Dependency array usage in React

Introduction

The dependency array tells React when to run some code again. It helps avoid running code too many times or missing updates.

When you want to run code only once when a component loads.

When you want to run code again only if some data changes.

When you want to avoid running code on every render to save time.

When you want to update something after a user changes a value.

When you want to fetch new data only if a filter or input changes.

Syntax

React

useEffect(() => {
  // code to run
}, [dependency1, dependency2]);

The array at the end is called the dependency array.

React runs the code inside useEffect only if one of the dependencies changes.

Examples

Empty array means run only once when the component appears.

React

useEffect(() => {
  console.log('Runs once when component mounts');
}, []);

Runs only when the count value changes.

React

useEffect(() => {
  console.log('Runs when count changes:', count);
}, [count]);

No dependency array means run on every render (not recommended usually).

React

useEffect(() => {
  console.log('Runs every render because no array');
});

Runs when either user or theme changes.

React

useEffect(() => {
  console.log('Runs when user or theme changes');
}, [user, theme]);

Sample Program

This component shows how dependency arrays control when effects run:

• The first effect runs only once when the component appears.
• The second effect runs only when count changes.
• The third effect runs only when name changes.

Try clicking the button or typing in the input to see console logs.

React

import React, { useState, useEffect } from 'react';

function Counter() {
  const [count, setCount] = useState(0);
  const [name, setName] = useState('');

  useEffect(() => {
    console.log('Component mounted');
  }, []);

  useEffect(() => {
    console.log(`Count changed to: ${count}`);
  }, [count]);

  useEffect(() => {
    console.log(`Name changed to: ${name}`);
  }, [name]);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increase Count</button>
      <p>Name: {name}</p>
      <input
        type="text"
        value={name}
        onChange={e => setName(e.target.value)}
        aria-label="Name input"
      />
    </div>
  );
}

export default Counter;

Important Notes

Time complexity: Effects run only when dependencies change, so efficient updates.

Space complexity: Minimal, just stores dependencies internally.

Common mistake: Forgetting to add dependencies causes effects to not update when expected.

Use dependency arrays to control when side effects run and avoid unnecessary work.

Summary

The dependency array controls when React runs your effect code.

Empty array means run once; no array means run every render.

Always list all variables your effect uses in the dependency array.

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