Resolving localStorage is not defined Error in Svelte SSR

Learn how to fix the `ReferenceError: localStorage is not defined` when using Svelte's server-side rendering by checking for a browser environment.
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Tackling the localStorage is not defined Error in Svelte SSR

When developing web applications with Svelte, particularly under server-side rendering (SSR), developers may encounter a frustrating error: ReferenceError: localStorage is not defined. This error arises when attempting to access localStorage during server-side operations, leading to unexpected behavior in your application. In this post, I’ll explain the root cause of this error and provide a clear solution that helps you effectively manage user information storage in your Svelte application.

Understanding the Problem

To understand why this error occurs, let's look at the context:

Server-Side Rendering: This process runs JavaScript on the server before sending the page to the user's browser. Since localStorage is a browser-specific feature, it is not available in this environment, leading to the ReferenceError when your Svelte app tries to access it.

The Error: Specifically, the error message states:

This indicates that your application is attempting to access localStorage during server-side rendering, which is not allowed. Interestingly, this issue appears only on the first load, as the second load occurs in the browser environment where localStorage is available.

The Solution: Check for Browser Environment

Step-by-Step Fix

Import Browser Environment: Use Svelte's built-in method to detect the browser environment.

Conditional Accessing: Wrap your localStorage related code in a condition that checks if the code is running in the browser.

[[See Video to Reveal this Text or Code Snippet]]

Key Changes Explained

Importing the Browser Checker: The line import { browser } from '$app/env'; allows us to easily determine if we are running in the browser.

Conditional Logic: Each time you access or manipulate localStorage, a check is made to see if the browser variable is true. This prevents any attempts to access localStorage when the code runs on the server.

Conclusion

By implementing these changes, you can prevent the ReferenceError: localStorage is not defined error from occurring in your Svelte application. This simple check for a browser environment not only resolves the problem but also enhances the overall robustness of your application when dealing with user data. Always remember the distinction between server-side and client-side environments while developing applications that utilize browser-specific features.

Implement these adjustments and watch your Svelte app handle user information seamlessly across page reloads—without any hitches!