useEffect React Hook When and How to Use It
Updated on 16th Dec, 23 9.1K Views

In this captivating journey, we will delve into the depths of useEffect, unraveling its mysteries and unlocking its remarkable capabilities. We will embark on a quest to understand when and how to employ this powerful hook.

Table of Contents:

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Introducing the useEffect Hook

The useEffect hook in React is a powerful tool that allows us to manage side effects within functional components. Side effects refer to any actions or operations that occur outside the scope of the component rendering, such as fetching data, subscribing to events, or updating the document title. 

Traditionally, managing side effects was done using class components and lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount. However, with the introduction of hooks in React 16.8, we can now achieve the same functionality in a more concise and declarative manner.

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Key Concepts of Using useEffects

To effectively utilize the useEffect hook in React, it’s important to understand the following key concepts:

  • Rendering Awareness: Before diving into the useEffect hook, it’s crucial to understand when the component will be rendered or re-rendered. Effects are executed after each rendering cycle.
  • Controlling Effect Execution: By default, effects always run after rendering. However, there’s an option to opt out of this behavior if desired.
  • Managing Dependencies: To skip or exclude an effect, it’s necessary to comprehend basic JavaScript concepts related to values. An effect will only run again if one or more values specified in its dependencies have changed since the last rendering cycle.
  • Avoiding Unnecessary Rerenders: Unnecessary repetitions of effects and re-rendering of components should be minimized. This can be achieved by being mindful of when effects are triggered and ensuring they run only when necessary.
  • Handling Function Definitions: Functions defined within the body of a function component are rebuilt with each render cycle. This can affect the behavior of functions used within effects. Strategies like moving functions outside the component, defining them inside the effect, or employing useCallback can help address this issue.
  • Understanding Stale Values: Familiarity with JavaScript concepts like closures is crucial. Without this understanding, resolving issues related to outdated props and state values in effects can become challenging. Strategies like using the effect dependency array or the useRef hook can be employed to tackle this situation.
  • React Hooks ESLint Plugin: It’s important not to disregard suggestions from the React Hooks ESLint plugin. Ignoring ESLint warnings, blindly removing dependencies, or carelessly using disabled comments can lead to errors. Paying attention to these suggestions is advisable.

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How to Use useEffect?

How to Use useEffect?
  • Basic Usage: The basic syntax of the useEffect hook consists of two arguments: a callback function and an optional dependency array. The callback function is where we define the side effect logic, and React executes it after the component has been rendered.
useEffect(() => {
  // Side effect logic goes here
For example, let's say we want to display a welcome message in the console whenever the component renders:
useEffect(() => {
  console.log("Welcome to the component!");
  • Cleaning Up Effects: In some cases, side effects may require cleanup when the component unmounts or when certain conditions change. This is especially important for operations like removing event listeners or canceling network requests to prevent memory leaks or unnecessary resource consumption. To achieve this, we can return a cleanup function from the useEffect callback.
useEffect(() => {
  // Side effect logic goes here
  return () => {
    // Cleanup logic goes here

For instance, let’s consider a scenario where we want to add an event listener to the window object and remove it when the component unmounts:

useEffect(() => {
  const handleScroll = () => {
    // Event handler logic
  window.addEventListener("scroll", handleScroll);
  return () => {
    window.removeEventListener("scroll", handleScroll);
}, []);
  • Controlling the Effect Dependency: The dependency array is an important aspect of useEffect as it allows us to control when the effect should be re-executed. By specifying dependencies, we ensure that the effect runs only when those dependencies change.
useEffect(() => {
  // Side effect logic goes here
}, [dependency1, dependency2]);

For example, suppose we have a component that fetches data from an API, and we want the effect to trigger only when the userId prop changes:

useEffect(() => {
  // Fetch data from API based on the userId prop
}, [userId]);

By specifying [userId] as the dependency array, the effect will only execute when the userId prop value changes.

  • Combining Multiple useEffect Hooks: In a single component, we can use multiple useEffect hooks to handle different side effects independently. This allows us to separate concerns and organize our code more effectively.
useEffect(() => { // Side effect logic 1 }, [dependency1]); useEffect(() => { // Side effect logic 2 }, [dependency2]);

By using multiple useEffect hooks, we can manage different side effects with their respective dependencies and keep our codebase clean and modular.

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Common Use Cases for useEffect

Some of the common use cases for useEffects are as follows:

Common Use Cases for useEffect
  • Fetching Data with useEffect: Fetching data from an API is a common use case for useEffect. We can leverage useEffect to trigger the data fetching operation and update the component state accordingly.
useEffect(() => {
  const fetchData = async () => {
    // Fetch data from the API
    // Update component state
}, []);

Bypassing an empty dependency array, we ensure that the effect runs only once when the component mounts.

