Things to Take Care before Going for ReactJS Interview

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Predict

6 min readJan 24, 2024

When preparing for a React interview, it’s essential to familiarize yourself with common React interview questions to enhance your chances of success.

In an interview for a React developer position, the potential employer will assess your knowledge and skills in web app development.

They will interview you with questions about your skills & knowledge in HTML, CSS, and JS. They’ll also ask you some framework-related questions about Angular, React, Vue, or any other related framework.

They also give you a coding challenge for testing your abilities in a specific domain. So, in this article, we will have a look at the most common questions that an employer can ask you in a technical interview round of React and JS development.

Let’s dive in!

Things to observe before attending a ReactJS Interview

Every candidate thinks- What skills is a potential employer looking for?

You will have to be prepared to show your prospective experience and skills with different tools, languages, and frameworks.

Interviewers also want to check that you have a robust understanding of the modern trends & technologies in web development.

They can ask you about your past projects and how you have solved different project-related challenges in the past.

They will also notice your problem-solving skills and discuss how you have tackled different challenges during the development process. Additionally, do not forget to highlight your technical strengths.

Now, let’s see some common questions that you are anticipated to face in a React technical interview.

What is VDOM or Virtual DOM?

VDOM or the Virtual DOM is a technique in programming where a virtual presentation of a UI is stored in the memory & is synched with the real DOM by using a library called ReactDOM.

VDOM is a copy of a real DOM; however, it’s not heavier than the real one. This feature makes the updating process of a VDOM smoother as compared to the real DOM.

When any updates are made in the React component’s state, the Virtual DOM gets updated. After that React will compare VDOM with real DOM and eventually update the real DOM to match it with the VDOM.

This process is called reconciliation. Moreover, reconciliation is helpful as it saves time because React will only update the parts of real DOM that don’t match the updated part of VDOM. So, a complete real DOM update isn’t required to make a small change.

How to build components in ReactJS?

In React, there are two ways to make a component- class components and function components.

Class components:

The class component uses ES6 to define any component. In the same function, a component can be written like this:

class Welcome extends React.Component {
render() {
return <h1>{`Hello, ${this.props.message}`}</h1>
} }

Function Components:

This is the easiest way to make a component. It uses pure JS functions that use props and return one React element.

function Welcome({ message }) {
return <h1>{`Hello, ${message}`}</h1>
}

Explain State in ReactJS

State in ReactJS is an object that stores information local to the component that can change the lifetime of that component.

Here’s a snippet that will show a state object that contains the property message that is rendered in the H1 tag:

class Welcome extends React.Component {
constructor(props) {
super(props)
this.state = {
message: 'Welcome to the world of React'
}
}
render() {
return (
<div>
<h1>{this.state.message}</h1>
</div>
)
} }

Explain Props in ReactJS

Props are a way of passing data from the parent component -> child component. This process passes the components using specific HTML attributes.

Note: Two more additional questions related to Props:

What’s Prop-drilling?

Prop-drilling is the method of passing values from multiple levels, i.e, grandparent to parent to child

Are props modifiable?

No, props are in read-only mode, and cannot be modified. The values stored in Props must be pure and cannot be changed or mutated directly.

When to leverage Context?

The context in React is made to share data that is global in a React components tree-like authenticated end-user, preferred language, or theme. For instance, in the code given below, we have manually threaded through the ‘theme’ prop for styling a specific button component:

const ThemeContext = createContext('light')
function App() {
return (
<div className="App">
<ThemeContext.Provider value={'red'}>
<Content/>
</ThemeContext.Provider>
</div>
);
}
function Content(){
const theme = useContext(ThemeContext)
return(
<div className={theme}>
<h2>Hello, World</h2>
</div>
);
}
export default App;

Explain ForEach, Map, Filter, and Reduce

The widely asked question in a ReactJS interview is about array methods. The interviewer will assess how experienced you are with the topic of array manipulation.

The .forEach()

ForEach doesn’t return an array. It iterates in an array and computes your logic written inside the foreach body.

let arr = [
{ id: 1, age: 12, name: 'ABC' },
{ id: 2, age: 24, name: 'XYZ' },
{ id: 3, age: 22, name: 'EFG' },
]
arr.forEach((el) => el.age+= 10);
console.log(arr);
// Output: 22 32 44

The .map()

In this method, everything is similar to the .foreach() method, but it will return a whole new array.

let arr = [
{ id: 1, age: 12, name: 'ABC' },
{ id: 2, age: 24, name: 'XYZ' },
{ id: 3, age: 22, name: 'EFG' },
]
let names = arr.map((el) => el.name)
console.log(names)
// Output: [ 'ABC', 'XYZ', 'EFG']

The .filter()

This method, as the name shows, helps in filtering out the values in an array that is based on Boolean conditions.

If the boolean becomes true, the result returns and is added to the final output array. If it’s false, it will be skipped. Filter, like the map method, returns an array.

let arr = [
{ id: 1, age: 12, name: 'ABC' },
{ id: 2, age: 24, name: 'XYZ' },
{ id: 3, age: 22, name: 'EFG' },
]
let tooYoung = arr.filter((el) => el.age <= 14);
console.log(tooYoung);
// Output: [ { id: 1, age: 12, name: 'ABC' } ]

The .reduce()

In simple terms, this method takes a previous value, accumulator, and current value into account.

The return type of this method is a single value. This method is useful when you start processing different values of the array and want to have accumulated results.

// Calculate the total age of all these three persons.
let arr = [
{ id: 1, age: 12, name: 'ABC'},
{ id: 2, age: 24, name: 'XYZ'},
{ id: 3, age: 22, name: 'EFG'},
]
let totalAge = arr.reduce((acc, currentObj) => acc + currentObj.age, 0)
console.log(totalAge)
// Output: 57

Explain React Hooks (almost every interviewer has asked this question)

Hooks are an addition to the React 16.8 version. They help you use different React features like State and many more without having to write a class. It’s a certainty that the majority of the latest React apps are developed using React hooks.

Additionally, hooks make your code clean, and easy-to-understand by humans and machines.

Here are some popular React hooks:

useEffect
useState
useContext
useReducer
useMemo
useCallback
useRef
useLayoutEffect
useImperativeHandle
useDebugValue

What are the 4 Stages that a React Component Goes Through?

Any ReactJS component has to go through the following 4 phases or let’s say stages:

The first step is initialization. Here, the component is constructed with a default state and given props
The second one is mounting- Here rendering of JSX returned components is done using the render method
The third step is updating- This stage occurs when any component state is changed and the app is updated as per the state change
The final stage is unmounting- in this stage, the lifecycle of the component ends and it’s detached from the DOM.

Final Verdict

At last, before you step into any ReactJS interview, it’s imperative to know and understand the key considerations to have a confident and skilled approach towards that interview.

Take your time to learn the core concepts of React, master important React libraries, learn what the ReactJS best practices going on in the industry, and master the concept of solving common challenges.

Moreover, stay tuned with the latest ReactJS advancements. By doing this, you will be able to bolster your modern knowledge about React. You can also show your genuine commitment to learning new things and keeping up with React trends. Happy learning and happy programming!