When I first learned React, everything made sense once I grasped the concept of the virtual DOM and functional components re-rendering. But then I switched to SvelteKit, and boy—did things get interesting! Initially, I thought SvelteKit worked pretty much like React, just with some minor syntax differences. Turns out, I was wrong.

Let’s take a walk through the complete lifecycle of a SvelteKit application, step-by-step, highlighting common mistakes (usually mine!) along the way.

Step 1: The Initial Request

When someone visits your SvelteKit website for the first time, the server receives this request (e.g., the homepage /). Here’s what happens behind the scenes:

• Server-side rendering (SSR) kicks in.
• The server runs your +page.server.ts (if present) and the corresponding .svelte file to generate HTML.
• This freshly baked HTML, combined with your JavaScript bundle, is sent to the user’s browser.

Common Mistake: Static HTML Confusion

Initially, I thought the server just sent a static index.html file each time. But that’s not correct. SvelteKit dynamically generates HTML for every request, tailoring content based on your load() function.

Step 2: Hydration (JavaScript Wakes Up)

Your browser receives the HTML and the JavaScript. Now, the magical-sounding process called “hydration” happens:

• Svelte attaches event listeners (like on:click) to your static HTML.
• The page becomes interactive as JavaScript binds itself to the existing DOM.

At this point, your website transforms from a static snapshot into a dynamic application.

Common Mistake: Thinking Hydration = Initial JavaScript Compilation

At first, I mistakenly believed that hydration was when SvelteKit compiled .svelte files into JavaScript at runtime. Nope! That compilation happens ahead-of-time during your build step (npm run build), not on each user request.

Hydration is more like waking up the JavaScript already sent to your browser. Think of it like rehydrating dried noodles—you already have them; you just add water (or in this case, JavaScript event bindings).

Step 3: Client-side Interactions (No Server Needed!)

Once hydrated, your app behaves like a single-page application (SPA). User interactions (clicking buttons, typing in forms, scrolling) happen entirely on the client side.

Imagine you have:

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<button on:click={() => count += 1}>Click me</button>

When someone clicks, the JavaScript instantly updates the DOM—no request to the server needed.

Common Mistake: Thinking Everything Goes Through the Server

Initially, I thought every interaction sent a request back to the server, like in traditional web apps. But with SvelteKit, once the page is hydrated, interactions are handled entirely in the browser unless you explicitly call the server (e.g., via fetch() or form actions).

Step 4: Navigating to Another Page (Client-side Routing)

Let’s say a user navigates from /home to /about. Here’s how SvelteKit handles that:

• SvelteKit intercepts the link click.
• It calls the load() function defined in your new route’s +page.ts or +page.server.ts.
• If the load function is client-side, it fetches data locally; if server-side, it fetches fresh JSON from your server.
• Your precompiled JavaScript then uses this JSON data to dynamically create and update the DOM nodes.

Common Mistake: Assuming HTML is Refetched on Every Navigation

Here’s another misconception I initially had: I thought each navigation would request fresh HTML from the server. Not so!

Instead, after the initial load, the HTML structure is dynamically constructed client-side by JavaScript functions compiled from your .svelte files during the initial build. The server only provides data (usually JSON), which is then plugged into your precompiled JavaScript template.

Step 5: Server Interactions (Only When Necessary)

SvelteKit doesn’t isolate you from the server completely. Sometimes you’ll need server-side actions, such as submitting forms, authentication, or API calls.

For example, a login form:

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<form method="POST">
  <input name="email">
  <button type="submit">Login</button>
</form>

• A form submission (POST request) triggers an action on the server.
• The server handles the request, sends a response, and the client updates accordingly.

Common Mistake: Overusing Server Requests

I initially sent too many requests to the server because I didn’t realize how efficiently SvelteKit could handle state and interactions client-side. Remember, keep things client-side unless you genuinely need server-side logic (e.g., security-sensitive operations).

Recap & Mental Model

Here’s your simple mental model for SvelteKit:

• First page load: HTML is dynamically rendered server-side and hydrated client-side.
• Subsequent interactions: Handled fully client-side by JavaScript.
• Navigation: Uses client-side routing; fetches only data, not full HTML.
• Server interaction: Occurs only explicitly via form submissions, fetch calls, or server-side load() functions.

Think of SvelteKit as a Compiler

This analogy finally made everything click for me:

• Your .svelte files: High-level source code.
• SvelteKit compiler: Turns source into optimized JavaScript.
• JavaScript code: “Machine instructions” that directly update your DOM without re-rendering everything (unlike React’s virtual DOM).

Understanding this process not only makes you a better SvelteKit developer, it also opens your eyes to why Svelte apps are often blazingly fast and responsive.