Why lifting state is needed in React - See It in Action

Why Lifting State Is Needed in React

📖 Scenario: Imagine you are building a simple React app where two components need to share and update the same information, like a temperature converter showing Celsius and Fahrenheit.

🎯 Goal: You will create two input components that share the same temperature value. You will lift the shared state up to a parent component so both inputs stay in sync when you type in either one.

📋 What You'll Learn

Create a parent component called TemperatureConverter with a state variable temperature initialized to an empty string.

Create two child components called CelsiusInput and FahrenheitInput that receive temperature and onTemperatureChange props.

In TemperatureConverter, write functions to convert temperatures between Celsius and Fahrenheit.

Lift the state up by storing the temperature in TemperatureConverter and passing it down to both inputs, so typing in one updates the other.

💡 Why This Matters

🌍 Real World

Many apps need to share data between components, like forms, filters, or settings panels. Lifting state helps keep data consistent and UI responsive.

💼 Career

Understanding lifting state is essential for React developers to build clean, maintainable, and bug-free user interfaces that share data properly.

1

Create the TemperatureConverter component with temperature state

Create a React functional component called TemperatureConverter. Inside it, create a state variable called temperature using useState and initialize it to an empty string "".

React

import React, { useState } from 'react';

function TemperatureConverter() {
  // Create temperature state here
  
  return (
    
      {/* Inputs will go here */}
    
  );
}

export default TemperatureConverter;

Hint

Use const [temperature, setTemperature] = useState("") inside the TemperatureConverter function.

2

Add CelsiusInput and FahrenheitInput components with props

Create two functional components called CelsiusInput and FahrenheitInput. Each should accept props temperature and onTemperatureChange. Render an input element of type text with its value set to temperature and onChange calling onTemperatureChange with the new value.

React

import React, { useState } from 'react';

function CelsiusInput({ temperature, onTemperatureChange }) {
  // Render input for Celsius
  
}

function FahrenheitInput({ temperature, onTemperatureChange }) {
  // Render input for Fahrenheit
  
}

function TemperatureConverter() {
  const [temperature, setTemperature] = useState("");

  return (
    
      {/* Inputs will go here */}
    
  );
}

export default TemperatureConverter;

Hint

Each input uses value={temperature} and calls onTemperatureChange with the new input value on change.

3

Add conversion functions and lift state in TemperatureConverter

Inside TemperatureConverter, add two functions: toCelsius and toFahrenheit that convert temperatures between Fahrenheit and Celsius. Add two handler functions: handleCelsiusChange and handleFahrenheitChange that update the temperature state with the converted value. Pass the correct temperature and handler props to CelsiusInput and FahrenheitInput so they stay in sync.

React

import React, { useState } from 'react';

function CelsiusInput({ temperature, onTemperatureChange }) {
  return (
    
      Enter temperature in Celsius:
       onTemperatureChange(e.target.value)}
        aria-label="Celsius temperature input"
      />
    
  );
}

function FahrenheitInput({ temperature, onTemperatureChange }) {
  return (
    
      Enter temperature in Fahrenheit:
       onTemperatureChange(e.target.value)}
        aria-label="Fahrenheit temperature input"
      />
    
  );
}

function TemperatureConverter() {
  const [temperature, setTemperature] = useState("");

  // Add conversion functions and handlers here

  return (
    
      {/* Render CelsiusInput and FahrenheitInput here */}
    
  );
}

export default TemperatureConverter;

Hint

Use conversion formulas and update the shared temperature state in the handlers. Pass converted values as props to inputs.

4

Complete the app with accessible labels and responsive layout

Add aria-label attributes to both input elements for accessibility. Wrap the inputs in semantic fieldset and legend tags. Add a container div with a simple flex style to place inputs side by side on wide screens and stacked on narrow screens.

React

import React, { useState } from 'react';

// Add CSS styles here or inline styles

function CelsiusInput({ temperature, onTemperatureChange }) {
  return (
    
      Enter temperature in Celsius:
       onTemperatureChange(e.target.value)}
        aria-label="Celsius temperature input"
      />
    
  );
}

function FahrenheitInput({ temperature, onTemperatureChange }) {
  return (
    
      Enter temperature in Fahrenheit:
       onTemperatureChange(e.target.value)}
        aria-label="Fahrenheit temperature input"
      />
    
  );
}

function TemperatureConverter() {
  const [temperature, setTemperature] = useState("");

  function toCelsius(fahrenheit) {
    return ((fahrenheit - 32) * 5) / 9;
  }

  function toFahrenheit(celsius) {
    return (celsius * 9) / 5 + 32;
  }

  function handleCelsiusChange(value) {
    setTemperature(value);
  }

  function handleFahrenheitChange(value) {
    const celsius = toCelsius(Number(value));
    if (!isNaN(celsius)) {
      setTemperature(celsius.toFixed(2));
    } else {
      setTemperature("");
    }
  }

  return (
    
      {/* Add container div with flex style here */}
    
  );
}

export default TemperatureConverter;

Hint

Use aria-label on inputs and wrap them in fieldset and legend. Add a flex container style to the parent div.

