Understanding useLayoutEffect in React

Aug 12, 2023

React provides a variety of hooks to cater to different use cases. The useLayoutEffect hook is one of them, and it's similar in many ways to useEffect, but with a crucial difference. This blog post aims to delve into the details of useLayoutEffect, its potential pitfalls, and how best to leverage it.

What is useLayoutEffect?

useLayoutEffect is essentially a version of useEffect that fires before the browser repaints the screen.

useLayoutEffect(setup, dependencies?);

When to use it?

The primary usage is when you need to measure the layout before the browser repaints the screen. An excellent example of this is a tooltip that adjusts its position based on its dimensions and available space.

Key Considerations

Performance Pitfalls

It's essential to note that useLayoutEffect can potentially hurt performance. Since it blocks the browser from painting, excessive usage might make your app feel laggy. It's advisable to prefer useEffect whenever possible.

Server-Side Rendering

A common error many face is: "useLayoutEffect does nothing on the server". This error arises because useLayoutEffect is designed to leverage layout information for rendering, but during server-side rendering, no such layout information is available. Hence, components that rely on layout specifics shouldn’t be rendered on the server.

Using it Properly

Remember, useLayoutEffect is a Hook. It should be called at the top level of your component or custom Hooks. Avoid calling it within loops or conditional expressions.

Practical Example

Let's explore a scenario where you're developing a Tooltip component. The Tooltip needs to adjust its position based on its height and available space:

function Tooltip( ) {
  const ref = useRef(null);
  const [tooltipHeight, setTooltipHeight] = useState(0);

  useLayoutEffect(() => {
    const { height } = ref.current.getBoundingClientRect();
    setTooltipHeight(height);
  }, []);
  // rendering logic here
}

In the example above, the tooltip:

Initially renders with tooltipHeight = 0.
Once React places it in the DOM, the code inside useLayoutEffect runs.
The height of the tooltip content is measured and triggers a re-render.
Finally, the Tooltip is rendered with the accurate height and positioned appropriately.

The primary benefit here is that the user never notices the intermediate state where the tooltip is wrongly positioned. It appears correctly right from the start.

useLayoutEffect vs. useEffect

When using useLayoutEffect, React ensures that all logic inside it and any state updates it triggers are executed before the browser paints the screen. This can be helpful in scenarios where we don't want the user to notice intermediate renders.

However, useEffect doesn't block the browser from painting. This could lead to noticeable "flickers" or shifts in the UI in situations where layout measurements change component positioning.

Conclusion

useLayoutEffect is a powerful tool in React's hook arsenal. While it can be incredibly beneficial in specific scenarios, like the Tooltip example, it's crucial to use it judiciously. Excessive or inappropriate usage can introduce performance bottlenecks. Always remember to measure, profile, and test the performance implications when integrating it into your application.

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