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How to Use React Hooks – useEffect, useState, and useContext Code Examples

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Sumit Upadhyay
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6 min read

React is a powerful JavaScript library for building user interfaces. And it has undergone significant changes over the years.

One of the most noteworthy additions is the introduction of hooks, which revolutionised the way developers manage state and side effects in functional components.

In this article, we'll explore three fundamental hooks for beginners: useState, useEffect, and useContext.

Introduction to React Hooks

Before hooks, stateful logic in React was primarily managed using class components.

With the advent of functional components and hooks, developers gained a more concise and expressive way to handle state and lifecycle methods. Hooks allow you to use state and other React features without writing a class.

What are React Hooks?

React hooks are functions that enable functional components to use state and lifecycle features that were previously only available in class components. They were introduced in React 16.8 to provide a more consistent way to manage stateful logic in functional components.

How to Use the useState Hook

The useState hook is perhaps the most basic and essential hook in React. It enables you to add state to your functional components, allowing them to keep track of data that changes over time. Let's dive into how useState works with a simple example.

Basic Usage of useState

import React, { useState } from 'react';

const Counter = () => {
  // Declare a state variable named 'count' with an initial value of 0
  const [count, setCount] = useState(0);

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

export default Counter;

In this example, we import the useState hook from the 'react' library. The useState function returns an array with two elements: the current state value (count) and a function (setCount) to update it. We initialize count to 0, and clicking the "Increment" button increases its value.

How to Use Multiple useState Hooks

You can use the useState hook multiple times in a single component to manage different pieces of state independently. Let's modify our Counter component to include a second piece of state.

import React, { useState } from 'react';

const Counter = () => {
  const [count, setCount] = useState(0);
  const [isEven, setIsEven] = useState(false);

  return (
    <div>
      <p>Count: {count}</p>
      <p>{isEven ? 'Even' : 'Odd'}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
      <button onClick={() => setIsEven(!isEven)}>Toggle Even/Odd</button>
    </div>
  );
};

export default Counter;

Now, our Counter component has two independent pieces of state: count and isEven. Clicking the "Toggle Even/Odd" button will switch the value of isEven.

How to Use the useEffect Hook

The useEffect hook is used to perform side effects in your functional components, such as fetching data, subscribing to external events, or manually changing the DOM. It combines the functionality of componentDidMount, componentDidUpdate, and componentWillUnmount in class components.

Basic Usage of useEffect

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

const DataFetcher = () => {
  const [data, setData] = useState(null);

  useEffect(() => {
    // Fetch data from an API
    fetch('https://api.example.com/data')
      .then((response) => response.json())
      .then((result) => setData(result))
      .catch((error) => console.error('Error fetching data:', error));
  }, []); // Empty dependency array means this effect runs once after the initial render

  return (
    <div>
      {data ? (
        <ul>
          {data.map((item) => (
            <li key={item.id}>{item.name}</li>
          ))}
        </ul>
      ) : (
        <p>Loading data...</p>
      )}
    </div>
  );
};

export default DataFetcher;

In this example, the useEffect hook is used to fetch data from an API when the component mounts. The second argument of useEffect is an array of dependencies. If the dependencies change between renders, the effect will run again. An empty array means the effect runs once after the initial render.

Cleaning Up in useEffect

Sometimes, side effects need to be cleaned up, especially when dealing with subscriptions or timers to prevent memory leaks. The useEffect hook can return a cleanup function that will be executed when the component unmounts.

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

const Timer = () => {
  const [seconds, setSeconds] = useState(0);

  useEffect(() => {
    const intervalId = setInterval(() => {
      setSeconds((prevSeconds) => prevSeconds + 1);
    }, 1000);

    // Cleanup function to clear the interval when the component unmounts
    return () => clearInterval(intervalId);
  }, []); // Empty dependency array for initial setup only

  return <p>Seconds: {seconds}</p>;
};

export default Timer;

In this example, the setInterval function is used to update the seconds state every second. The cleanup function returned by useEffect clears the interval when the component is unmounted.

How to Use the useContext Hook

The useContext hook is used to consume values from a React context. Context provides a way to pass data through the component tree without having to pass props manually at every level. Let's explore how useContext works with a simple example.

How to Create a Context

First, let's create a context to hold a user's authentication status.

import React, { createContext, useContext, useState } from 'react';

const AuthContext = createContext();

export const AuthProvider = ({ children }) => {
  const [isAuthenticated, setIsAuthenticated] = useState(false);

  const login = () => {
    setIsAuthenticated(true);
  };

  const logout = () => {
    setIsAuthenticated(false);
  };

  return (
    <AuthContext.Provider value={{ isAuthenticated, login, logout }}>
      {children}
    </AuthContext.Provider>
  );
};

export const useAuth = () => {
  return useContext(AuthContext);
};

In this example, we create an AuthContext using createContext and provide an AuthProvider component. The AuthProvider component wraps its children with the context provider and includes functions for logging in and out.

How to Use useContext

Now, let's use the useContext hook in a component to access the authentication status.

import React from 'react';
import { useAuth } from './AuthContext';

const AuthStatus = () => {
  const { isAuthenticated, login, logout } = useAuth();

  return (
    <div>
      <p>User is {isAuthenticated ? 'logged in' : 'logged out'}</p>
      <button onClick={login}>Login</button>


      <button onClick={logout}>Logout</button>
    </div>
  );
};

export default AuthStatus;

Here, the useAuth hook is used to access the values provided by the AuthContext. The AuthStatus component displays the user's login status and provides buttons to log in and out.

Putting It All Together

Let's create a more complex example that combines useState, useEffect, and useContext in a single component. Suppose we have a component that fetches user data from an API and displays it, taking into account the user's authentication status.

import React, { useState, useEffect } from 'react';
import { useAuth } from './AuthContext';

const UserProfile = () => {
  const { isAuthenticated } = useAuth();
  const [userData, setUserData] = useState(null);

  useEffect(() => {
    if (isAuthenticated) {
      // Fetch user data when authenticated
      fetch('https://api.example.com/user')
        .then((response) => response.json())
        .then((result) => setUserData(result))
        .catch((error) => console.error('Error fetching user data:', error));
    }
  }, [isAuthenticated]); // Run the effect when isAuthenticated changes

  return (
    <div>
      {isAuthenticated ? (
        <div>
          <h2>Welcome, {userData ? userData.name : 'User'}!</h2>
          <p>Email: {userData ? userData.email : 'Loading...'}</p>
        </div>
      ) : (
        <p>Please log in to view your profile.</p>
      )}
    </div>
  );
};

export default UserProfile;

In this example, the UserProfile component uses the useAuth hook to check the user's authentication status. If authenticated, it fetches the user data and displays a personalized welcome message. If not authenticated, it prompts the user to log in.

Conclusion

React hooks, including useState, useEffect, and useContext, have transformed the way developers write components by providing a more intuitive and flexible approach to managing state and side effects.

As you continue your journey with React, mastering these hooks will empower you to build more efficient and maintainable applications.

Remember, practice is key to becoming proficient with React hooks. Experiment with different scenarios, explore additional hooks like useReducer and useCallback, and stay up-to-date with the React documentation for any new features or best practices. Happy coding!

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