Props as read-only data in React

Introduction

Props let components get information from their parent. They are read-only so components don't change data they don't own.

Passing a user name from a parent to a child component to display it.

Sending a color value to style a button without changing it inside the button.

Sharing a list of items from a parent to a child for showing in a list.

Giving a child component a callback function to notify the parent about an event.

Syntax

React

function ChildComponent(props) {
  return <div>{props.message}</div>;
}

// Usage
<ChildComponent message="Hello!" />

Props are passed like HTML attributes when using a component.

Inside the component, props are read-only and should not be changed.

Examples

Pass a simple string prop to show a greeting.

React

function Greeting(props) {
  return <h1>Hello, {props.name}!</h1>;
}

<Greeting name="Alice" />

Pass multiple props to customize button text and color.

React

function Button(props) {
  return <button style={{ backgroundColor: props.color }}>{props.label}</button>;
}

<Button color="blue" label="Click me" />

Pass an array prop to render a list of items.

React

function List(props) {
  return <ul>{props.items.map(item => <li key={item}>{item}</li>)}</ul>;
}

<List items={["Apple", "Banana", "Cherry"]} />

Sample Program

This simple app shows how a child component receives a message prop and displays it without changing it.

React

import React from 'react';

function DisplayMessage(props) {
  // Trying to change props.message here would be wrong
  return <p>{props.message}</p>;
}

export default function App() {
  return (
    <div>
      <DisplayMessage message="Props are read-only!" />
    </div>
  );
}

Important Notes

Never try to modify props inside a component; it can cause bugs and unexpected behavior.

If you need to change data, use component state or context instead.

Props help keep components predictable and easy to understand.

Summary

Props pass data from parent to child components.

Props are read-only inside the receiving component.

Use state if you need to change data within a component.

Practice

1. In React, what does it mean that props are read-only?

easy

A. You cannot change props inside the child component.

B. You can modify props directly in the child component.

C. props are only used for styling components.

D. props are used to store component state.

Solution

1. Step 1: Understand the role of props

   Props are data passed from a parent component to a child component in React.

2. Step 2: Recognize props immutability

   Props are read-only, meaning the child component cannot change them directly.

3. Final Answer:

   You cannot change props inside the child component. -> Option A

4. Quick Check:

   Props are read-only = D [OK]

Hint: Remember: props come from parent, child can't change them [OK]

Common Mistakes:

• Trying to modify props directly inside child component
• Confusing props with state
• Thinking props are for styling only

2. Which of the following is the correct way to access a prop named title inside a functional component?

easy

A. const title = this.props.title;

B. const title = getProps.title;

C. const title = state.title;

D. const title = props.title;

Solution

1. Step 1: Identify functional component syntax

   In functional components, props are passed as a parameter named props.

2. Step 2: Access prop correctly

   You access a prop by props.title. Using this.props is for class components.

3. Final Answer:

   const title = props.title; -> Option D

4. Quick Check:

   Functional component props access = B [OK]

Hint: Use props.parameterName inside functional components [OK]

Common Mistakes:

• Using this.props in functional components
• Confusing state with props
• Trying to use undefined variables like getProps

3. What will be the output of this React component?

function Greeting(props) {
  props.name = "Alice";
  return <h1>Hello, {props.name}!</h1>;
}

// Usage: <Greeting name="Bob" />

medium

A. Hello, Bob!

B. Hello, Alice!

C. Error: Cannot assign to props.name

D. Hello, undefined!

Solution

1. Step 1: Understand props mutability

   Although props are read-only and mutating them is an anti-pattern, JavaScript allows changing the object property within the component function.

2. Step 2: Determine rendered output

   The assignment props.name = "Alice" mutates the value before the return, so the output is "Hello, Alice!".

3. Final Answer:

   Hello, Alice! -> Option B

4. Quick Check:

   Props mutation affects render output = B [OK]

Hint: Mutating props technically works but violates React's read-only principle [OK]

Common Mistakes:

• Thinking props mutation doesn't affect output
• Expecting an error from assignment
• Confusing props with state

4. Identify the error in this React functional component:

function Counter(props) {
  props.count = props.count + 1;
  return <div>Count: {props.count}</div>;
}

medium

A. Props are being mutated directly, which is not allowed.

B. The component should use this.state instead of props.

C. The return statement is missing parentheses.

D. The component should not return JSX.

Solution

1. Step 1: Check for props mutation

   The line props.count = props.count + 1; tries to change props directly, which is not allowed.

2. Step 2: Understand correct data handling

   Props are read-only; to change data, use state or callbacks, not direct mutation.

3. Final Answer:

   Props are being mutated directly, which is not allowed. -> Option A

4. Quick Check:

   Direct props mutation = error = A [OK]

Hint: Never assign to props; use state for changes [OK]

Common Mistakes:

• Trying to mutate props directly
• Confusing props with state usage
• Ignoring React's immutability rules

5. You want to create a component that shows a user's name and allows updating it inside the component. Which approach correctly respects that props are read-only?

hard

A. Modify the props.name directly inside the component to update the name.

B. Use a global variable to store the name and ignore props.

C. Copy props.name to a state variable and update the state to change the name.

D. Pass a new name prop from the child component to the parent.

Solution

1. Step 1: Understand props immutability

   Props cannot be changed inside the component; they are read-only.

2. Step 2: Use state for data that changes

   Copy the prop value to a state variable, then update the state to reflect changes.

3. Final Answer:

   Copy props.name to a state variable and update the state to change the name. -> Option C

4. Quick Check:

   Use state to update data, not props = C [OK]

Hint: Use state for changes, keep props read-only [OK]

Common Mistakes:

• Trying to change props directly
• Using global variables instead of state
• Passing props from child to parent (not possible)