Practice

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

easy

A. To improve the styling of components

B. To share the same state between multiple child components

C. To avoid using props in child components

D. To make each child component have its own separate state

Solution

1. Step 1: Understand what lifting state means

   Lifting state means moving state from child components to their common parent.

2. Step 2: Identify the purpose of lifting state

   This is done so multiple children can share and update the same state, keeping data consistent.

3. Final Answer:

   To share the same state between multiple child components -> Option B

4. Quick Check:

   Sharing state = lifting state [OK]

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

Common Mistakes:

• Thinking lifting state is for styling
• Believing lifting state means separate states
• Confusing lifting state with removing props

2. Which of the following is the correct way to lift state up in React?

easy

A. Use global variables instead of React state

B. Keep the state inside each child component separately

C. Move the state to the nearest common parent and pass it down as props

D. Remove state and use only static values

Solution

1. Step 1: Recall the pattern of lifting state

   State should be moved to the nearest common parent component to share it.

2. Step 2: Check the options for correct syntax

   Passing state down as props from the parent is the correct React pattern.

3. Final Answer:

   Move the state to the nearest common parent and pass it down as props -> Option C

4. Quick Check:

   Lift state = parent state + props down [OK]

Hint: Lift state to parent, pass as props down [OK]

Common Mistakes:

• Keeping state only in children
• Using global variables instead of React state
• Removing state completely

3. Consider this React code snippet:

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

function ChildA({ count }) {
  return <div>Count: {count}</div>;
}

function ChildB({ setCount }) {
  return <button onClick={() => setCount(c => c + 1)}>Increment</button>;
}

What will happen when the button in ChildB is clicked?

medium

A. Nothing will change because state is not shared

B. The button will disappear

C. An error will occur because setCount is not passed correctly

D. The count displayed in ChildA will increase by 1

Solution

1. Step 1: Understand state lifting in the code

   The state count is in Parent and passed down to ChildA and ChildB.

2. Step 2: Analyze what happens on button click

   Clicking button calls setCount which updates count in Parent, causing ChildA to re-render with new count.

3. Final Answer:

   The count displayed in ChildA will increase by 1 -> Option D

4. Quick Check:

   Shared state update re-renders children [OK]

Hint: State in parent updates all children using it [OK]

Common Mistakes:

• Thinking state is local to ChildB only
• Assuming no re-render happens on state change
• Confusing props with state setters

4. You have two sibling components that need to share a counter state. You put the state inside one sibling and try to pass it to the other directly. Why is this a problem?

medium

A. Because siblings cannot directly share state; state must be lifted to their parent

B. Because React does not allow state in functional components

C. Because state can only be stored in global variables

D. Because props cannot be passed between siblings

Solution

1. Step 1: Understand React state sharing rules

   State lives inside a component and cannot be directly shared between sibling components.

2. Step 2: Identify correct sharing method

   To share state, it must be lifted to the nearest common parent, which passes it down as props.

3. Final Answer:

   Because siblings cannot directly share state; state must be lifted to their parent -> Option A

4. Quick Check:

   Siblings share state via parent [OK]

Hint: Siblings share state only via parent component [OK]

Common Mistakes:

• Thinking siblings can share state directly
• Believing React forbids state in functional components
• Confusing props passing between siblings

5. You have three components: Grandparent, Parent, and two siblings Child1 and Child2. Both children need to update and display the same piece of state. Where should you lift the state to and why?

hard

A. Lift the state to Parent because it is the nearest common ancestor of the children

B. Lift the state to Grandparent to make it global for all components

C. Keep the state inside Child1 and pass it to Child2 directly

D. Create separate states in both children and sync them manually

Solution

1. Step 1: Identify the nearest common ancestor of the siblings

   The siblings Child1 and Child2 share the Parent component as their closest common parent.

2. Step 2: Decide where to lift state for sharing

   Lifting state to Parent allows both children to access and update the same state easily without unnecessary complexity.

3. Final Answer:

   Lift the state to Parent because it is the nearest common ancestor of the children -> Option A

4. Quick Check:

   Lift state to nearest common parent [OK]

Hint: Lift state to nearest common parent of siblings [OK]

Common Mistakes:

• Lifting state too high unnecessarily
• Trying to share state directly between siblings
• Duplicating state in multiple components