Dependency array usage in React - Mini Project: Build & Apply

Understanding Dependency Array Usage in React useEffect

📖 Scenario: You are building a simple React component that shows a counter and updates the document title whenever the counter changes.

🎯 Goal: Create a React functional component that uses useEffect with a dependency array to update the document title only when the counter changes.

📋 What You'll Learn

Create a state variable called count initialized to 0 using useState

Create a variable called increment that increases count by 1 when called

Use useEffect with a dependency array containing count to update document.title

Render a button that calls increment when clicked and displays the current count

💡 Why This Matters

🌍 Real World

Updating the document title based on user interaction is common in web apps to show notifications or status.

💼 Career

Understanding useEffect and dependency arrays is essential for React developers to manage side effects efficiently and avoid bugs.

1

Set up the initial React component with state

Create a React functional component called Counter. Inside it, use useState to create a state variable called count initialized to 0.

React

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

function Counter() {
    // Create state variable count with initial value 0
    // Your code here
}

Hint

Use const [count, setCount] = useState(0); inside the Counter function.

2

Add an increment function to update the count

Inside the Counter component, create a function called increment that calls setCount to increase count by 1.

React

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

function Counter() {
    const [count, setCount] = useState(0);
    // Create increment function to add 1 to count
    // Your code here
}

Hint

Define increment as an arrow function that calls setCount(count + 1).

3

Use useEffect with dependency array to update document title

Inside the Counter component, add a useEffect hook that updates document.title to `Count: ${count}`. Include count in the dependency array.

React

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

function Counter() {
    const [count, setCount] = useState(0);
    const increment = () => setCount(count + 1);
    // Add useEffect to update document.title when count changes
    // Your code here
}

Hint

Use useEffect(() => { document.title = `Count: ${count}`; }, [count]); to update the title only when count changes.

4

Render a button to increment count and display count value

Inside the Counter component, return a <button> element that shows the text Count: {count} and calls increment when clicked.

React

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

function Counter() {
    const [count, setCount] = useState(0);
    const increment = () => setCount(count + 1);

    useEffect(() => {
        document.title = `Count: ${count}`;
    }, [count]);

    // Return a button that shows count and calls increment on click
    // Your code here
}

Hint

Return a button element with onClick={increment} and text Count: {count}.

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