Top 25 + React 19 Props Interview Questions and Answers
Our previous article “React 19 Interview Questions and Answers “covered 25 basic interview questions on React, helping you build a strong foundation for React-based interviews. If you haven’t gone through it yet, I highly recommend checking it out to strengthen your basics before diving into this one.
This article will focus specifically on Props in React, one of the most fundamental concepts that every React developer should understand. Let’s get started!
1. What are props in React, and how do they differ from state? |
In React, props and state are essential for every application. Whenever you need to pass data from one component to another, these are the fundamental tools you’ll use. Props in React are a way to pass data to components, much like properties, and can contain various types of values, including single values and objects. An important point about props is that they are immutable, meaning they cannot be updated from within the component. They are primarily used for rendering purposes. |
2. How do you pass data from a parent component to a child component in React? |
Parent to Child Communication in React :Step 1: Pass Data as a Prop from Parent: In the Parent Component, pass data to the child component using props. Like : function Parent() { const message = "Hello from Parent!"; // Data to pass return <Child myMessage={message} />; } Step 2: Receive Props in the Child Component: In the Child Component, accept the prop as an argument. function Child({ myMessage }) { return <h2>{myMessage}</h2>; } |
3. Explain how you would handle a scenario where you need to send data from a child component back to the parent component. |
Child to Parent Communication in React :Since data flows unidirectionally from parent to child in React, sending data back to the parent requires defining a function in the parent component, passing it as a prop to the child, and then calling that function from the child component. The function can accept data as parameters, enabling the parent to receive information from the child. |
4. What are default props, and why would you use them? |
Default props provide default values for props in case they are not passed by the parent component. They help prevent errors and ensure that a component still functions even if certain props are not explicitly defined by the parent.
Earlier, you could use defaultProps like this (refer to the image below), but if you use it the same way now, you will get an error (refer to the screenshot). To avoid this error, you need to use the following method instead: Output : |
5. If a child component receives a prop but doesn’t use it, what impact does that have on the component or the application? |
If a child component receives an unused prop, there’s no direct impact on functionality or performance. However, it can lead to unclear code, as the unused prop can confuse developers trying to understand the codebase. This could also increase maintenance complexity |
6. Can props be modified within a child component? Why or why not? |
No, props cannot be modified directly within a child component because they are immutable. React follows a one-way data flow, meaning props are read-only and are intended only for receiving and displaying data passed from the parent component. However, if you need to modify the data, you can initialize a state in the child component using the value of the prop. The state is mutable, and you can update it as needed within the child component. But remember, this modified state will not affect the original prop in the parent component. |
7. How would you conditionally render a child component based on the value of a prop in the parent component? |
You can conditionally render a child component by using JavaScript conditional expressions in the parent component’s JSX. For example, “{condition && <ChildComponent />}” renders “ChildComponent” only if the “condition” is true. Like import './App.css' function App() { const [showChild1, setShowChild1] = useState(true); const toggleChild = () => { setShowChild1(!showChild1); }; return ( <> <h1>Conditional Rendering Example</h1> <button onClick={toggleChild}> {showChild1 ? "Show Child2" : "Show Child1"} </button> {showChild1 ? <Child1 /> : <Child2 />} </> ) } function Child1() { return <div>This is Child1</div>; } function Child2() { return <div>This is Child2</div>; } export default App |
8. What happens if a child component expects a specific type of prop but receives the wrong type? How can you handle this in React? |
If a child component receives a prop of the wrong type, it can lead to unexpected behavior, errors, or even crashes. To handle this, React provides a way to enforce prop types using a package called prop-types. By using the below steps you can handle it, Step 1 : install prop-types by the following command: npm install prop-types Step 2 : Use the above library in your project like : Step 3: In the above file, when you call the Child component and give the wrong information then you will get errors like |
9. Explain how you would pass multiple values, such as an object or an array, from a parent to a child component in React |
You can pass complex data types like arrays or objects as props. For instance, “<ChildComponent data={myObject} />” or “<ChildComponent items={myArray} />”. The child component then accesses the data via “props.data” or “props.items”.
