Passing props to components in React

Introduction

Props let you send information from one component to another in React. This helps components share data and work together.

You want to show a user's name inside a greeting component.

You need to customize a button's label and color from its parent component.

You want to reuse a card component but show different content each time.

You want to pass a function to handle a button click inside a child component.

Syntax

React

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

function ParentComponent() {
  return <ChildComponent message="Hello!" />;
}

Props are passed like HTML attributes when using a component.

Inside the child, props is an object holding all passed values.

Examples

This passes the name 'Alice' to the Welcome component, which shows it.

React

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

function App() {
  return <Welcome name="Alice" />;
}

Props customize the button's label and background color.

React

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

function App() {
  return <Button label="Click me" color="blue" />;
}

A function is passed as a prop to handle the button click.

React

function AlertButton({ onAlert }) {
  return <button onClick={onAlert}>Alert</button>;
}

function App() {
  const showAlert = () => alert('Button clicked!');
  return <AlertButton onAlert={showAlert} />;
}

Sample Program

This app shows two greetings with different names by passing props to the Greeting component.

React

import React from 'react';

function Greeting(props) {
  return <h2>Hello, {props.user}!</h2>;
}

export default function App() {
  return (
    <main>
      <Greeting user="Sam" />
      <Greeting user="Lee" />
    </main>
  );
}

Important Notes

Props are read-only inside the child component; do not try to change them.

Use destructuring like function Greeting({ user }) for cleaner code.

Always give props meaningful names to keep code clear.

Summary

Props send data from parent to child components.

They look like HTML attributes when used.

Inside the child, props are accessed as an object.

Practice

1. What is the main purpose of props in React components?

easy

A. To handle user events like clicks

B. To store local state inside a component

C. To pass data from a parent component to a child component

D. To style components with CSS

Solution

1. Step 1: Understand the role of props

   Props are used to send data from one component to another, specifically from parent to child.

2. Step 2: Differentiate props from other React features

   State stores local data, events handle interactions, and CSS styles components, but props specifically pass data down.

3. Final Answer:

   To pass data from a parent component to a child component -> Option C

4. Quick Check:

   Props = data passing [OK]

Hint: Props always send data down from parent to child [OK]

Common Mistakes:

• Confusing props with state
• Thinking props handle events
• Believing props style components

2. Which of the following is the correct way to pass a prop named title with value "Hello" to a component Header?

easy

A.

B.

C.

D.

Solution

1. Step 1: Recall JSX prop syntax

   Props are passed like HTML attributes: name="value" or name={expression}.

2. Step 2: Check each option

   uses correct string syntax.

   misses quotes around string.

   uses invalid quotes.

   uses colon which is invalid in JSX.

3. Final Answer:

   <Header title="Hello" /> -> Option B

4. Quick Check:

   Props look like HTML attributes [OK]

Hint: Use quotes for string props in JSX [OK]

Common Mistakes:

• Omitting quotes for string values
• Using colons instead of equals
• Adding extra quotes around prop names

3. What will be rendered by this React component?

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

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

medium

A. Hello, Alice!

B. Hello, {props.name}!

C. Hello, name!

D. Error: props is undefined

Solution

1. Step 1: Understand how props are used inside the component

   The component receives props and accesses props.name to display the name.

2. Step 2: Check the usage of the component

   The component is called with name="Alice", so props.name is "Alice".

3. Final Answer:

   <h1>Hello, Alice!</h1> -> Option A

4. Quick Check:

   props.name = Alice [OK]

Hint: Props values replace placeholders inside JSX [OK]

Common Mistakes:

• Printing literal {props.name} instead of value
• Forgetting to pass the prop
• Assuming props is undefined

4. Identify the error in this code snippet:

function Welcome(props) {
  return <h2>Welcome, {props.user}!</h2>;
}

// Usage:
<Welcome user=John />

medium

A. Missing quotes around string prop value 'John'

B. Props object is not passed correctly

C. Component name should be lowercase

D. JSX tags are not closed properly

Solution

1. Step 1: Check the prop usage in JSX

   In JSX, string prop values must be quoted, e.g., user="John".

2. Step 2: Identify the error in usage

   The code uses user=John without quotes, causing a syntax error.

3. Final Answer:

   Missing quotes around string prop value 'John' -> Option A

4. Quick Check:

   String props need quotes [OK]

Hint: Always quote string props in JSX [OK]

Common Mistakes:

• Omitting quotes for string props
• Using lowercase component names
• Leaving JSX tags unclosed

5. Given this parent and child component code, what will be the output?

function Child({ message }) {
  return <p>Message: {message || 'No message'}</p>;
}

function Parent() {
  const msg = '';
  return <Child message={msg} />;
}

hard

A. Message:

B. Error: message prop missing

C. Message: undefined

D. Message: No message

Solution

1. Step 1: Analyze the prop value passed

   The parent passes msg which is an empty string '' as the message prop.

2. Step 2: Understand the child component's rendering logic

   The child uses {message || 'No message'}. Since empty string is falsy, it falls back to 'No message'.

3. Final Answer:

   <p>Message: No message</p> -> Option D

4. Quick Check:

   Falsy empty string triggers fallback [OK]

Hint: Falsy props like '' use fallback values in JSX [OK]

Common Mistakes:

• Assuming empty string is truthy
• Expecting empty string to display as is
• Thinking missing prop causes error here