Streaming with ReadableStream
Stream large responses progressively using the Web Streams API in +server.js.
Streaming with ReadableStream is a free Sveltejs Academy lesson on CoddyKit. This is lesson 4 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.
Why Stream
Large responses can be sent progressively so the client starts receiving data immediately.
ReadableStream API
Use the standard Web Streams API to build a streaming response.
const stream = new ReadableStream({
async start(controller) {
for (const chunk of source) {
controller.enqueue(new TextEncoder().encode(chunk));
}
controller.close();
}
});
return new Response(stream);Server-Sent Events
Stream text/event-stream data for live updates.
Chunked Transfer
HTTP delivers each enqueued chunk over the wire as soon as it is ready.
Use Cases
Real-time logs, AI text generation, file uploads/downloads, progress events.
Client Side
Use response.body.getReader() on the client to consume the stream.
Backpressure
The stream pauses producing when the consumer is slow. Avoid blocking in the producer.
Closing
Always call controller.close() when done. Errors should call controller.error().
Adapters
Streaming support varies by adapter. adapter-node works; some edge runtimes have limits.
Performance
Streaming improves Time to First Byte and perceived speed dramatically.
Combine With load
SvelteKit's streaming load returns Promises that resolve over time, distinct from raw streams.
Quick Check
What is the main benefit of streaming?
Recap
Wrap a ReadableStream in a Response to stream chunks progressively from +server.js.
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Frequently Asked Questions
Is the “Streaming with ReadableStream” lesson free?
Yes — the full text of “Streaming with ReadableStream” 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 with ReadableStream”?
Stream large responses progressively using the Web Streams API in +server.js. 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 4 of 4.
How long does the “Streaming with ReadableStream” 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
- +server.js: GET POST PUT DELETE
- Request and RequestEvent
- Returning JSON Responses
- Streaming with ReadableStream