Great question! Let's break down the difference between hook files and server files in SvelteKit, and when you'd use each.

Hook Files

Hook files (hooks.server.js, hooks.client.js, hooks.js) are special files that allow you to intercept and modify SvelteKit's behavior at specific points in the application lifecycle. They run app-wide, meaning they affect all routes and pages in your application.

Key characteristics of hook files:

1. Global scope: They apply to your entire application
2. Lifecycle events: They run at specific moments (like when a request is received or when an error occurs)
3. Three types:
   • Server hooks: Run only on the server
   • Client hooks: Run only in the browser
   • Universal hooks: Run in both environments

Example use cases for hooks:

• Modifying all incoming requests (like adding headers)
• Handling errors globally
• Initializing database connections when the server starts
• Custom routing logic

Server Files

Server files (like +page.server.js or +layout.server.js) are route-specific files that handle server-side logic for individual pages or layouts. They run only on the server and are used for:

1. Data loading: Fetching data from databases or APIs
2. Form actions: Handling form submissions
3. Server-only operations: Using private environment variables or accessing the filesystem

Key differences:

• Scope: Server files are route-specific, while hooks are app-wide
• Purpose: Server files handle page-specific logic, hooks handle global behavior
• Execution: Server files run per-request, hooks can run at startup or per-request

When to Use Which

Use hook files when you need to:

• Modify behavior across your entire app
• Handle errors globally
• Initialize shared resources (like database connections)
• Implement custom routing logic

Use server files when you need to:

• Load data for a specific page
• Handle form submissions for a specific route
• Access server-only resources for a particular page

Example Comparison

Hook File Example (global request modification):

// src/hooks.server.js
export async function handle({ event, resolve }) {
  // Add a custom header to ALL responses
  event.request.headers.set('x-custom-header', 'hello');
  return resolve(event);
}

Server File Example (page-specific data loading):

// src/routes/blog/[slug]/+page.server.js
import { getPost } from '$lib/server/database';

export async function load({ params }) {
  // Load data for a specific blog post
  return {
    post: await getPost(params.slug)
  };
}

Remember:

• Hooks are for global, app-wide behavior
• Server files are for route-specific server logic
• You'll often use both together in a SvelteKit application

Does this help clarify the distinction? Let me know if you have any other questions!