Effect execution timing in React - Performance & Optimization

Performance: Effect execution timing

MEDIUM IMPACT

This affects how quickly side effects run after rendering, impacting interaction responsiveness and visual updates.

Running side effects after every render regardless of dependencies

React

useEffect(() => {
  fetchData();
}, []);

Effect runs only once after initial render, reducing unnecessary work and improving responsiveness.

📈 Performance GainSingle reflow and paint, reduces blocking time by 90%

Running side effects after every render regardless of dependencies

React

useEffect(() => {
  fetchData();
});

Effect runs after every render, causing repeated network requests and blocking main thread.

📉 Performance CostTriggers multiple reflows and repaints, blocks rendering for 50-100ms per fetch

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
useEffect with empty deps	Minimal, runs once	Single reflow	Low paint cost	[OK] Good
useEffect without deps	Runs every render	Multiple reflows	High paint cost	[X] Bad
useLayoutEffect heavy work	Blocks DOM updates	Blocks reflow	Blocks paint	[X] Bad
useEffect heavy work	Runs after paint	No blocking reflow	Paint not blocked	[!] OK

Rendering Pipeline

Effect execution timing determines when side effects run relative to rendering stages, influencing layout, paint, and user interaction responsiveness.

→Commit phase

→Layout

→Paint

⚠️ BottleneckBlocking effects during commit phase delay paint and user interaction readiness.

Core Web Vital Affected

INP

This affects how quickly side effects run after rendering, impacting interaction responsiveness and visual updates.

Optimization Tips

1Use useEffect for side effects that don't block rendering.

2Always specify dependencies to avoid repeated effect executions.

3Avoid heavy computations inside useLayoutEffect to prevent blocking paint.

Performance Quiz - 3 Questions

Which hook timing is best to avoid blocking the initial paint?

A useLayoutEffect

B useEffect

C useState

D useMemo

DevTools: Performance

How to check: Record a performance profile while interacting with the app. Look for long tasks during commit phase and effect execution timing.

What to look for: Check if effects run before or after paint and if they cause long blocking tasks delaying interaction readiness.

Practice

1. In React, when does the useEffect hook run its effect function by default?

easy

A. Only when the component unmounts

B. Before the component renders

C. Only once when the component mounts

D. After every render of the component

Solution

1. Step 1: Understand default useEffect behavior

   By default, useEffect runs after every render, including the first render and all updates.

2. Step 2: Compare with other lifecycle timings

   It does not run before render or only once unless dependencies are specified.

3. Final Answer:

   After every render of the component -> Option D

4. Quick Check:

   Default effect runs after render [OK]

Hint: Default useEffect runs after every render unless dependencies given [OK]

Common Mistakes:

• Thinking useEffect runs before render
• Assuming it runs only once without dependencies
• Confusing cleanup timing with effect timing

2. Which of the following is the correct syntax to run a useEffect only once when the component mounts?

easy

A. useEffect(() => { /* effect */ }, [true])

B. useEffect(() => { /* effect */ })

C. useEffect(() => { /* effect */ }, [])

D. useEffect(() => { /* effect */ }, [null])

Solution

1. Step 1: Identify dependency array usage

   An empty array [] means the effect runs only once after the first render (mount).

2. Step 2: Check other options

   Omitting dependencies runs every render; non-empty arrays cause effect to run on changes.

3. Final Answer:

   useEffect(() => { /* effect */ }, []) -> Option C

4. Quick Check:

   Empty dependency array = run once on mount [OK]

Hint: Empty array runs effect once on mount [OK]

Common Mistakes:

• Omitting dependency array runs effect every render
• Using non-empty arrays thinking it runs once
• Confusing null or true as empty array

3. Consider this React component:

function Counter() {
  const [count, setCount] = React.useState(0);
  React.useEffect(() => {
    console.log('Effect:', count);
    return () => console.log('Cleanup:', count);
  }, [count]);
  return  setCount(count + 1)}>Increment;
}

What will be logged to the console after clicking the button twice?

medium

A. Effect: 0, Cleanup: 0, Effect: 1, Cleanup: 1, Effect: 2

B. Effect: 0, Effect: 1, Effect: 2

C. Effect: 0, Cleanup: 0, Cleanup: 1, Effect: 2

D. Cleanup: 0, Cleanup: 1, Effect: 2

Solution

1. Step 1: Understand effect and cleanup with dependencies

   Effect runs after mount and after each count change. Cleanup runs before effect re-runs or unmount.

2. Step 2: Trace logs on two clicks

   Initial mount: Effect: 0
   First click: Cleanup: 0, Effect: 1
   Second click: Cleanup: 1, Effect: 2

3. Final Answer:

   Effect: 0, Cleanup: 0, Effect: 1, Cleanup: 1, Effect: 2 -> Option A

4. Quick Check:

   Effect runs after render; cleanup before next effect [OK]

Hint: Cleanup runs before next effect when dependencies change [OK]

Common Mistakes:

• Ignoring cleanup logs
• Assuming cleanup runs after effect
• Missing initial effect run on mount

4. What is wrong with this React effect code?

React.useEffect(() => {
  const id = setInterval(() => console.log('Tick'), 1000);
  return clearInterval(id);
}, []);

medium

A. The effect will run multiple times because dependencies are missing

B. The cleanup function should be a function, not the result of clearInterval

C. setInterval should be replaced with setTimeout

D. The effect will never run because of missing dependencies

Solution

1. Step 1: Check cleanup function syntax

   The cleanup must be a function returning a function, but here it calls clearInterval immediately.

2. Step 2: Correct cleanup usage

   It should return a function: return () => clearInterval(id); to run cleanup on unmount.

3. Final Answer:

   The cleanup function should be a function, not the result of clearInterval -> Option B

4. Quick Check:

   Cleanup must be a function returning a function [OK]

Hint: Cleanup must be a function, not a direct call [OK]

Common Mistakes:

• Returning cleanup call instead of function
• Confusing dependencies with cleanup
• Thinking setInterval needs setTimeout

5. You want a React effect to run only when either propA or stateB changes, but not on every render. Which dependency array is correct?

React.useEffect(() => {
  // effect code
}, ???);

hard

A. [propA, stateB]

B. []

C. [propA || stateB]

D. [propA && stateB]

Solution

1. Step 1: Understand dependency array purpose

   The effect runs when any value in the array changes compared to last render.

2. Step 2: Analyze options

   [propA, stateB] lists both dependencies separately, so effect runs if either changes.
   Options C and D use logical operators, which evaluate to single values and won't track changes properly.
   [] runs effect only once on mount.

3. Final Answer:

   [propA, stateB] -> Option A

4. Quick Check:

   List all dependencies separately in array [OK]

Hint: List each dependency separately in array to track changes [OK]

Common Mistakes:

• Using logical operators inside dependency array
• Leaving dependency array empty when dependencies exist
• Assuming single combined value tracks all changes