Passing an object like : function Parent() { const user = { name: "Shaurya", age: 5, location: "New Delhi" }; return ( <div> <h1>Passing an Object</h1> <Child user={user} /> </div> ); } function Child({ user }) { return ( <div> <p>Name: {user.name}</p> <p>Age: {user.age}</p> <p>Location: {user.location}</p> </div> ); } export default Parent; Passing as a list: function Parent() { const items = ["Atul", "Ajit", "Amit"]; return ( <div> <h1>Passing an Array</h1> <Child items={items} /> </div> ); } function Child({ items }) { return ( <ul> {items.map((item, index) => ( <li key={index}>{item}</li> ))} </ul> ); } export default Parent; |
10. What would you do if you need to pass a function as a prop to a child component? How does this affect the behavior of the child component |
You can pass a function as a prop by defining it in the parent component and passing it down, such as “<ChildComponent handleClick={this.handleClick} />”. This allows the child component to execute the function in the parent when needed, enabling communication from the child to the parent. |
11. How can you ensure that certain props are required in a component? |
To ensure that certain props are required in a React component, you can use the prop-types library. By specifying the expected types and marking certain props as required, you can enforce the correct usage of props and catch errors during development. For code snippet refer to question number 8. |
12. Can you explain the concept of “prop drilling” and how to avoid it? |
Prop drilling occurs when you need to pass data from a higher-level component down to a deeply nested child component through multiple layers of intermediary components. This can make your code harder to maintain and understand, as each intermediate component needs to pass along the props even if they don’t directly use them. Example of Prop drilling // Grandparent component const Grandparent = () => { const data = "I need to be passed down"; return ( <div> <Parent data={data} /> </div> ); }; // Parent component const Parent = ({ data }) => { return ( <div> <Child data={data} /> </div> ); }; // Child component const Child = ({ data }) => { return <div>{data}</div>; }; You can avoid Prop Drilling by the use of React’s Context API, State Management Libraries like Redux, MobX and Custom Hooks. |
13. What is the significance of using the “key” prop when rendering a list of elements? |
The key prop is crucial when rendering a list of elements in React for several reasons: Uniqueness: Each key should be unique among sibling elements to help React identify which items have changed, been added, or removed. This helps React in efficiently updating the UI, as it can directly map these changes to the specific items. Performance: By using unique keys, React can optimize the rendering process by reusing existing elements instead of re-rendering them from scratch. This improves the performance of your application, especially for large lists. State Preservation: Keys help in preserving component states between re-renders. When keys are stable and consistent, React can maintain the state of each component correctly, even when the order of elements in the list changes. |
14. Describe how you would handle a scenario where a component requires multiple props, and they are only conditionally available. |
Provide default values for props so the component behaves predictably even when some props are missing. Use default parameters in the function signature or defaultProps for class components.
function Test({ title = "Default Title", description = "No description provided" }) { return ( <div> <h1>{title}</h1> <p>{description}</p> </div> ); } export default Test; |
15. How would you handle passing a prop that could change over time, like a theme or language setting? |
When passing a prop that could change over time, such as a theme or language setting, the most efficient way to handle it in React is by using state management and ensuring the updates propagate correctly. You have lot of approaches to achieve this like Redux, ContextAPI, custom hooks. Let’s do it with Context API:
import React, { useState } from "react"; function Parent() { const [theme, setTheme] = useState("light"); const toggleTheme = () => { setTheme((prevTheme) => (prevTheme === "light" ? "dark" : "light")); }; return ( <div> <button onClick={toggleTheme}> Switch to {theme === "light" ? "Dark" : "Light"} Mode </button> <Child theme={theme} /> </div> ); } function Child({ theme }) { return <div style={{ backgroundColor: theme === "light" ? "#fff" : "#333", color: theme === "light" ? "#000" : "#fff" }}> This is a {theme} themed component. </div>; } export default Parent; |
16. Explain the concept of “lifting state up” in React and its relationship with props. |
Lifting state up refers to the process of moving state from a child component to the closest common parent component so that it can be shared between multiple child components. This concept is crucial when multiple components need to synchronize or communicate by sharing the same state.
In React, data flows unidirectionally (top-down) from parent to child components using props. When you lift the state up, you centralize the management of that state in a parent component and pass it down to children as props, allowing those children to use or modify it indirectly. Example : import React, { useState } from "react"; function Parent() { const [value, setValue] = useState(""); return ( <div> <Child1 value={value} onChange={(e) => setValue(e.target.value)} /> <Child2 value={value} /> </div> ); } function Child1({ value, onChange }) { return <input value={value} onChange={onChange} />; } function Child2({ value }) { return <p>Value: {value}</p>; } export default Parent; |
17. What happens if you accidentally overwrite an existing prop inside a component? |
In React, props are immutable, meaning they are read-only and cannot be directly modified within a child component. If you attempt to overwrite a prop, React will not throw an error but your changes will not take effect, and the original value will remain intact. However, if you inadvertently overwrite a prop by assigning it to a new value (e.g., within a variable of the same name), you may experience unexpected behavior, especially if you’re using that variable later in the component. |
18. How do you optimize the performance of a component that depends on multiple props, ensuring it doesn’t re-render unnecessarily? |
In React, a component re-renders whenever its state or props change. While this is a core feature, unnecessary re-renders can degrade performance, especially for components with complex logic or heavy DOM updates. Below are strategies to optimize a component that depends on multiple props.
