Svelte Compiler

What Is the Svelte Online Compiler?

The Svelte Online Compiler lets you write and compile Svelte components directly in your browser with instant live preview. It uses the Svelte 4 compiler loaded via CDN to transform your component code into optimized JavaScript. You get a single-pane editor for your Svelte component (including script, markup, and scoped styles) and a live output iframe. Reactive declarations, event handling, and stores are supported so you can experiment with Svelte features without installing any local tooling.

How It Works

1. Write your Svelte component code in the left-hand editor panel, including <script>, markup, and <style> blocks.
2. Click the Run button in the header toolbar.
3. The Svelte 4 compiler is loaded from a CDN and compiles your component source into optimized DOM-targeting JavaScript.
4. The compiled output is loaded inside a sandboxed iframe using an import map for Svelte internals.
5. Your component renders in the output preview on the right.

Step-by-Step Example

Follow these steps to create a reactive counter with a task list:

1. In the editor, add a <script> block and declare let count = 0; and let tasks = ["Sketch", "Build", "Ship"];.
2. Below the script, write markup with an <h1> heading, a paragraph displaying {count}, and a button with on:click={() => count += 1}.
3. Use {#each tasks as task} to loop through the task array and render each item as a list element.
4. Add a <style> block at the bottom to style the card, button, and list layout.
5. Click Run to compile and render. The counter appears in the output panel and clicking the button updates the count reactively.

Use Cases

• Learning Svelte — Explore reactive declarations, template syntax, and component patterns without installing SvelteKit or Node.js.
• Prototyping components — Quickly build and test individual Svelte components before adding them to a full project.
• Testing reactivity — Experiment with the $: reactive syntax, writable stores, and derived values in real time.
• Sharing examples — Write self-contained Svelte demos to share with colleagues or in tutorials.
• Teaching — Demonstrate Svelte concepts in workshops, classrooms, or online courses with zero environment setup.

Limitations & Notes

• No npm package installation is available. Only Svelte core modules are loaded via CDN.
• SvelteKit features like routing, server-side rendering, and load functions are not available.
• The compiler runs in the browser, so opening the file directly (file://) may fail due to CORS. Use a local server or hosted version.
• Multi-file projects and component imports are not supported. All code lives in a single editor.
• Advanced transitions and animations have basic support but may not cover all Svelte transition features.
• API calls and fetch requests from the iframe may be blocked by CORS policies.
• All execution happens in the browser. There is no server-side rendering or Node.js runtime.

Sources & References

• Svelte official documentation — Official guides, tutorials, and API reference for Svelte.
• Svelte tutorial — Interactive step-by-step tutorial for learning Svelte.
• Svelte API reference — Complete API reference for Svelte component and runtime APIs.
• Svelte REPL official — The official Svelte online playground and REPL.
• MDN Web Components — Reference for web component standards and APIs.

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