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nicko
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Beginner
Asked: 2025-02-20T00:57:13+00:00 In:

How do you handle side effects in React?

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An explanation of managing side effects in React.

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  1. Chloe Stewart
    Teacher

    In React, side effects refer to operations that can affect something outside the scope of a component, such as:

    • Data fetching
    • Subscriptions (like WebSocket connections or event listeners)
    • Manual DOM manipulations
    • Timers (setTimeout, setInterval)
    • Updating external libraries or services

    React provides a couple of ways to handle side effects efficiently:

    1. useEffect Hook

    The primary tool for handling side effects in React is the useEffect hook. It allows you to perform side effects in function components.

    Basic Syntax:

    useEffect(() => {
// Your side effect logic here
}, [dependencies]);


    • useEffect runs after every render by default, but it can be controlled using the dependency array to run only when specific variables change.

    Example 1: Data Fetching in useEffect:

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

function DataFetchingComponent() {
const [data, setData] = useState(null);
const [loading, setLoading] = useState(true);

useEffect(() => {
// Side effect: fetching data when the component mounts
fetch('https://jsonplaceholder.typicode.com/posts')
.then((response) => response.json())
.then((data) => {
setData(data);
setLoading(false);
})
.catch((error) => console.error('Error fetching data:', error));
}, []); // Empty dependency array means this effect runs once when the component mounts

if (loading) return <div>Loading...</div>;
return <ul>{data.map(item => <li key={item.id}>{item.title}</li>)}</ul>;
}

export default DataFetchingComponent;
    • When to run: The useEffect here runs once after the component mounts because the dependency array is empty ([]).
    • Side Effect: We perform a data fetch using fetch, and once the data is fetched, it updates the component state.

    Example 2: Running Effect on Component Updates:

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

function UpdateEffectComponent() {
const [count, setCount] = useState(0);

useEffect(() => {
// Side effect: logging whenever `count` changes
console.log(`Count has changed to: ${count}`);
}, [count]); // This effect will run when `count` changes

return (
<div>
<p>Count: {count}</p>
<button onClick={() => setCount(count + 1)}>Increment</button>
</div>
);
}

export default UpdateEffectComponent;
    • When to run: The useEffect runs every time the count state changes because count is listed in the dependency array.

    Example 3: Cleanup with useEffect (Unsubscribing, Clearing Timers, etc.):

    When performing side effects like subscriptions or timers, you may need to clean up when the component unmounts or before the effect runs again. This is done using the cleanup function.

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

function TimerComponent() {
const [time, setTime] = useState(0);

useEffect(() => {
// Side effect: setting up a timer
const intervalId = setInterval(() => {
setTime((prevTime) => prevTime + 1);
}, 1000);

// Cleanup function: clearing the timer when the component unmounts
return () => clearInterval(intervalId);
}, []); // Effect runs only once (on mount)

return <p>Time elapsed: {time} seconds</p>;
}

    export default TimerComponent;

    • When to run: This effect runs only once (on mount), and it sets up a timer using setInterval.
    • Cleanup: The cleanup function (clearInterval) ensures that the timer is cleared when the component unmounts, preventing memory leaks.

    2. useLayoutEffect Hook

    useLayoutEffect works similarly to useEffect, but it runs synchronously after all DOM mutations. It is useful when you need to measure DOM elements or make changes to the DOM before the browser paints the screen.

    • Use Case: If you need to perform layout calculations or adjust visual elements right after the render but before the browser paints, you can use useLayoutEffect.
    import React, { useState, useLayoutEffect } from 'react';

function LayoutEffectComponent() {
const [width, setWidth] = useState(0);

useLayoutEffect(() => {
// Side effect: measuring the width of a DOM element
const element = document.getElementById('box');
setWidth(element.getBoundingClientRect().width);
}, []);

return (
<div>
<div id="box" style={{ width: '200px', height: '100px', background: 'lightblue' }}>
Box with width {width}px
</div>
</div>
);
}

export default LayoutEffectComponent;
    • Difference from useEffect: useLayoutEffect runs synchronously after the DOM has been updated but before the browser paints. It is useful when you need to do measurements or adjustments based on the layout.

    3. useCallback and useMemo for Optimizing Side Effects

    • useCallback: Used to memoize functions so they don’t get recreated on every render, preventing unnecessary side effects when functions are passed to child components.
    import React, { useState, useCallback } from 'react';

function ParentComponent() {
const [count, setCount] = useState(0);

// useCallback helps prevent unnecessary re-creations of this function
const increment = useCallback(() => {
setCount((prevCount) => prevCount + 1);
}, []); // `increment` function is memoized

return <ChildComponent increment={increment} />;
}

    • useMemo: Used to memoize computed values so they don’t get recalculated unless necessary, optimizing side effects based on computed data.
    import React, { useState, useMemo } from 'react';

function ExpensiveComputationComponent() {
const [number, setNumber] = useState(1);

// Memoize the result of the expensive computation
const factorial = useMemo(() => {
const calculateFactorial = (n) => (n <= 1 ? 1 : n * calculateFactorial(n - 1));
return calculateFactorial(number);
}, [number]);

return (
<div>
<p>Factorial of {number}: {factorial}</p>
<button onClick={() => setNumber(number + 1)}>Increase</button>
</div>
);
}

    4. Managing External Libraries / APIs (e.g., Analytics)

    You might need to handle external services or APIs (e.g., analytics tracking) as side effects. These can be added inside a useEffect hook to run after the component is rendered.

    import React, { useEffect } from 'react';

function AnalyticsComponent() {
useEffect(() => {
// Side effect: Sending pageview data to an analytics service
const analyticsService = {
trackPageView: (url) => console.log(`Tracking page view for ${url}`)
};
analyticsService.trackPageView(window.location.pathname);
}, []); // Runs only once (on mount)

return <div>Analytics sent for current page</div>;
}

    Summary of Side Effect Management in React:

    • useEffect is the main hook to manage side effects such as data fetching, subscriptions, and timers.
    • useLayoutEffect is similar to useEffect but runs synchronously after the DOM is updated.
    • Memoization with useCallback and useMemo can help optimize performance by preventing unnecessary recalculations and function re-creations.
    • Always ensure proper cleanup (e.g., unsubscribing, clearing intervals) to avoid memory leaks and unnecessary side effects.

    This approach allows you to handle side effects in a clean and efficient way within your React components.