Dependency array usage in React - Performance & Optimization

Performance: Dependency array usage

HIGH IMPACT

This affects how often React re-runs effects, impacting rendering speed and responsiveness.

Running a side effect only when specific values change

React

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

Effect runs only when userId changes, reducing unnecessary work and improving responsiveness.

📈 Performance Gaintriggers effect only on dependency change, reducing CPU load and improving INP

Running a side effect only when specific values change

React

useEffect(() => {
  // expensive operation
  fetchData();
});

Effect runs after every render, causing repeated expensive operations and blocking UI updates.

📉 Performance Costtriggers effect on every render, increasing CPU usage and blocking rendering frequently

Performance Comparison

Pattern	Effect Runs	CPU Usage	Render Blocking	Verdict
No dependency array	Runs every render	High	Blocks rendering frequently	[X] Bad
Empty dependency array	Runs once	Low	Minimal blocking	[OK] Good
Correct dependency array	Runs on relevant changes	Minimal	Minimal blocking	[OK] Good

Rendering Pipeline

React schedules effects after rendering. Dependency arrays control when effects run, affecting CPU work and paint timing.

→Commit phase

→Effect execution

→Paint

⚠️ BottleneckEffect execution causing CPU blocking and delayed paint

Core Web Vital Affected

INP

This affects how often React re-runs effects, impacting rendering speed and responsiveness.

Optimization Tips

1Always specify dependencies to control when effects run.

2Include all variables used inside the effect in the dependency array.

3Avoid empty or missing dependency arrays unless effect should run only once.

Performance Quiz - 3 Questions

What happens if you omit the dependency array in useEffect?

A Effect runs only once on mount.

B Effect never runs.

C Effect runs after every render, causing potential performance issues.

D Effect runs only when dependencies change.

DevTools: React DevTools Profiler

How to check: Record a profiling session while interacting with the component. Look for frequent effect executions and re-renders.

What to look for: High number of effect calls or re-renders indicates missing or incorrect dependency arrays.

Practice

1. What does an empty dependency array [] in a useEffect hook mean?

easy

A. The effect runs only when a state changes.

B. The effect runs after every render.

C. The effect never runs.

D. The effect runs only once after the first render.

Solution

1. Step 1: Understand the role of dependency array

   The dependency array tells React when to run the effect based on changes in listed variables.

2. Step 2: Interpret empty array meaning

   An empty array means no variables to watch, so React runs the effect only once after the first render.

3. Final Answer:

   The effect runs only once after the first render. -> Option D

4. Quick Check:

   Empty array = run once [OK]

Hint: Empty array means run effect once after mount [OK]

Common Mistakes:

• Thinking empty array runs effect every render
• Confusing empty array with no array
• Assuming effect never runs with empty array

2. Which of the following is the correct syntax for a useEffect hook that runs only once after the component mounts?

easy

A. useEffect(() => { console.log('Hello'); });

B. useEffect(() => { console.log('Hello'); }, []);

C. useEffect(() => { console.log('Hello'); }, [{}]);

D. useEffect(() => { console.log('Hello'); }, [() => {}]);

Solution

1. Step 1: Identify correct useEffect syntax

   The second argument is the dependency array controlling when the effect runs.

2. Step 2: Check which option uses empty array

   useEffect(() => { console.log('Hello'); }, []); uses an empty array, so effect runs once after mount, which is correct.

3. Final Answer:

   useEffect(() => { console.log('Hello'); }, []); -> Option B

4. Quick Check:

   Empty array syntax = useEffect(() => { console.log('Hello'); }, []); [OK]

Hint: Empty array in second argument runs effect once [OK]

Common Mistakes:

• Omitting dependency array runs effect every render
• Using non-constant values in dependency array incorrectly
• Confusing dependency array with effect callback

3. Consider this code:

const [count, setCount] = useState(0);
useEffect(() => {
  console.log('Count is', count);
}, [count]);

setCount(1);
setCount(2);

What will be logged to the console?

medium

A. Count is 0, Count is 2

B. Count is 0 only

C. Count is 2 only

D. Count is 0, Count is 1, Count is 2

Solution

1. Step 1: Understand effect dependency on count

   The effect runs after first render (count=0) and whenever count changes.

2. Step 2: Analyze state updates

   Two state updates happen quickly: setCount(1) then setCount(2). React batches them, so only final count=2 triggers effect after initial render.

3. Final Answer:

   Count is 0, Count is 2 -> Option A

4. Quick Check:

   Effect runs on initial and final count change [OK]

Hint: Effect runs after render with latest dependency value [OK]

Common Mistakes:

• Assuming effect runs for every intermediate state update
• Ignoring batching of state updates
• Thinking effect runs before first render

4. What is wrong with this useEffect usage?

const [count, setCount] = useState(0);
useEffect(() => {
  console.log(count);
}, []);

medium

A. Effect will run only once and log initial count, ignoring updates.

B. Effect will never run because dependency array is empty.

C. Effect will run on every render causing infinite loop.

D. Syntax error due to missing dependencies.

Solution

1. Step 1: Check dependency array usage

   Empty array means effect runs once after first render only.

2. Step 2: Understand effect behavior with state

   Effect logs count only once with initial value, ignoring later count changes.

3. Final Answer:

   Effect will run only once and log initial count, ignoring updates. -> Option A

4. Quick Check:

   Empty array ignores state changes [OK]

Hint: Empty array ignores state changes inside effect [OK]

Common Mistakes:

• Thinking effect runs on every count change
• Assuming empty array stops effect from running
• Confusing syntax error with logical error

5. You have a useEffect that depends on two props: userId and filter. Which dependency array is correct to ensure the effect runs only when these props change?

useEffect(() => {
  fetchData(userId, filter);
}, ???);

hard

A. []

B. [fetchData]

C. [userId, filter]

D. [userId]

Solution

1. Step 1: Identify variables used inside effect

   The effect uses userId and filter inside its callback.

2. Step 2: List all used variables in dependency array

   To rerun effect when either userId or filter changes, both must be in the dependency array.

3. Final Answer:

   [userId, filter] -> Option C

4. Quick Check:

   List all used variables in dependencies [OK]

Hint: Include all used variables in dependency array [OK]

Common Mistakes:

• Leaving dependency array empty causing stale data
• Including functions that don't change as dependencies
• Listing only some used variables