When to Use useEffect and useLayoutEffect in React
Tags: useeffect, uselayouteffect, webdev, javascript
Published: 2023-11-28 05:09:12

React Hooks, introduced in React 16.8, have completely changed the way we write components in React. They allow us to use state and other React features without writing a class. Two of these hooks, `useEffect` and `useLayoutEffect`, are used to perform side effects in function components. But when should we use each one? Let's explore both hooks and find out.

## What is useEffect?

The `useEffect` hook is used to perform side effects in function components. A side effect could be anything that affects something outside of the scope of the current function being executed. Examples include data fetching, setting up a subscription, manually changing the DOM, and so on.

Here is an example of `useEffect`:

```jsx
useEffect(() => {
  document.title = `You clicked ${count} times`;
}, [count]); // Only re-run the effect if count changes
```

The function passed to `useEffect` will run after the render is committed to the screen. Think of effects as an escape hatch from React’s purely functional world into the imperative world.

## What is useLayoutEffect?

The `useLayoutEffect` hook has the same signature as `useEffect`. However, it fires synchronously after all DOM mutations. This can be useful when you need to make new updates and measurements after the DOM updates, but before the browser has a chance to "paint" those changes, such as reading layout from the DOM or synchronously re-rendering.

Here is an example of `useLayoutEffect`:

```jsx
useLayoutEffect(() => {
  ref.current.style.color = 'blue';
}, []); // Only run once
```

## When to use useEffect vs useLayoutEffect?

The primary difference between `useEffect` and `useLayoutEffect` lies in the timing of execution. `useEffect` runs asynchronously and after a render is painted to the screen. `useLayoutEffect`, on the other hand, runs synchronously after a render but before the screen is updated.

If you’re migrating code from a class component, note `useLayoutEffect` fires in the same phase as `componentDidMount` and `componentDidUpdate`.

Here are some general rules of thumb:

- If you need to mutate the DOM and/or do measurements and then make further updates, you'd want to use `useLayoutEffect` to ensure these updates occur before the browser has a chance to paint. This can help prevent flickering on the screen.
- For other cases, including data fetching and subscriptions, `useEffect` is the way to go. It won't block the painting process, leading to better perceived performance.

Always remember, each tool has its place. Understanding the difference between `useEffect` and `useLayoutEffect` allows us to make better decisions about which to use when, for the best possible user experience.

Remember, while `useLayoutEffect` can prevent flickering, using it excessively can lead to performance problems, as it blocks visual updates. So, unless you need to make updates and measurements before the browser "paints", stick with `useEffect`.

In conclusion, understanding the difference between `useEffect` and `useLayoutEffect` is crucial to ensure your React app's performance. Use the right hook at the right time, and you're on your way to creating a smooth and efficient React application.