React Differences Between useEffect vs useLayoutEffect

React hooks are an essential part of creating functional components in React. They are used to add state, side effects, and other custom logic to your application. Two of the most popular hooks are useEffect and useLayoutEffect. Although they might seem similar, they are quite different in how they behave and operate.

In this article, we'll explore these two hooks' differences and when to use them appropriately.

What are useEffect and useLayoutEffect?

useEffect is a hook that allows you to run side-effects after the component has rendered. Side-effects may include fetching data from a remote server, registering an event listener or setting up a timer. You can also use useEffect to keep track of changes to specific variables in your state.

useLayoutEffect is another hook that allows you to run side-effects after the component has rendered, but before the browser paints the screen. That means that any side-effect you run with useLayoutEffect will block the browser's rendering process, which might impact the performance.

Differences between useEffect and useLayoutEffect

The main difference between useEffect and useLayoutEffect is the timing of when their effects are executed. As we mentioned earlier, useEffect runs the side effects after the component has rendered and painted on the screen. In contrast, useLayoutEffect runs the side-effects before the app gets painted on the screen, just before the user sees the output.

Another difference is how they handle render-blocking issues caused by their effects. useEffect handles these issues asynchronously, meaning that the function returned from useEffect runs after the rendering cycle finishes. useLayoutEffect, on the other hand, handles these issues synchronously, meaning that it runs before the painting process but can cause blocking if the effect takes a long time to execute.

When to use useEffect

If you don't have to manipulate the DOM or perform heavy calculations, useEffect is the hook you should use. This is because the effect you run with useEffect is asynchronous, meaning it won't block the rendering process, resulting in better performance. useEffect should be used when you want to perform cleanup tasks such as removing event listeners or when you want to update your state.

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

export default function App() {
  const [count, setCount] = useState(0);

  useEffect(() => {
    console.log("Component mounted");
    return () => console.log("Component unmounted");
  }, []);

  return (
    <>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>Click me</button>
    </>
  );
}

The above example logs the message "Component mounted" when the component has rendered and logs "Component unmounted" when the component is being removed from the DOM.

When to use useLayoutEffect

useLayoutEffect should be used when you want to perform any action that is tightly coupled with the DOM elements or impacts the UI. This is because useLayoutEffect runs just before the app gets painted on the screen, so you can use it to read and manipulate the layout of the elements in the DOM.

import React, { useLayoutEffect, useState } from "react";

export default function App() {
  const [count, setCount] = useState(0);
  const [color, setColor] = useState("white");

  useLayoutEffect(() => {
    if (count % 2 === 0) {
      setColor("pink");
    } else {
      setColor("green");
    }
  }, [count]);

  return (
    <>
      <p style={{ backgroundColor: color }}>
        You clicked {count} times and the background color is {color}
      </p>
      <button onClick={() => setCount(count + 1)}>Click me</button>
    </>
  );
}

In this example, the color of the paragraph's background changes based on the count's value. This is achieved by the useLayoutEffect effect changing the color state variable. Since we want to run the effect only when the count state changes, we pass the count variable as a dependency to the effect.

Conclusion

In summary, useEffect and useLayoutEffect are two powerful hooks that you can use to manage functionality in your React components. Knowing the differences between these hooks will allow you to choose the hook that better suits your needs. Remember that, as always, the performance impact is something to consider when using these hooks.

FAQs

1. What is the difference between useEffect and useLayoutEffect?
   • useEffect runs the side effects after rendering, while useLayoutEffect runs side effects before rendering.
2. How do I choose the right hook between useEffect and useLayoutEffect?
   • If your effect doesn't manipulate the UI, use useEffect. If it does, use useLayoutEffect.
3. Can I use useEffect and useLayoutEffect together in the same component?
   • Yes, you can. However, you should be careful with those hooks and use them appropriately.
4. How many times does useLayoutEffect run?
   • useLayoutEffect runs after every render, always before the browser paints the screen.
5. How do I stop useEffect from running every time my component renders?
   • You can pass a dependency array to useEffect to tell it when to run. If it's empty, it will only run when the component mounts and unmounts.