What are the differences between using useEffect and useLayoutEffect

Question

What are the key differences between using useEffect and useLayoutEffect in React and in what scenarios should you prefer one over the other for handling side effects in your components??

Adarsh Nigam · Answer

Key Differences Between `useEffect` and `useLayoutEffect` in React: Execution Timing: - `useEffect`: Runs after the browser has painted the UI. It’s asynchronous and doesn’t block the rendering process. - `useLayoutEffect`: Runs synchronously after all DOM mutations but before the browser paints. It can block rendering until the effect is complete. Primary Use Cases: - **`useEffect`:** - Best for non-UI blocking operations like data fetching subscriptions or logging. - Suitable for tasks that don’t need to alter the DOM before the paint. - **`useLayoutEffect`:** - Ideal for operations that need to happen before the browser paints such as reading layout measurements or synchronously changing DOM elements. - Helps avoid visual inconsistencies like flickering during initial renders. - **Performance Considerations:** - `useEffect`: Generally better for most side effects as it avoids blocking the UI leading to smoother user experiences. - `useLayoutEffect`: Should be used sparingly as it can impact performance if overused particularly in complex components. **When to Prefer One Over the Other:** - **Use `useEffect`** for tasks that are not critical to the layout and visual consistency like API calls or setting up subscriptions. - **Use `useLayoutEffect`** when you need to make DOM measurements or manipulate the DOM synchronously to avoid layout shifts or flickering.