events are triggered reactions to certain actions, such as mouseover, mouse clicks, key presses, and so on. Handling these events is similar to handling events on DOM elements. But there are some syntactic differences, for example:

• Events are named using camel case, not just lowercase letters.
• Events are passed as functions, not strings.

The event argument contains a set of properties related to the event. Each event type contains its own properties and behavior, which can only be accessed through its event handler.

class Display extends React.Component({
show(evt) {
// code
},
render() {
// Render the div with an onClick prop (value is a function)
return (

<div onClick={this.show}>Click Me!</div>
);
}
});

Synthetic events are objects that act as a cross-browser wrapper around a native browser event. They combine the behavior of different browsers into one API. This is done to ensure that events display consistent properties across browsers.

Refs is short for References in React. This is an attribute that helps to store a reference to a specific React element or component that will be returned by the component's render config function. It is used to return references to a specific element or component as returned by the render () function. They come in handy when we need DOM measurements or to add methods to components.

Below are the cases when links should be used:

1. When you need to control focus, choose to play text or media
2. To start the required animation
3. Integration with third-party DOM libraries

We can split the code into modules using the export and import properties. They help to write components separately to different files.

React forms are similar to HTML forms. But in React, the state is contained in the component's state property and is only updated via setState (). Thus, elements cannot directly update their state, and their dispatch is handled by a JavaScript function. This function has full access to the data that the user enters into the form.

A higher-order component is an advanced way to reuse component logic. Basically, this is a template that inherits from the compositional nature of React. HOCs are custom components that include another component. They can accept any dynamically provided child component, but they will not modify or copy any behavior from their input components. We can say that HOCs are "pure" components.

HOC can be used for many tasks such as:

• Code reuse, logic, and bootstrap abstraction
• High jacking rendering
• Abstraction and state manipulation
• Manipulation props

Pure components are the simplest and fastest components you can write. They can replace any component that only has render (). These components improve the simplicity of your code and the performance of your application.

Keys are used for identifying unique virtual DOM elements with associated data that controls the user interface. They help React optimize rendering by reusing all existing elements in the DOM. These keys must be a unique number or string that React uses to simply reorder the elements instead of re-rendering them. This leads to an increase in the performance of the application.

Below are some of the main problems with the MVC framework:

• DOM manipulation was very expensive
• Apps were slow and ineffective
• There was a huge memory loss
• Due to cyclic dependencies, a complex model was created from models and views.

Flux is an architectural pattern that provides unidirectional data flow. It controls derived data and provides communication between multiple components through a central repository that has authority over all data. Any update of data in the application should happen only here. Flux provides application stability and reduces runtime errors.