Blog

etc. This function must be kept pure i.e., it must return the same result each time it is invoked.

13. How can you embed two or more components into one?

We can embed components into one in the following way:
class MyComponent extends React.Component{
render(){
return(
Hello
);
}
}
class Header extends React.Component{
render(){
return


Header Component

};
}
ReactDOM.render(
, document.getElementById('content')
);

14. What is Props?

Props are shorthand for Properties in React. They are read-only components that must be kept pure i.e. immutable. They are always passed down from the parent to the child components throughout the application. A child component can never send a prop back to the parent component. This helps in maintaining the unidirectional data flow and are generally used to render the dynamically generated data.

15. What is a state in React and how is it used?

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

16. Differentiate between states and props.

17. How can you update the state of a component?

State of a component can be updated using this.setState().
class MyComponent extends React.Component {
constructor() {
super();
this.state = {
name: 'Maxx',
id: '101'
}
}
render()
{
setTimeout(()=>{this.setState({name:'Jaeha', id:'222'})},2000)
return (

Hello {this.state.name}

Your Id is {this.state.id}

);
}
}
ReactDOM.render(
, document.getElementById('content')
);

18. What is the arrow function in React? How is it used?

Arrow functions are more of a brief syntax for writing the function expression. They are also called ‘fat arrow‘ (=>) the functions. These functions allow to bind the context of the components properly since in ES6 auto binding is not available by default. Arrow functions are most useful while working with higher-order functions.
//General way
render() {
return(

);
}
//With Arrow Function
render() {
return(
this.handleOnChange(e) } />
);
}

19. Differentiate between stateful and stateless components.

20. What are the different phases of React component’s lifecycle?

There are three different phases of React component’s lifecycle:

1. Initial Rendering Phase: This is the phase when the component is about to start its life journey and make its way to the DOM.
2. Updating Phase: Once the component gets added to the DOM, it can potentially update and re-render only when a prop or state change occurs. That happens only in this phase.
3. Unmounting Phase: This is the final phase of a component’s life cycle in which the component is destroyed and removed from the DOM.

21. Explain the lifecycle methods of React components in detail.

Some of the most important lifecycle methods are:

1. componentWillMount()–Executed just before rendering takes place both on the client as well as server-side.
2. componentDidMount()–Executed on the client side only after the first render.
3. componentWillReceiveProps() – Invoked as soon as the props are received from the parent class and before another render is called.
4. shouldComponentUpdate()–Returns true or false value based on certain conditions. If you want your component to update, return true else return false. By default, it returns false.
5. componentWillUpdate()– Called just before rendering takes place in the DOM.
6. componentDidUpdate()–Called immediately after rendering takes place.
7. componentWillUnmount()– Called after the component is unmounted from the DOM. It is used to clear up the memory spaces.

22. What is an event in React?

In React, events are the triggered reactions to specific actions like mouse hover, mouse click, key press, etc. Handling these events are similar to handling events in DOM elements. But there are some syntactical differences like:

1. Events are named using camel case instead of just using the lowercase.
2. Events are passed as functions instead of strings.

The event argument contains a set of properties, which are specific to an event. Each event type contains its own properties and behavior which can be accessed via its event handler only.

23. How do you create an event in React?

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

this.show}>Click Me!

);
}
});

24. What are synthetic events in React?

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

25. What do you understand by refs in React?

Refs is the short hand for References in React. It is an attribute which helps to store a reference to a particular React element or component, which will be returned by the components render configuration function. It is used to return references to a particular element or component returned by render(). They come in handy when we need DOM measurements or to add methods to the components.
class ReferenceDemo extends React.Component{
display() {
const name = this.inputDemo.value;
document.getElementById('disp').innerHTML = name;
}
render() {
return(

Name: "text" ref={input => this.inputDemo = input} />
"Click" onClick={this.display}>Click

Hello "disp"> !!!

);
}
}

26. List some of the cases when you should use Refs.

Following are the cases when refs should be used:

• When you need to manage focus, select text or media playback
• To trigger imperative animations
• Integrate with third-party DOM libraries

27. How do you modularize code in React?

We can modularize code by using the export and import properties. They help in writing the components separately in different files.
//ChildComponent.jsx
export default class ChildComponent extends React.Component {
render() {
return(

This is a child component

);
}
}
//ParentComponent.jsx
import ChildComponent from './childcomponent.js';
class ParentComponent extends React.Component {
render() {
return(

);
}
}

28. How are forms created in React?

React forms are similar to HTML forms. But in React, the state is contained in the state property of the component and is only updated via setState(). Thus the elements can’t directly update their state and their submission is handled by a JavaScript function. This function has full access to the data that is entered by the user into a form.
handleSubmit(event) {
alert('A name was submitted: ' + this.state.value);
event.preventDefault();
}
render() {
return (


Name:



);
}

29. What do you know about controlled and uncontrolled components?

30. What are Higher-Order Components(HOC)?

Higher-Order Component is an advanced way of reusing the component logic. Basically, it’s a pattern that is derived from React’s compositional nature. HOC are custom components that wrap another component within it. They can accept any dynamically provided child component but they won’t modify or copy any behavior from their input components. You can say that HOC is ‘pure’ components.

31. What can you do with HOC?

HOC can be used for many tasks like:

• Code reuse, logic, and bootstrap abstraction
• Render High jacking
• State abstraction and manipulation
• Props manipulation

32. What are Pure Components?

Pure components are the simplest and fastest components that can be written. They can replace any component which only has a render(). These components enhance the simplicity of the code and the performance of the application.

