Why lifting state is needed in React - Performance Evidence

Performance: Why lifting state is needed

MEDIUM IMPACT

This affects how efficiently React components update and share data, impacting interaction responsiveness and rendering speed.

Sharing state between sibling components

React

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

// State is lifted and shared, keeping data consistent

Single source of truth reduces redundant renders and keeps components in sync efficiently.

📈 Performance GainReduces re-renders and improves INP by minimizing state updates

Sharing state between sibling components

React

function Parent() {
  const [countA, setCountA] = React.useState(0);
  const [countB, setCountB] = React.useState(0);
  return (
    <>
      <ComponentA count={countA} setCount={setCountA} />
      <ComponentB count={countB} setCount={setCountB} />
    </>
  );
}

// ComponentA and ComponentB cannot share state easily

State is duplicated in parent for each component, causing inconsistent data and extra renders when syncing is needed.

📉 Performance CostTriggers multiple re-renders and state updates, increasing INP latency

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Duplicated state in multiple parents	Multiple state updates causing many DOM changes	Triggers reflows for each component update	Higher paint cost due to frequent updates	[X] Bad
Lifted state in common ancestor	Single state update triggers minimal DOM changes	Single reflow per update	Lower paint cost with batched updates	[OK] Good

Rendering Pipeline

When state is lifted, React updates a single state source causing fewer component re-renders. This reduces the number of times React recalculates the virtual DOM and applies changes to the real DOM.

→Reconciliation

→Commit (DOM Update)

⚠️ BottleneckExcessive re-renders due to duplicated or scattered state

Core Web Vital Affected

INP

This affects how efficiently React components update and share data, impacting interaction responsiveness and rendering speed.

Optimization Tips

1Lift shared state to the closest common ancestor component.

2Avoid duplicating state across sibling components to prevent redundant renders.

3Use React DevTools Profiler to identify unnecessary re-renders caused by scattered state.

Performance Quiz - 3 Questions

What is a main performance benefit of lifting state in React?

A Increases bundle size

B Reduces unnecessary component re-renders

C Triggers more DOM reflows

D Blocks rendering on every input

DevTools: React DevTools Profiler

How to check: Record interactions and observe component re-render counts and durations when state changes.

What to look for: Look for fewer re-renders and shorter update times when state is lifted versus duplicated.

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