JavaScript Web Storage: `localStorage` and `sessionStorage`

Table of Contents

1. [Fundamental Concepts](#fundamental - concepts)
   • [What is Web Storage?](#what - is - web - storage)
   • [Differences between localStorage and sessionStorage](#differences - between - localstorage - and - sessionstorage)
2. [Usage Methods](#usage - methods)
   • [Storing Data](#storing - data)
   • [Retrieving Data](#retrieving - data)
   • [Removing Data](#removing - data)
   • [Clearing All Data](#clearing - all - data)
3. [Common Practices](#common - practices)
   • [Storing User Preferences](#storing - user - preferences)
   • [Caching API Responses](#caching - api - responses)
4. [Best Practices](#best - practices)
   • [Data Serialization](#data - serialization)
   • [Error Handling](#error - handling)
   • [Security Considerations](#security - considerations)
5. Conclusion
6. References

Fundamental Concepts

What is Web Storage?

Web Storage is a client - side storage mechanism introduced in HTML5. It provides a way to store data on the user’s browser in a simple key - value format. Unlike cookies, web storage has a larger storage capacity (usually around 5MB per origin) and the data is not sent to the server with every HTTP request, which can improve performance.

Differences between localStorage and sessionStorage

• localStorage:
  • The data stored in localStorage persists even after the browser is closed and reopened. It has no expiration date unless explicitly removed by the web application or the user.
  • The data is shared across all tabs and windows of the same origin.
• sessionStorage:
  • The data stored in sessionStorage is only available for the duration of the page session. When the page session ends (i.e., the tab or window is closed), the data is cleared.
  • The data is isolated to the specific tab or window where it was created. Different tabs or windows of the same origin do not share sessionStorage data.

Usage Methods

Storing Data

To store data in localStorage or sessionStorage, you can use the setItem() method. The method takes two parameters: a key and a value. The value must be a string.

// Storing data in localStorage
localStorage.setItem('username', 'john_doe');

// Storing data in sessionStorage
sessionStorage.setItem('userLoggedIn', 'true');

Retrieving Data

To retrieve data, you can use the getItem() method. It takes the key as a parameter and returns the corresponding value. If the key does not exist, it returns null.

// Retrieving data from localStorage
const username = localStorage.getItem('username');
console.log(username);

// Retrieving data from sessionStorage
const isLoggedIn = sessionStorage.getItem('userLoggedIn');
console.log(isLoggedIn);

Removing Data

To remove a specific key - value pair, you can use the removeItem() method. It takes the key as a parameter.

// Removing data from localStorage
localStorage.removeItem('username');

// Removing data from sessionStorage
sessionStorage.removeItem('userLoggedIn');

Clearing All Data

To clear all the data stored in localStorage or sessionStorage, you can use the clear() method.

// Clearing localStorage
localStorage.clear();

// Clearing sessionStorage
sessionStorage.clear();

Common Practices

Storing User Preferences

You can use localStorage to store user preferences such as theme selection, font size, or language settings. This way, the user’s preferences will be remembered even when they revisit the website.

// Storing user theme preference
const theme = 'dark';
localStorage.setItem('theme', theme);

// Retrieving user theme preference on page load
const storedTheme = localStorage.getItem('theme');
if (storedTheme) {
    // Apply the theme
    document.body.classList.add(storedTheme);
}

Caching API Responses

You can use localStorage to cache API responses to reduce the number of requests to the server. For example, if you have an API that returns a list of products, you can store the response in localStorage and check if the data is already available before making another API call.

const apiUrl = 'https://api.example.com/products';

// Check if the data is already in localStorage
const cachedData = localStorage.getItem('products');
if (cachedData) {
    const products = JSON.parse(cachedData);
    // Use the cached products
    console.log(products);
} else {
    // Make an API call
    fetch(apiUrl)
      .then(response => response.json())
      .then(data => {
            // Store the data in localStorage
            localStorage.setItem('products', JSON.stringify(data));
            // Use the fetched products
            console.log(data);
        });
}

Best Practices

Data Serialization

Since localStorage and sessionStorage can only store strings, if you want to store complex data types such as objects or arrays, you need to serialize them using JSON.stringify() before storing and deserialize them using JSON.parse() when retrieving.

const user = {
    name: 'John Doe',
    age: 30
};

// Serialize and store the object in localStorage
localStorage.setItem('user', JSON.stringify(user));

// Retrieve and deserialize the object
const storedUser = localStorage.getItem('user');
if (storedUser) {
    const parsedUser = JSON.parse(storedUser);
    console.log(parsedUser.name);
}

Error Handling

When working with localStorage and sessionStorage, errors can occur, such as the storage quota being exceeded. You should handle these errors gracefully in your code.

try {
    localStorage.setItem('largeData', 'a very long string...');
} catch (e) {
    if (e instanceof DOMException && (
        e.code === 22 ||
        e.code === 1014 ||
        e.name === 'QuotaExceededError' ||
        e.name === 'NS_ERROR_DOM_QUOTA_REACHED')
    ) {
        console.error('Storage quota exceeded');
    } else {
        console.error('An error occurred:', e);
    }
}

Security Considerations

• Data Sensitivity: Do not store sensitive information such as passwords or credit card numbers in localStorage or sessionStorage. Since the data is stored on the client - side, it can be accessed by JavaScript code running on the same origin, which poses a security risk.
• Cross - Site Scripting (XSS): Be aware of XSS attacks. Malicious scripts injected through XSS can access and modify the data stored in localStorage and sessionStorage. Make sure to sanitize all user input and output to prevent XSS vulnerabilities.

Conclusion

localStorage and sessionStorage are powerful tools in JavaScript for client - side storage. They offer a simple and efficient way to store data on the user’s browser, with different persistence and scope characteristics. By understanding their fundamental concepts, usage methods, common practices, and best practices, you can use them effectively in your web applications to improve performance, store user preferences, and cache data. However, it’s important to handle errors and security considerations carefully to ensure the reliability and security of your application.

References

• MDN Web Docs - Web Storage API
• HTML5 Rocks - Using the Web Storage API