Common lifting state patterns in React - Mini Project: Build & Apply

Common Lifting State Patterns in React

📖 Scenario: You are building a simple React app where two sibling components need to share and update the same data. This is a common situation where lifting state up helps keep data consistent.

🎯 Goal: Build two sibling components that share a counter value. The counter state should live in their common parent. One sibling increments the counter, and the other displays the current count.

📋 What You'll Learn

Create a parent component called CounterParent with a state variable count initialized to 0

Create a child component called Incrementer that has a button to increase count by 1

Create a child component called DisplayCount that shows the current count value

Lift the count state up to CounterParent and pass necessary props to children

💡 Why This Matters

🌍 Real World

Many React apps need multiple components to share and update the same data. Lifting state up is a simple and common pattern to keep data consistent and synchronized.

💼 Career

Understanding lifting state is essential for React developers to build maintainable and scalable user interfaces where components communicate effectively.

1

Create the parent component with initial state

Create a React functional component called CounterParent. Inside it, use useState to create a state variable called count initialized to 0. Return a div with the text Parent Component.

React

import React, { useState } from 'react';

function CounterParent() {
  // Your code here
}

export default CounterParent;

Hint

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

2

Add child components Incrementer and DisplayCount

Inside the same file, create two new functional components: Incrementer and DisplayCount. For now, have Incrementer return a button with text Increment. Have DisplayCount return a div with text Count: 0. Update CounterParent to render both components inside a div.

React

import React, { useState } from 'react';

function CounterParent() {
  const [count, setCount] = useState(0);
  return (
    
      {/* Render Incrementer and DisplayCount here */}
    
  );
}

function Incrementer() {
  // Your code here
}

function DisplayCount() {
  // Your code here
}

export default CounterParent;

Hint

Define two new functions Incrementer and DisplayCount that return the specified JSX elements.

3

Lift state: Pass count and setCount as props

Modify CounterParent to pass count as a prop named count to DisplayCount. Pass setCount as a prop named setCount to Incrementer. Update DisplayCount to accept count as a prop and display it inside the div. Update Incrementer to accept setCount as a prop and add an onClick handler to the button that calls setCount with count => count + 1.

React

import React, { useState } from 'react';

function CounterParent() {
  const [count, setCount] = useState(0);
  return (
    
      {/* Pass setCount to Incrementer and count to DisplayCount */}
    
  );
}

function Incrementer({ setCount }) {
  // Your code here
}

function DisplayCount({ count }) {
  // Your code here
}

export default CounterParent;

Hint

Pass props from CounterParent and use them inside the child components.

4

Add accessibility and final touches

Add an aria-label attribute with value Increment counter to the button in Incrementer for accessibility. Wrap the two child components inside a section element with aria-live="polite" in CounterParent to announce count changes to screen readers.

React

import React, { useState } from 'react';

function CounterParent() {
  const [count, setCount] = useState(0);
  return (
    
      {/* Your code here */}
    
  );
}

function Incrementer({ setCount }) {
  return  setCount(count => count + 1)}>Increment;
}

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

export default CounterParent;

Hint

Use aria-label on the button and wrap children in a section with aria-live="polite".

Practice

1. What is the main reason to lift state up in React?
lifting state means moving state to a common parent component.

easy

A. To make components independent

B. To share state between sibling components

C. To avoid using props

D. To store state only in child components

Solution

1. Step 1: Understand lifting state concept

   Lifting state means moving state to a common parent so multiple children can access it.

2. Step 2: Identify the purpose of lifting state

   This is done to share state between sibling components and keep data in sync.

3. Final Answer:

   To share state between sibling components -> Option B

4. Quick Check:

   Lifting state = sharing state [OK]

Hint: Lifting state shares data between siblings via parent [OK]

Common Mistakes:

• Thinking lifting state makes components independent
• Believing lifting state removes props usage
• Assuming state stays only in children

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

easy

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

B. <Child state=setState />

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

D. <Child state=setState() />

Solution

1. Step 1: Recall React props syntax

   Props are passed as name={value}. To pass state and updater, use state={state} and setState={setState}.

2. Step 2: Check each option's syntax

   <Child state={state} setState={setState} /> correctly passes both as props. Others misuse assignment or call functions incorrectly.

3. Final Answer:

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

4. Quick Check:

   Props pass state and updater correctly [OK]

Hint: Props use curly braces for variables [OK]

Common Mistakes:

• Passing functions without curly braces
• Calling updater functions instead of passing them
• Swapping state and updater names

3. Given this parent component code:

function Parent() {
  const [count, setCount] = React.useState(0);
  return (
    <>
      <Display count={count} />
      <Button onClick={() => setCount(count + 1)} />
    </>
  );
}

What will happen when the Button is clicked?

medium

A. Nothing happens because count is not updated

B. The Button component will re-render but count stays the same

C. The count displayed by Display increases by 1

D. An error occurs because setCount is not passed to Button

Solution

1. Step 1: Understand state and props flow

   The Parent holds count state and passes count to Display and an onClick handler to Button.

2. Step 2: Analyze Button click effect

   Clicking Button calls setCount(count + 1), updating state and causing Parent to re-render with new count.

3. Final Answer:

   The count displayed by Display increases by 1 -> Option C

4. Quick Check:

   State update triggers re-render and display update [OK]

Hint: State update in parent re-renders children with new props [OK]

Common Mistakes:

• Thinking Button needs count prop to update
• Believing setCount must be passed to Display
• Assuming no re-render happens on state change

4. Identify the error in this code snippet where a parent tries to lift state:

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

medium

A. setValue prop is incorrectly assigned the state value instead of the updater function

B. value state is not initialized properly

C. Child component is missing parentheses

D. React.useState should be React.state

Solution

1. Step 1: Check props passed to Child

   Parent passes value={value} correctly but setValue={value} incorrectly passes the state value instead of the updater function.

2. Step 2: Understand correct usage

   setValue should be passed as setValue={setValue} to allow Child to update state.

3. Final Answer:

   setValue prop is incorrectly assigned the state value instead of the updater function -> Option A

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:

• Confusing state variable with updater function
• Forgetting curly braces around variables
• Assuming React.useState is incorrect

5. You have two sibling components, InputA and InputB, both need to update and display the same text value. Which pattern correctly lifts state to their parent to keep them in sync?

hard

A. Use local variables in siblings without state

B. Store text state separately in each sibling, update independently

C. Store text state in one sibling and pass it down to the other sibling

D. Store text state in parent, pass text and setText to both siblings as props

Solution

1. Step 1: Understand shared state need

   Both siblings must show and update the same text, so state must be shared.

2. Step 2: Apply lifting state pattern

   Lift text state to their common parent and pass text and setText as props to both siblings.

3. Step 3: Evaluate other options

   Storing state separately or in one sibling breaks sync; local variables don't persist or trigger re-renders.

4. Final Answer:

   Store text state in parent, pass text and setText to both siblings as props -> Option D

5. Quick Check:

   Lift state to parent for shared updates [OK]

Hint: Lift shared state to parent, pass state and updater down [OK]

Common Mistakes:

• Keeping separate states in siblings causing out-of-sync
• Passing state only one way between siblings
• Using local variables instead of state