Streaming from Server with +server.js
Emit chunks progressively from an API route using a ReadableStream.
Streaming from Server with +server.js is a free Sveltejs Academy lesson on CoddyKit. This is lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Sveltejs Academy learning path, and your progress syncs across the web and the CoddyKit app. The Sveltejs Academy course includes 4 lessons in total.
Server-Sent Streams
Inside +server.js, return a Response wrapping a ReadableStream.
Basic Stream
Build a stream that emits chunks over time.
export function GET() {
const stream = new ReadableStream({
async start(controller) {
for (const chunk of source) {
controller.enqueue(new TextEncoder().encode(chunk));
}
controller.close();
}
});
return new Response(stream);
}Content Type
Set the appropriate content type: text/plain, text/event-stream, application/json (NDJSON), etc.
Use Cases
AI text generation, real-time logs, large file downloads, progressive data updates.
Backpressure
The stream throttles producing when the consumer is slow, preventing memory bloat.
Client Consumption
Use response.body.getReader() in the client to read the stream.
Server-Sent Events
For one-way push from server to client, SSE is a great fit. Encode events as text/event-stream.
WebSockets Alternative
For bidirectional, choose WebSockets via a separate server or adapter that supports them.
Cancellation
Handle stream cancellation in the start function to free resources.
Adapter Support
Most adapters support streaming; check your adapter's docs for edge cases.
Performance
Streaming reduces TTFB and lets clients process partial data sooner.
Quick Check
What stream class do you use in +server.js?
Recap
Use ReadableStream in +server.js for chunked, low-latency server responses.
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Frequently Asked Questions
Is the “Streaming from Server with +server.js” lesson free?
Yes — the full text of “Streaming from Server with +server.js” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Sveltejs Academy course, upgrade to CoddyKit PRO. The Sveltejs Academy course includes 4 lessons in total.
What will I learn in “Streaming from Server with +server.js”?
Emit chunks progressively from an API route using a ReadableStream. You practise Sveltejs Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Sveltejs Academy?
No prior experience is required. Sveltejs Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 4.
How long does the “Streaming from Server with +server.js” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Sveltejs Academy lesson?
Yes. Every Sveltejs Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Streaming Data with Promises in load
- Deferred Loading and Suspense-Like UX
- Streaming from Server with +server.js
- Progressive Hydration