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19. What’s the difference between spreading props (“{…props}”) and explicitly passing each prop? |
Spreading props means passing all properties of an object (typically the props object) to a child component using the spread operator (…). In this you can pass all properties to a child using spread operator.
Example : function Child(props) { return ( <div> <h1>{props.title}</h1> <p>{props.content}</p> </div> ); } function Parent() { const childProps = { title: "Hello", content: "This is a demo." }; return <Child {...childProps} />; } In explicitly passing, you pass each prop explicitly by specifying its name and value, like function Parent() { return <Child title="Hello" content="This is a demo." />; } |
20. How can you make sure that a required prop is always provided by the parent component? |
To make sure a required prop is always provided, you can use PropTypes in React. This helps catch issues where a parent component might fail to pass a necessary prop. “prop-types” is a package that allows you to define the expected types and requirements of props in a component.
import React from "react"; import PropTypes from "prop-types"; function Child({ name }) { return <h1>Hello, {name}!</h1>; } Child.propTypes = { name: PropTypes.string.isRequired, // Ensures `name` is a required string }; function Parent() { return <Child name="Mahesh" />; // Works fine // return <Child />; // This will throw a warning in the console } export default Parent; What Happens?
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21. Can you dynamically determine which props to pass to a child component at runtime? If yes, how? |
Yes, in React, you can dynamically determine and pass props to a child component at runtime based on various conditions such as user interactions, API responses, or state changes. You can pass different props based on a condition. Like :
import React, { useState } from "react"; function Child({ message }) { return <h1>{message}</h1>; } function Parent() { const [isGreeting, setIsGreeting] = useState(true); return ( <div> <Child message={isGreeting ? "Hello, Study!" : "Goodbye, Study!"} /> <button onClick={() => setIsGreeting(!isGreeting)}>Toggle Message</button> </div> ); } export default Parent; |
22. What are the problems of passing a large object or array as a prop? How can you minimize potential issues? |
Problems of Passing Large Objects/Arrays:
Optimizations to Minimize Issues:
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23. How would you handle deeply nested props to avoid “prop drilling”? |
Prop drilling occurs when you pass props through multiple layers of components just to reach a deeply nested child. This can make the code hard to maintain. You can avoid it by using ContextAPI, Redux, Component Composition, Custom Hooks etc. Let’s take a look of context API:
import React, { createContext, useContext } from "react"; // 1. Create Context const UserContext = createContext(); // 2. Parent Component (Provider) function Parent() { const user = { name: "Atul", role: "Teacher" }; return ( <UserContext.Provider value={user}> <ChildA /> </UserContext.Provider> ); } // 3. Deeply Nested Component (Consuming Context) function ChildC() { const user = useContext(UserContext); return <h1>Hello, {user.name}! Your role is {user.role}.</h1>; } // 4. Intermediate Components (No Prop Drilling Needed) function ChildA() { return <ChildB />; } function ChildB() { return <ChildC />; } export default Parent; |
24. What happens to the props passed to its child components if a parent component’s state changes? |
In React, props are derived from the parent component’s state or variables. If the parent component’s state changes, the props passed to its child components will also update accordingly, causing the child components to re-render with the new prop values.
React’s Data Flow Reactivity: When the parent state updates:
Example : import React, { useState } from "react"; function Parent() { const [count, setCount] = useState(0); const increment = () => setCount(count + 1); return ( <div> <h1>Parent Component</h1> <button onClick={increment}>Increment</button> <Child count={count} /> </div> ); } function Child({ count }) { return <h2>Child Component: Count is {count}</h2>; } export default Parent; |
25. What are the advantages of using props over global state management? |
Using props instead of global state management has several advantages, especially in simpler or more modular React applications. Here are some key benefits:
1. Simplicity and Clarity: Props offer a straightforward way to pass data from parent to child components, making the data flow easy to understand and trace. |
Summary :
In this article, we explored 25 essential interview questions on Props in React, covering everything from basic concepts to advanced use cases. Props play a crucial role in React by enabling component communication, making them a fundamental topic for any React developer.
We discussed how props work, passing and updating props, default props, prop validation, destructuring, and performance considerations. Understanding these concepts will help you write better React components and confidently tackle interview questions.
If you haven’t checked out our previous article on 25 basic React interview questions, make sure to go through it for a complete understanding of React fundamentals. Stay tuned for more in-depth articles to strengthen your React knowledge