Common lifting state patterns in React - Performance & Optimization

Performance: Common lifting state patterns

MEDIUM IMPACT

This affects interaction responsiveness and rendering speed by controlling how often components re-render when state changes.

Sharing state between sibling components

React

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

const ChildA = React.memo(({ setCount }) => {
  console.log('ChildA rendered');
  return <button onClick={() => setCount(c => c + 1)}>Increment</button>;
});

const ChildB = React.memo(({ count }) => {
  console.log('ChildB rendered');
  return <div>{count}</div>;
});

Using React.memo and passing only needed props reduces unnecessary re-renders, improving responsiveness.

📈 Performance GainEach child re-renders only when its relevant props change; reduces re-renders from 2 to 1 per update.

Sharing state between sibling components

React

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

function ChildA({ count, setCount }) {
  return <button onClick={() => setCount(count + 1)}>Increment</button>;
}

function ChildB({ count }) {
  return <div>{count}</div>;
}

Lifting state to the immediate parent causes both children to re-render on every state change, even if only one needs updating.

📉 Performance CostTriggers 2 re-renders per update; increases interaction latency.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Lifting state too high	Many components re-render	Multiple reflows per update	High paint cost due to many updates	[X] Bad
Lifting state minimally	Only necessary components re-render	Single reflow per update	Low paint cost	[OK] Good

Rendering Pipeline

When state is lifted too high, React re-renders many components unnecessarily, causing more layout and paint work. Optimizing state placement reduces the number of components React must update.

→Reconciliation

→Layout

→Paint

⚠️ BottleneckReconciliation stage due to excessive component re-renders

Core Web Vital Affected

INP

This affects interaction responsiveness and rendering speed by controlling how often components re-render when state changes.

Optimization Tips

1Lift state only to the nearest common ancestor that needs it.

2Use React.memo to prevent unnecessary re-renders of child components.

3Avoid lifting state higher than necessary to reduce re-render scope.

Performance Quiz - 3 Questions

What is a performance downside of lifting state too high in a React component tree?

A State becomes inaccessible to child components

B Unnecessary re-renders of many components on state change

C The app bundle size increases significantly

D CSS styles fail to apply correctly

DevTools: React DevTools Profiler

How to check: Record a profiling session while interacting with the component. Look for which components re-render on state changes.

What to look for: Components re-rendering unnecessarily indicate poor state lifting. Aim for minimal re-renders on input or state updates.

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