Parent-child data flow in React

Introduction

Parent-child data flow helps components share information clearly. The parent sends data down to the child, so the child can use or show it.

When a parent component needs to give information to a child component to display.

When you want to control a child component's behavior from the parent.

When you want to keep data in one place and share it with many children.

When you want to update the child component when the parent data changes.

Syntax

React

function Parent() {
  const data = 'Hello from parent';
  return <Child message={data} />;
}

function Child(props) {
  return <p>{props.message}</p>;
}

The parent passes data as props to the child.

The child receives data via the props object.

Examples

Parent sends a userName prop to child, which shows a welcome message.

React

function Parent() {
  const name = 'Alice';
  return <Child userName={name} />;
}

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

Using destructuring in child props to get number directly.

React

function Parent() {
  const count = 5;
  return <Child number={count} />;
}

function Child({ number }) {
  return <p>Count is {number}</p>;
}

Sample Program

This example shows a parent component sending a greeting message to its child. The child displays the message inside a paragraph.

React

import React from 'react';

function Parent() {
  const greeting = 'Hi there!';
  return (
    <div>
      <h2>Parent Component</h2>
      <Child message={greeting} />
    </div>
  );
}

function Child(props) {
  return (
    <div>
      <h3>Child Component</h3>
      <p>Message from parent: {props.message}</p>
    </div>
  );
}

export default Parent;

Important Notes

Data flows only one way: from parent to child.

Children cannot change props; they are read-only.

Use props to keep components simple and reusable.

Summary

Parent sends data to child using props.

Child receives data via props and uses it to render.

This keeps data flow clear and easy to follow.

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