27. What is the difference between `useLayoutEffect` and `useEffect` in React?

Understanding useLayoutEffect vs useEffect in React

In React, both useEffect and useLayoutEffect are hooks that let you perform side effects in function components. However, they have distinct differences in when they are executed, which can impact your application's performance and behavior.

useEffect

• Timing: The useEffect hook is executed after the browser has painted the screen. This means it runs asynchronously and does not block the browser from updating the screen.
• Use Cases: It's suitable for operations that do not require blocking the UI update, such as data fetching, setting up subscriptions, or manually changing the DOM (if you don't need to perform a measurement).

import React, { useEffect } from 'react';

function ExampleComponent() {
  useEffect(() => {
    console.log('useEffect after render');
    // Perform operations like data fetching here
  }, []); // The empty array means this effect runs once after the initial render

  return <div>Hello World</div>;
}

useLayoutEffect

• Timing: The useLayoutEffect hook is executed synchronously after React has performed all DOM mutations but before the browser has painted. This means it runs before the screen is updated, allowing you to make measurements or changes to the DOM that should appear before the screen is updated.
• Use Cases: It's ideal for operations that need to be completed before the browser painting, such as reading layout from the DOM or synchronously re-rendering the component.

import React, { useLayoutEffect } from 'react';

function ExampleComponent() {
  useLayoutEffect(() => {
    console.log('useLayoutEffect before paint');
    // Perform operations like synchronously reading from the DOM
  }, []); // The empty array means this effect runs once after the initial render

  return <div>Hello World</div>;
}

Key Differences

1. Execution Timing: useLayoutEffect blocks the painting of the screen until its code is executed, which can be beneficial for certain types of operations but may cause performance issues if used improperly. In contrast, useEffect does not block painting.
2. Performance Impact: Due to its synchronous nature, useLayoutEffect can lead to slower rendering if not used cautiously, especially with complex or lengthy operations.

Overall, the choice between these hooks depends on the specific needs of your application. For most use cases, useEffect is sufficient and preferred for non-blocking operations. Use useLayoutEffect only when you need to perform operations that affect the layout and must occur before the screen is updated.