useEffect vs. useLayoutEffect

Key takeaways:

  • useEffect runs asynchronously after the browser has painted, while useLayoutEffect runs synchronously before the browser paints, allowing for immediate DOM updates.

  • useEffect is used for side effects like data fetching that don’t impact the visual output, while useLayoutEffect is used for tasks requiring immediate DOM measurements or updates, such as focusing an input field

  • useLayoutEffect can cause layout thrashing if overused, so it should be applied only when necessary for visual updates.

When working with React, managing side effects in functional components can be efficiently handled using hooks like useEffect and useLayoutEffect. Both of these hooks serve the purpose of running code after render, but they’re designed for different scenarios and can impact the performance and behavior of our application in unique ways.

What are hooks?

Hooks in programming are a way to add functionality to components in React.js, a popular JavaScript library for building user interfaces. They allow us to use state and other React features without writing a class. Think of them as special functions that let us “hook into” React’s life cycle and state management. With hooks, we can manage state, perform side effects, and tap into React features within functional components, making our code more modular and easier to understand. The following table shows the comparison between using hooks and not using hooks in React.

With Hooks

Without Hooks

Allows stateful logic inside functional components

Stateful logic requires class components

Enables reusing stateful logic across components

Enables the use of stateful logic duplication or higher-order components

Simplifies code and reduces nesting

Might lead to complex class hierarchies and lifecycle methods

Provides a cleaner and more concise component structure

Requires more verbose syntax and boilerplate code

Types of hooks

Here are some of the core hooks provided by React for managing state, side effects, context, and more within functional components:

  1. useState

  2. useEffect

  3. useContext

  4. useReducer

  5. useCallback

  6. useMemo

  7. useRef

  8. useImperativeHandle

  9. useLayoutEffect

  10. useDebugValue

In this Answer, we’ll discuss and compare the useEffect and useLayoutEffect hooks.

The useEffect hook

The useEffect hook in React is a function that allows us to perform side effects in functional components. It’s used to manage actions that need to be performed after the component has been rendered, such as data fetching, subscriptions, or DOM manipulation. Examples of its use include fetching data from an API, subscribing to a WebSocket connection, setting up event listeners, or updating the document title based on the component state. By default, the useEffect hook runs both after the first render and after every update.

Syntax

The code snippet below shows the syntax of the useEffect hook.

useEffect(() => {
    // Side effect logic
    return () => {
        // Cleanup logic
    };
}, [dependencies]);
Syntax of the useEffect hook

In the code snippet above:

  • () => {}: This is the first argument to the useEffect hook. It’s the side logic that we want to execute after each render of the component.

  • return () => {}: This is an optional part of the useEffect hook. It defines a cleanup function that runs when the component unmounts or before rerunning the effect if any of the dependencies changes.

  • [dependencies]: This is the second argument of the useEffect hook. It’s an array of values that determines when the effect should run. If any value in this array changes between renders, the effect reruns. If the array is empty, the effect runs only once after the initial render.

Example

The coding playground below shows the useEffect hook in action. Once the code has been executed, click the link next to the text “Your app can be found at:” to open the application. The text is located at the bottom of the coding playground below.

import React, { useState, useEffect } from 'react';

const App = () => {
  const [count, setCount] = useState(0);

  useEffect(() => {
    document.title = `Count: ${count}`;
  }, [count]);

  return (
    <div>
      <p> Count: {count} </p>
      <button onClick={() => setCount(count + 1)}> Increment Count </button>
    </div>
  );
}

export default App;
Coding playground

In the code snippet above:

  • Line 1: We import the React library, along with the useState and useEffect hooks.

  • Line 3: We define a functional component named App.

  • Line 4: We define a state variable count using the useState hook. We set the initial value of count to 0.

  • Lines 6–8: We define the useEffect hook. Within this hook, we update the document title to include the current value of the count variable. The effect will run after the first render of the App component and subsequently every time the value of the count variable changes.

  • Lines 10–14: We define the UI of the App component. The UI renders the current value of the count variable. It also renders a button to update the value of the count variable.

  • Line 18: We export the App component.

The useLayoutEffect hook

The useLayoutEffect hook in React is similar to the useEffect hook. The only difference is that the useLayoutEffect hook runs synchronously after all DOM mutations are applied. This makes it suitable for tasks that require immediate DOM measurements or modifications before the browser paints, ensuring smoother user experiences. For instance, the useLayoutEffect hook can be used when we want to focus on an input field immediately after it’s rendered. This ensures that the input field is focused without any delay, providing a smoother user experience. To summarize, the useLayoutEffect hook executes once after the initial render and before the browser paints, and subsequently each time the dependencies within the dependency array change (just like the useEffect hook).

Syntax

The syntax of the useLayoutEffect hook is similar to the useEffect hook. Both hooks have exactly the same structure. The code snippet below shows the syntax of the useLayoutEffect hook.

useLayoutEffect(() => {
    // Side effect logic
    return () => {
        // Cleanup logic
    };
}, [dependencies]);
Syntax of the useLayoutEffect hook

Example

The coding playground below shows the useLayoutEffect hook in action. Once the code has been executed, click the link next to the text “Your app can be found at:” to open the application. The text is located at the bottom of the coding playground below.

Note: Open the “Console” window using “Inspect” to see the console logs being executed in the coding playground below.

import React, { useState, useEffect, useLayoutEffect } from 'react';

const App = () => {
  const [count, setCount] = useState(0);

  useEffect(() => {
    console.log('useEffect hook called!');
    document.title = `Count: ${count}`;
  }, [count]);

  useLayoutEffect(() => {
    console.log('useLayoutEffect hook called!');
  }, [count]);

  return (
    <div>
      <p> Count: {count} </p>
      <button onClick={() => setCount(count + 1)}> Increment Count </button>
    </div>
  );
}

export default App;
Coding playground

Code explanation

  • Line 1: We import the React library, along with the useState, useEffect, and useLayoutEffect hooks.

  • Line 3: We define a functional component named App.

  • Line 4: We define the state variable count using the useState hook. We set the initial value of count to 0.

  • Lines 6–9: We define the useEffect hook. Within this hook, we update the document title to include the current value of the count variable. The effect will run after the first render of the App component and subsequently every time the value of the count variable changes. We also log the useEffect hook called! message to the console whenever the effect is executed.

  • Lines 11–13: We define the useLayoutEffect hook. Within this hook, we log useLayoutEffect hook called! to the console whenever the effect is executed. The effect will run after the first render of the App component and before the browser paints, and subsequently every time the value of the count variable changes.

  • Lines 15–20: We define the UI of the App component. The UI renders the current value of the count variable. It also renders a button to update the value of the count variable.

  • Line 23: We export the App component.

Note: Within the “Console” window, notice that each time the value of the count variable changes, the useLayoutEffect is called before the useEffect hook. This is because the useLayoutEffect is executed before the browser paints whereas the useEffect hook is executed after the browser paints.

The following table summarizes the difference between the useEffect and useLayoutEffect hooks:

useEffect

useLayoutEffect

Execution timing

Runs asynchronously after the first render and browser paint and, subsequently, every time the dependencies within the dependency array change

Runs synchronously after the first render and before browser paint and, subsequently, every time the dependencies within the dependency array change

DOM mutations

After browser paint

Before browser paint

Use cases

Data fetching and side effects with no visual impact

Sets focus on an input field or element before rendering. Generally used for visual effects to provide a smoother user experience

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