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Mastering React's `useEffect` Hook

Published: at 12:00 AM

React has revolutionized the way we think about UI development, turning it into a more component-centric approach. One of the hooks that stands out in making this approach seamless is the useEffect hook. In this post, we’ll dive deep into understanding this powerful tool.

What is useEffect?

At its core, useEffect allows you to perform side effects in function components. Think of side effects as any action that interacts with the outside of your component, such as data fetching, manual DOM manipulations, or subscriptions.

In class components, we had lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount to handle these side effects. useEffect encompasses all these lifecycle methods into a unified API.

Basic Usage

The basic structure of useEffect looks like this:

useEffect(() => {
  // Your side effect code here

By default, this effect will run after every render of your component.

Dependency Array

The real power of useEffect comes when you introduce the dependency array:

useEffect(() => {
  // Runs when `someValue` changes
}, [someValue]);

If someValue changes between renders, the code inside useEffect will run. If it remains the same, React will skip this effect, optimizing performance.

An empty dependency array [] means the effect will only run once, similar to componentDidMount.

Cleanup with useEffect

Some side effects need to be cleaned up to prevent memory leaks, like event listeners or subscriptions. useEffect allows you to return a function for this purpose:

useEffect(() => {
  const handleResize = () => console.log(window.innerWidth);
  window.addEventListener("resize", handleResize);

  return () => {
    window.removeEventListener("resize", handleResize);
}, []);

This ensures that the event listener is removed when the component is no longer in use.

Practical Example: Theme Toggler

Let’s build a simple theme toggler using useEffect:

const [theme, setTheme] = useState("light");

useEffect(() => {
  document.body.className = theme;
}, [theme]);

Here, whenever our theme state changes, the class of the document’s body will update, allowing us to switch between light and dark themes using CSS.


The useEffect hook is a testament to React’s commitment to creating a more intuitive and powerful UI development experience. By mastering this hook, you can ensure your UI is truly reactive and can handle any side effects your application needs.