# useLayoutEffect - when to use it?

I recently attended an interview for React JS and the one question that I answered without actually ever implementing it was about the useLayoutEffect hook. I have heard about it from my colleague who had mentioned using it for some animation. I just looked for its definition then and it never occurred to me to implement it or to play around it until the day of the interview, where I could only answer what I read and couldn't really code to put out an example of it. So, after spending a good two hours on it, I am sharing the code snippets and explaining them here for an insight into the useLayoutEffect hook and how it is different from the useEffect hook, with examples.

`useLayoutEffect` is a React Hook that is similar to the `useEffect` Hook, but it runs synchronously immediately after all DOM mutations before the browser has a chance to paint those changes on the screen. This is useful for performing actions that depend on the layout or dimensions of the DOM elements, such as animating transitions or calculating measurements.

The signature `useLayoutEffect` is the same as `useEffect`: it takes two arguments, a function and an optional array of dependencies. The function argument will be executed after the initial render and after every update to the component, while the dependency array determines whether or not the effect should be re-run based on changes to its dependencies.

Here is an example of using `useLayoutEffect` to perform a simple animation:

```javascript
import { useState, useLayoutEffect } from 'react';

function App() {
  const [show, setShow] = useState(false);

  useLayoutEffect(() => {
    const el = document.getElementById('box');
    el.style.opacity = '1';
    el.style.transform = 'translateX(0)';
  }, [show]);

  return (
    <div>
      <button onClick={() => setShow(!show)}>Toggle Box</button>
      {show && <div id="box" style={{ opacity: 0, transform: 'translateX(-50px)' }}>This is a box.</div>}
    </div>
  );
}
```

In this example, we are using `useLayoutEffect` to update the styles of the `#box` element when the `show` state changes. The `useLayoutEffect` hook runs synchronously after every update, so the DOM changes will be immediately reflected in the browser, allowing for smooth animations without flickering or delays.

It is important to note that because `useLayoutEffect` runs synchronously, it can potentially cause performance issues or jank if the effect takes too long to execute. In these cases, it may be better to use the `useEffect` Hook instead, which runs asynchronously after the browser has painted the changes on the screen. However, for certain use cases such as animations or layout-dependent effects, `useLayoutEffect` can be a useful tool for achieving a smooth and responsive user experience.

1. Updating CSS Styles
    

```javascript
import { useState, useEffect, useLayoutEffect } from 'react';

function App() {
  const [color, setColor] = useState('red');

  useEffect(() => {
    const el = document.getElementById('box');
    el.style.backgroundColor = color;
  }, [color]);

  useLayoutEffect(() => {
    const el = document.getElementById('box');
    el.style.borderColor = color;
  }, [color]);

  return (
    <div id="box" style={{ width: '100px', height: '100px', border: '5px solid black' }}>
      This is a box.
    </div>
  );
}
```

In this example, we are using `useEffect` to update the background colour of the `#box` element whenever the `colour` state changes, and we are using `useLayoutEffect` to update the border colour of the same element. Because `useLayoutEffect` runs synchronously before the browser has a chance to paint the changes, the border colour will be updated before the background colour, resulting in a brief flicker where the border colour is visible before the background colour is updated.

1. Measuring Element Dimensions
    

```javascript
import { useState, useEffect, useLayoutEffect } from 'react';

function App() {
  const [width, setWidth] = useState(null);

  useEffect(() => {
    const el = document.getElementById('box');
    setWidth(el.clientWidth);
  }, []);

  useLayoutEffect(() => {
    const el = document.getElementById('box');
    setWidth(el.clientWidth);
  }, []);

  return (
    <div id="box" style={{ width: '50%' }}>
      This is a box. Its width is {width}px.
    </div>
  );
}
```

In this example, we are using both `useEffect` and `useLayoutEffect` to measure the width of the `#box` element and update the `width` state accordingly. Because `useEffect` runs asynchronously after the browser has painted the changes, it may not always reflect the most up-to-date measurements of the element. In contrast, `useLayoutEffect` runs synchronously before the browser paints the changes, ensuring that the `width` state is always up-to-date with the latest measurements.

Overall, the choice between `useEffect` and `useLayoutEffect` depends on the specific use case and the desired behavior of the effect. If the effect relies on the layout or dimensions of DOM elements or needs to perform synchronous updates to the DOM, `useLayoutEffect` may be more appropriate. Otherwise, `useEffect` is generally a safer and more performant choice.