Sharing state between components in React - Mini Project: Build & Apply

Sharing state between components

📖 Scenario: You are building a simple React app that shows a counter. The counter value is shared between two components: one to display the count and one to increase it.This is like having a shared scoreboard where one person can see the score and another can update it.

🎯 Goal: Create two React functional components that share the same counter state. One component will show the current count, and the other will have a button to increase the count.

📋 What You'll Learn

Use React functional components

Use the useState hook to hold the counter value

Share the counter state between two components via props

Update the counter from the increment button component

Display the current counter value in the display component

💡 Why This Matters

🌍 Real World

Sharing state between components is common in React apps, like sharing user info or app settings across different parts of the UI.

💼 Career

Understanding how to share and update state between components is a key skill for React developers building interactive web apps.

1

Create the initial counter state

Create a React functional component called CounterApp. Inside it, create a state variable called count with initial value 0 using the useState hook.

React

import React, { useState } from 'react';

function CounterApp() {
  // Create count state with useState(0) here
  
}

Hint

Use const [count, setCount] = useState(0); inside the CounterApp function.

2

Create the display component

Inside CounterApp, create a new functional component called DisplayCount that accepts a prop called count and returns a <div> showing the text "Current count: " followed by the count value.

React

import React, { useState } from 'react';

function CounterApp() {
  const [count, setCount] = useState(0);

  // Create DisplayCount component here
}

Hint

Define DisplayCount inside CounterApp and use {count} inside the returned <div>.

3

Create the increment button component

Inside CounterApp, create a new functional component called IncrementButton that accepts a prop called onIncrement. It should return a <button> with the text "Increase". When clicked, it should call the onIncrement function.

React

import React, { useState } from 'react';

function CounterApp() {
  const [count, setCount] = useState(0);

  function DisplayCount({ count }) {
    return Current count: {count};
  }

  // Create IncrementButton component here
}

Hint

Use onClick={onIncrement} on the button inside IncrementButton.

4

Use the components and share state

Inside CounterApp's return statement, render <DisplayCount /> passing the count state as a prop, and <IncrementButton /> passing a function that increases count by 1 as the onIncrement prop.

React

import React, { useState } from 'react';

function CounterApp() {
  const [count, setCount] = useState(0);

  function DisplayCount({ count }) {
    return Current count: {count};
  }

  function IncrementButton({ onIncrement }) {
    return Increase;
  }

  // Render DisplayCount and IncrementButton here
}

Hint

Use <DisplayCount count={count} /> and <IncrementButton onIncrement={() => setCount(count + 1)} /> inside the return.

Practice

1. What is the main reason to share state between React components?

easy

A. To keep data consistent and allow multiple components to use the same information

B. To make each component have its own separate data

C. To avoid using props in components

D. To prevent components from rendering

Solution

1. Step 1: Understand the purpose of shared state

   Sharing state means keeping data in one place so multiple components can access and update it.

2. Step 2: Identify the benefit of shared state

   This helps keep the app consistent and easier to manage because all components see the same data.

3. Final Answer:

   To keep data consistent and allow multiple components to use the same information -> Option A

4. Quick Check:

   Sharing state = data consistency [OK]

Hint: Shared state means one source of truth for data [OK]

Common Mistakes:

• Thinking each component should have its own separate state always
• Confusing props with state sharing
• Believing sharing state prevents rendering

2. Which of the following is the correct way to pass state and its updater from a parent to a child component in React?

easy

A. <Child state=setState />

B. <Child state={state} setState={setState} />

C. <Child>{setState}</Child>

D. <Child state>{state}</Child>

Solution

1. Step 1: Recall how props are passed in JSX

   Props are passed as attributes with values, like propName={value}.

2. Step 2: Identify correct syntax for passing state and updater

   The parent passes both state and setState as props: <Child state={state} setState={setState} />.

3. Final Answer:

   <Child state={state} setState={setState} /> -> Option B

4. Quick Check:

   Props syntax = attribute=value [OK]

Hint: Props use attribute={value} syntax [OK]

Common Mistakes:

• Using children tags incorrectly to pass props
• Passing updater as a child instead of prop
• Missing curly braces around JavaScript expressions

3. Given this React code, what will be displayed when the button is clicked?

function Parent() {
  const [count, setCount] = React.useState(0);
  return <Child count={count} setCount={setCount} />;
}

function Child({ count, setCount }) {
  return <>
    <p>Count: {count}</p>
    <button onClick={() => setCount(count + 1)}>Increment</button>
  </>;
}

medium

A. Count: 0 initially, but button does nothing

B. Count: 1 initially, then stays the same on click

C. Count: 0 initially, then increases by 1 each click

D. Error because setCount is not defined

Solution

1. Step 1: Understand initial state and props

   The parent sets count to 0 initially and passes count and setCount to Child.

2. Step 2: Analyze button click behavior

   Clicking the button calls setCount(count + 1), increasing count by 1 each time.

3. Final Answer:

   Count: 0 initially, then increases by 1 each click -> Option C

4. Quick Check:

   useState count increments on click [OK]

Hint: State updates cause re-render with new value [OK]

Common Mistakes:

• Thinking count starts at 1
• Assuming button does not update state
• Confusing setCount usage or scope

4. Identify the error in this React code that tries to share state between components:

function Parent() {
  const [value, setValue] = React.useState('');
  return <Child value={value} setValue={value} />;
}

function Child({ value, setValue }) {
  return <input value={value} onChange={e => setValue(e.target.value)} />;
}

medium

A. value state is not initialized properly

B. onChange handler syntax is invalid

C. Child component should not receive props

D. setValue prop is passed incorrectly as value instead of the updater function

Solution

1. Step 1: Check props passed from Parent

   Parent passes setValue={value} instead of setValue={setValue}, so setValue is a string, not a function.

2. Step 2: Understand effect on Child component

   Child tries to call setValue as a function on input change, but it is a string, causing an error.

3. Final Answer:

   setValue prop is passed incorrectly as value instead of the updater function -> Option D

4. Quick Check:

   Updater function must be passed, not state value [OK]

Hint: Pass updater function, not state value, as setState prop [OK]

Common Mistakes:

• Passing state value instead of updater function
• Ignoring prop names mismatch
• Assuming onChange syntax is wrong

5. You want to share a counter state between two sibling components in React. Which approach correctly shares the state so both siblings update together?

function Parent() {
  const [count, setCount] = React.useState(0);
  return (
    <>
      <SiblingA count={count} setCount={setCount} />
      <SiblingB count={count} setCount={setCount} />
    </>
  );
}

What is the key reason this works?

hard

A. State is lifted to the common parent and passed down, so both siblings share the same source

B. Each sibling has its own state, so they update independently

C. Using context API is required for siblings to share state

D. State is stored inside each sibling, but synchronized automatically

Solution

1. Step 1: Understand state lifting

   The parent holds the state and updater, passing them as props to both siblings.

2. Step 2: Explain shared state effect

   Because both siblings receive the same state and updater, changes in one update the parent's state, causing both to re-render with the new value.

3. Final Answer:

   State is lifted to the common parent and passed down, so both siblings share the same source -> Option A

4. Quick Check:

   Lift state up for shared data [OK]

Hint: Lift state to parent to share between siblings [OK]

Common Mistakes:

• Thinking siblings can share state without lifting or context
• Believing context API is always required
• Assuming state sync happens automatically without shared source