Defensive Isomorphic Design with Browser Checks

Snippet

Defensive Isomorphic Design with Browser Checks

In Server-Side Rendering (SSR) environments, accessing browser-specific globals like 'window' or 'localStorage' outside of lifecycle hooks will cause server crashes. Using the '$app/environment' browser flag or 'onMount' ensures code is only executed in the appropriate context, maintaining isomorphic stability.

snippet.js
javascript
import { browser } from '$app/environment';
import { onMount } from 'svelte';
 
let storageValue;
 
// Expert Pattern: Prevent SSR crashes
if (browser) {
  storageValue = localStorage.getItem('user_prefs');
}
 
onMount(() => {
  // Guaranteed to run only on client
  const width = window.innerWidth;
  console.log('Client-only logic:', width);
});

svelte

Breakdown

import { browser }

SvelteKit-specific boolean that is true only when running in the user's browser.

if (browser) { ... }

Protects top-level script execution from referencing APIs that don't exist in Node.js.

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Defensive Isomorphic Design with Browser Checks

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