Parent-child data flow in React - Mini Project: Build & Apply

Parent-child data flow in React

📖 Scenario: You are building a simple React app where a parent component holds a message and passes it to a child component to display.

🎯 Goal: Create a parent component that stores a message string and passes it as a prop to a child component. The child component should display the message inside a paragraph.

📋 What You'll Learn

Create a parent component called Parent with a message state variable

Create a child component called Child that accepts a message prop

Pass the message from Parent to Child as a prop

Render the Child component inside Parent

Display the message inside a <p> tag in Child

💡 Why This Matters

🌍 Real World

Passing data from parent to child components is a fundamental pattern in React apps for building reusable UI parts.

💼 Career

Understanding parent-child data flow is essential for React developers to manage component communication and state effectively.

1

Create the Parent component with a message state

Create a React functional component called Parent. Inside it, create a state variable called message using useState and set its initial value to "Hello from Parent".

React

import React, { useState } from 'react';

function Parent() {
  // Your code here
}

Hint

Use useState to create the message state inside Parent.

2

Create the Child component that accepts a message prop

Create a React functional component called Child that accepts a prop named message. Inside the component, return a <p> element that displays the message prop.

React

import React, { useState } from 'react';

function Parent() {
  const [message, setMessage] = useState("Hello from Parent");
  return null;
}

// Your code here

Hint

Use destructuring in the function parameter to get the message prop.

3

Pass the message prop from Parent to Child

Inside the Parent component's return statement, render the Child component and pass the message state as a prop named message.

React

import React, { useState } from 'react';

function Parent() {
  const [message, setMessage] = useState("Hello from Parent");
  // Your code here
}

function Child({ message }) {
  return {message};
}

Hint

Use JSX to render <Child message={message} /> inside Parent.

4

Export the Parent component as default

Add an export statement to export the Parent component as the default export of the module.

React

import React, { useState } from 'react';

function Parent() {
  const [message, setMessage] = useState("Hello from Parent");
  return ;
}

function Child({ message }) {
  return {message};
}

// Your code here

Hint

Use export default Parent; at the end of the file.

Practice

1. In React, how does a parent component send data to its child component?

easy

A. By importing the child's variables

B. By passing data as props to the child component

C. By using the child's setState method

D. By directly modifying the child's state

Solution

1. Step 1: Understand data flow in React

   React uses one-way data flow where parents send data down to children.

2. Step 2: Identify how data is passed

   Parents pass data to children using props, which are like function arguments.

3. Final Answer:

   By passing data as props to the child component -> Option B

4. Quick Check:

   Parent to child data flow = props [OK]

Hint: Remember: props flow down from parent to child [OK]

Common Mistakes:

• Trying to change child's state directly from parent
• Using child's setState from parent
• Importing child's variables instead of passing props

2. Which of the following is the correct syntax to pass a prop named title with value "Hello" from a parent to a child component Child?

easy

A. <Child>title="Hello"</Child>

B. <Child {title: "Hello"} />

C. <Child title="Hello" />

D. <Child>{title: "Hello"}</Child>

Solution

1. Step 1: Recall JSX prop syntax

   Props are passed as attributes inside the component tag, like HTML attributes.

2. Step 2: Match correct syntax

   The correct way is <Child title="Hello" /> which sets prop title to "Hello".

3. Final Answer:

   <Child title="Hello" /> -> Option C

4. Quick Check:

   Props look like HTML attributes [OK]

Hint: Props look like HTML attributes inside component tags [OK]

Common Mistakes:

• Using object syntax inside JSX tags
• Placing props inside children instead of attributes
• Using curly braces incorrectly for props

3. Given the code below, what will be rendered inside the <Child /> component?

function Parent() {
  const message = "Hi there!";
  return <Child text={message} />;
}

function Child(props) {
  return <h1>{props.text}</h1>;
}

medium

A. Hi there!

B. text

C. {message}

D. Nothing will render

Solution

1. Step 1: Identify prop passed from Parent

   Parent passes prop named text with value "Hi there!" to Child.

2. Step 2: Check Child rendering

   Child renders <h1> with content from props.text, which is "Hi there!".

3. Final Answer:

   <h1>Hi there!</h1> -> Option A

4. Quick Check:

   props.text = "Hi there!" renders inside <h1> [OK]

Hint: Child renders what parent sends via props [OK]

Common Mistakes:

• Confusing prop name with string literal
• Expecting variable name to render instead of value
• Thinking nothing renders without explicit return

4. What is wrong with the following code that tries to pass data from parent to child?

function Parent() {
  const data = "Hello";
  return <Child data=data />;
}

function Child(props) {
  return <div>{props.data}</div>;
}

medium

A. Parent should use state instead of variable

B. Child component cannot receive props named data

C. Child must destructure props before use

D. The prop value should be inside curly braces: data={data}

Solution

1. Step 1: Check JSX prop syntax

   In JSX, to pass a JavaScript variable as prop value, use curly braces.

2. Step 2: Identify error in code

   Code uses data=data without braces, so it passes string "data" instead of variable value.

3. Final Answer:

   The prop value should be inside curly braces: data={data} -> Option D

4. Quick Check:

   JSX props need curly braces for variables [OK]

Hint: Use curly braces to pass variables as props [OK]

Common Mistakes:

• Omitting curly braces around variables in JSX props
• Thinking child cannot receive certain prop names
• Believing state is required to pass props

5. You want to pass a list of items from a parent to a child component and render them as a list. Which approach correctly implements this parent-child data flow?

function Parent() {
  const items = ["Apple", "Banana", "Cherry"];
  return <Child items={items} />;
}

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

hard

A. This code correctly passes the list and renders it in the child

B. The parent should convert the list to a string before passing

C. Child should not use map; it should render items directly

D. Key prop is not needed when rendering lists

Solution

1. Step 1: Check how data is passed

   Parent passes array items as prop named items using curly braces, which is correct.

2. Step 2: Check child rendering

   Child uses map to create <li> elements for each item with unique key, which is best practice.

3. Final Answer:

   This code correctly passes the list and renders it in the child -> Option A

4. Quick Check:

   Array props + map in child = correct list rendering [OK]

Hint: Pass arrays as props and map in child to render lists [OK]

Common Mistakes:

• Passing arrays as strings instead of arrays
• Not using keys when rendering lists
• Trying to render arrays directly without map