33. What is the significance of keys in React?

Keys are used for identifying unique Virtual DOM Elements with their corresponding data driving the UI. They help React to optimize the rendering by recycling all the existing elements in the DOM. These keys must be a unique number or string, using which React just reorders the elements instead of re-rendering them. This leads to an increase in the application’s performance.

React Redux – React Interview Questions

34. What were the major problems with the MVC framework?

Following are some of the major problems with the MVC framework:

• DOM manipulation was very expensive
• Applications were slow and inefficient
• There was huge memory wastage
• Because of circular dependencies, the complicated model was created around models and views

35. Explain Flux.

Flux is an architectural pattern that enforces the uni-directional data flow. It controls derived data and enables the communication between multiple components using a central Store that has authority for all data. Any update in data throughout the application must occur here only. Flux provides stability to the application and reduces run-time errors.

36. What is Redux?

Redux is one of the hottest libraries for front-end development in today’s marketplace. It is a predictable state container for JavaScript applications and is used for the entire applications state management. Applications developed with Redux are easy to test and can run in different environments showing consistent behavior.

37. What are the three principles that Redux follows?

1. Single source of truth: The state of the entire application is stored in an object/ state tree within a single store. The single state tree makes it easier to keep track of changes over time and debug or inspect the application.
2. The state is read-only: The only way to change the state is to trigger an action. An action is a plain JS object describing the change. Just like the state is the minimal representation of data, the action is the minimal representation of the change to that data.
3. Changes are made with pure functions: In order to specify how the state tree is transformed by actions, you need pure functions. Pure functions are those whose return value depends solely on the values of their arguments.

38. What do you understand by “Single source of truth”?

Redux uses ‘Store’ for storing the application’s entire state at one place. So all the component’s state are stored in the Store and they receive updates from the Store itself. The single state tree makes it easier to keep track of changes over time and debug or inspect the application.

39. List down the components of Redux.

Redux is composed of the following components:

1. Action – It’s an object that describes what happened.
2. Reducer –  It is a place to determine how the state will change.
3. Store – State/ Object tree of the entire application is saved in the Store.
4. View – Simply displays the data provided by the Store.

40. Show how the data flows through Redux?

41. How are Actions defined in Redux?

Actions in React must have a type property that indicates the type of ACTION being performed. They must be defined as a String constant and you can add more properties to it as well. In Redux, actions are created using the functions called Action Creators. Below is an example of Action and Action Creator:
function addTodo(text) {
return {
type: ADD_TODO,
text
}
}

42. Explain the role of the Reducer.

Reducers are pure functions that specify how the application’s state changes in response to an ACTION. Reducers work by taking in the previous state and action, and then it returns a new state. It determines what sort of update needs to be done based on the type of the action, and then returns new values. It returns the previous state as it is if no work needs to be done.

43. What is the significance of Store in Redux?

A store is a JavaScript object which can hold the application’s state and provide a few helper methods to access the state, dispatch actions, and register listeners. The entire state/ object tree of an application is saved in a single store. As a result of this, Redux is very simple and predictable. We can pass middleware to the store to handle the processing of data as well as to keep a log of various actions that change the state of stores. All the actions return a new state via reducers.

44. How is Redux different from Flux?

45. What are the advantages of Redux?

Advantages of Redux are listed below:

• Predictability of outcome – Since there is always one source of truth, i.e. the store, there is no confusion about how to sync the current state with actions and other parts of the application.
• Maintainability – The code becomes easier to maintain with a predictable outcome and strict structure.
• Server-side rendering – You just need to pass the store created on the server, to the client-side. This is very useful for initial render and provides a better user experience as it optimizes the application performance.
• Developer tools – From actions to state changes, developers can track everything going on in the application in real-time.
• Community and ecosystem – Redux has a huge community behind it which makes it even more captivating to use. A large community of talented individuals contribute to the betterment of the library and develop various applications with it.
• Ease of testing – Redux’s code is mostly functions that are small, pure, and isolated. This makes the code testable and independent.
• Organization – Redux is precise about how code should be organized, this makes the code more consistent and easier when a team works with it.

46. What is React Router?

React Router is a powerful routing library built on top of React, which helps in adding new screens and flows to the application. This keeps the URL in sync with the data that’s being displayed on the web page. It maintains a standardized structure and behavior and is used for developing single-page web applications. React Router has a simple API.

47. Why is the switch keyword used in React Router v4?

Although a <div> is used to encapsulate multiple routes inside the Router. The ‘switch’ keyword is used when you want to display only a single route to be rendered amongst the several defined routes. The <switch> tag when in use matches the typed URL with the defined routes in sequential order. When the first match is found, it renders the specified route. Thereby bypassing the remaining routes.

48. Why do we need a Router in React?

A Router is used to define multiple routes and when a user types a specific URL, if this URL matches the path of any ‘route’ defined inside the router, then the user is redirected to that particular route. So basically, we need to add a Router library to our app that allows creating multiple routes with each leading to us a unique view.

49. List down the advantages of React Router.

A few advantages are:

1. Just like how React is based on components, in React Router v4, the API is ‘All About Components’. A Router can be visualized as a single root component (<BrowserRouter>) in which we enclose the specific child routes (<route>).
2. No need to manually set History value: In React Router v4, all we need to do is wrap our routes within the <BrowserRouter> component.
3. The packages are split: Three packages one each for Web, Native, and Core. This supports the compact size of our application. Its easy to switch over based on a similar coding style.

50. How is React Router different from conventional routing?