UseEffect vs UseLayoutEffect

 

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As a React Developer, it's important to understand the difference between useEffect and useLayoutEffect hooks in React. Understanding them gives a better picture of how these two hooks work and when we can use them!

Both useEffect and useLayoutEffect are React hooks that allow you to run code after rendering a component. However, the main difference between the two hooks is when they run.

What is the difference between a useEffect and a useLayoutEffect?

A useEffect is called a “post-render” hook, meaning that it doesn’t prevent the browser from painting the screen. For example, you would use a useEffect to fetch data from the API, update a component’s state, or set up an event listener. A useLayoutEffect is a hook that is called “pre-render”, meaning that it’s used before the browser can paint the screen. In other words, it prevents the browser from changing the screen’s size or position until the code within the hook is finished running. For example, you might use a useLayoutEffect to measure a DOM element’s size/position before it’s been painted. I have explained the difference between a render and a painting at the bottom of this article (in the bonus section).

What does this suggest?

This essentially indicates that pre-render operations utilize LayoutEffect, and post-render uses useEffect.

Example of useEffect:

In this example, when the component mounts, useEffect is used to retrieve data from an API. React is instructed to only run this effect once when the component mounts via the [] parameter. Following the data fetch, we use the newly obtained information to update the component's state, causing the component to be rendered again.

Example of useLayoutEffect:

This React code defines a Tooltip component that uses the useState, useRef, and useLayoutEffect hooks. It creates a reference (ref) to the tooltip element, and state variables for the tooltip height (tooltipHeight) and another variable (someVariable). The useLayoutEffect hook calculates the tooltip's height using the getBoundingClientRect method, updates the tooltipHeight state, and sets a new value for someVariable based on the calculated height. This process occurs once when the component mounts ([] as the dependency) to ensure accurate measurements before rendering.

What is the difference between a useEffect and a useLayoutEffect in React?

The main difference between the useEffect and the useLayoutEffect hooks is that they execute asynchronously. This means that there is a delay between the render of the component and the execution of the side effect. This is useful for side effects that don't depend on the component or DOM, like measuring the size of the element or animating the component based on the position of the element. For side effects that do depend on the component and DOM, like fetching data from the API or updating the global state, the useLayoutEffect hook executes immediately after the render but before any changes have been made to the DOM.

If you need to run a side effect synchronously after the component is drawn and it depends on the component's layout or the DOM, you should use useLayoutEffect rather than useEffect. UseEffect can be used in place of this if you don't require the side effect to happen right away when the component is rendered.

Bonus

Rendering vs. Painting

When it comes to software development, there are two ways to go about it: rendering and painting. Rendering is the process of creating HTML, CSS and JavaScript code to represent a visual element, like a button or form, in your web application. On the other end of the spectrum, painting is the process of changing the pixels on your screen to reflect any visual changes your application has made. You can think of it as taking the output of your rendering process and turning it into a set of instructions that your graphics hardware can then use to update your screen.

Painting a screen is the process of updating the physical display to reflect the changes made by the application, whereas rendering a component is the process of creating the code that represents a visual component.

Conclusion

Both useEffect and useLayoutEffect are hooks in React that handle side effects, but they differ in terms of when they are executed during the component lifecycle. Use useEffect for asynchronous and less critical tasks, as it runs after the browser has painted. On the other hand, opt for useLayoutEffect when you need to perform synchronous operations before the browser paints, ensuring changes are applied before the user sees the updated UI. Choose the appropriate hook based on your specific use case and the timing requirements of your side effects.