  • Event Listeners and Clean-up: Adding and removing event listeners is another common scenario. We can use useEffect to attach event listeners and clean them up when the component unmounts.

useEffect(() => { const handleClick = () => { // Event handler logic }; document.addEventListener(“click”, handleClick); return () => { document.removeEventListener(“click”, handleClick); }; }, []);

  • Handling Animations and Transitions: When dealing with animations or transitions, useEffect can be utilized to trigger certain actions at specific points in the animation lifecycle.
useEffect(() => {
  // Perform actions before animation
  const animationStart = () => {
    // Animation start logic
  // Perform actions after animation
  const animationEnd = () => {
    // Animation end logic
  element.addEventListener("animationstart", animationStart);
  element.addEventListener("animationend", animationEnd);
  return () => {
    element.removeEventListener("animationstart", animationStart);
    element.removeEventListener("animationend", animationEnd);
}, []);
  • Updating Document Title: When the document title needs to change dynamically based on the component’s state or other variables, useEffect can be employed to update the title accordingly.
useEffect(() => {
  document.title = "New Page Title";
}, [dependency]);

When the dependency array is incorporated, it guarantees that the title will be modified whenever there are changes in the dependency.

  • Integration with External Libraries: In some cases, we might need to integrate React components with external libraries that require initialization or clean-up. useEffect can be used to manage these interactions.
useEffect(() => {
  // Initialize external library
  return () => {
    // Clean up external library
}, []);

By utilizing useEffect, we can seamlessly integrate React components with external libraries and handle their initialization and clean-up.

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Best Practices for Using useEffect

  • Minimizing the Effect Dependency Array: When working with useEffect, it’s crucial to minimize the effect dependency array. The dependency array specifies the values that the effect depends on, triggering a re-run of the effect whenever any of those values change. It’s important to include only the necessary dependencies to avoid unnecessary re-renders.

Consider the following example:

useEffect(() => { // Effect logic }, [dependency1, dependency2]);

In this case, if the effect relies only on dependency1 and not dependency2, it’s better to exclude dependency2 from the array. This optimization prevents the effect from being needlessly re-executed when dependency2 changes.

  • Handling Dependencies Correctly: It’s crucial to handle dependencies correctly when working with useEffect. The effect should only run when the relevant dependencies change. Missing dependencies or including unnecessary ones can lead to unintended behavior or performance issues.

To illustrate, let’s say we have a function that retrieves information from an API and modifies the state. It’s imperative for the function to depend on the API endpoint, rather than the specific state variable that’s being modified.

const [data, setData] = useState([]);
useEffect(() => {
    .then(response => response.json())
    .then(data => setData(data));
}, []);

In this case, since the effect doesn’t depend on any variable, an empty dependency array is provided. This ensures that the effect runs only once, simulating the behavior of componentDidMount.

  • Optimizing Performance with Memoization: To optimize performance, we can utilize memoization techniques in conjunction with useEffect. Memoization allows us to cache expensive computations and avoid unnecessary re-computations.

For example, let’s say we have an effect that performs a heavy computation based on a dependency. We can use the useMemo hook to memoize the computation and include it as a dependency in the effect.

const [dependency, setDependency] = useState(0);
const memoizedValue = useMemo(() => {
  // Expensive computation based on dependency
  return computeValue(dependency);
}, [dependency]);
useEffect(() => {
  // Effect that uses the memoized value
}, [memoizedValue]);

By memoizing the value and including it as a dependency, we ensure that the effect only runs when the memoized value changes, avoiding unnecessary re-executions of the expensive computation.

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Handling Custom Hooks and useEffect

Custom hooks are a powerful tool for code reuse and abstraction. They can encapsulate complex logic, including the use of useEffect. When using custom hooks that contain useEffect, the effect will be triggered whenever the component using the hook mounts, updates, or unmounts.

For example, let’s consider a custom hook called useScrollPosition that tracks the window scroll position:

const useScrollPosition = () => {
  useEffect(() => {
    const handleScroll = () => {
      // Scroll position logic
    window.addEventListener('scroll', handleScroll);
    return () => {
      window.removeEventListener('scroll', handleScroll);
  }, []);
const MyComponent = () => {
  // Component rendering logic

In this case, the useScrollPosition custom hook encapsulates the scroll position logic within useEffect. By using the hook in MyComponent, the scroll position will be tracked and updated accordingly.

Debugging useEffect

Debugging effects can sometimes be challenging, especially when dealing with complex dependencies or unexpected behavior. React provides a helpful tool called the “dependency array” warning to aid in debugging.

If you encounter an issue where an effect is being triggered too frequently or not at all, React will display a warning indicating that dependencies might be missing or unnecessary. This warning can help identify and resolve issues related to effect dependencies.


The useEffect hook is a powerful tool in React for managing side effects and orchestrating component lifecycles. By mastering the useEffect hook and incorporating it into their development workflow, developers can streamline their code, reduce errors, and create more efficient and effective applications that meet the needs of today’s users. Its versatility and flexibility make it an essential part of modern web development and a must-have for any developer looking to build dynamic and responsive web applications. 
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