Out-of-order HTML streaming via Declarative Partial Updates.
Define named slots, fill them in any order, and the browser renders content as it arrives. No client-side JavaScript required.
npm install stream-slots
Servers stream HTML top-to-bottom, so a slow database query for the header blocks everything below it. You end up either waiting for all the data before sending anything, or sending a shell and hydrating with client JS later. DPU gives you a third option: stream the shell immediately, then push each section into the page as its data resolves, in any order, with zero client JavaScript.
stream-slots wraps that in a clean API:
import { createPage } from 'stream-slots';
const page = createPage({
title: 'Dashboard',
slots: [
'header',
{ name: 'metrics', fallback: 'Loading metrics...' },
'chart',
'footer'
]
});
const stream = page.stream();
// Fill slots out of order. Footer before header!
page.fill('footer', '<footer>© 2026</footer>');
page.fill('header', '<h1>Dashboard</h1>');
// Async data? No problem.
const data = await fetchMetrics();
page.fill('metrics', renderMetrics(data));
page.fill('chart', renderChart(data));
page.close();
return new Response(stream, {
headers: { 'Content-Type': 'text/html' }
});
The browser sees the page shell right away (with "Loading metrics..." as a placeholder), then each slot fills in as its <template for="..."> fragment arrives.
Server Browser
------ -------
1. Send HTML shell with placeholders -> Fallback content shown
<?start name="header"><?end>
2. Data for "footer" ready first -> Footer fills in
<template for="footer">...</template>
3. Data for "header" ready -> Header fills in
<template for="header">...</template>
4. Remaining slots fill as ready -> Progressive rendering!
Under the hood, stream-slots uses processing instructions (<?start name="x">) and <template for="x"> elements from the DPU patching spec. Chrome 148+ handles this natively when the experimental flag is on. No polyfill needed.
createPage(options)Creates a new streaming page with a full HTML shell.
Options:
| Option | Type | Default | Description |
|---|---|---|---|
title | string | '' | Page <title> |
head | string | '' | Extra HTML for <head> (stylesheets, meta tags) |
lang | string | 'en' | HTML lang attribute |
charset | string | 'utf-8' | Charset meta tag |
slots | (string | { name, fallback })[] | required | Slot declarations. Strings or objects with optional fallback content. |
body | string | auto | Custom body HTML. Use {{slots}} as the placeholder. |
Returns a Page object:
| Method / Property | Description |
|---|---|
stream() | Returns a ReadableStream<Uint8Array>. The shell is enqueued right away. Call once. |
fill(name, html) | Fill a slot with HTML. Enqueues a <template for="name"> chunk. |
append(name, html, keepOpen?) | Append HTML to a slot. Leaves a <?marker> for further appends when keepOpen is true (default). |
error(name, err) | Fill a slot with a styled error message. |
abort(err) | Abort the entire stream with an error. |
close() | Close the stream cleanly. |
isClosed | boolean - whether the stream has been closed or aborted. |
filledSlots | Set<string> - which slots have been filled so far. |
declaredSlots | Set<string> - all declared slot names. |
createStream()Creates a lightweight stream for DPU fills without generating an HTML shell. Use this when you write your own HTML and only need the fill/append mechanism.
import { createStream } from 'stream-slots';
const { stream, fill, append, close } = createStream();
// Write your own shell however you want
res.write(myCustomShell);
// Pipe the DPU fills
Readable.fromWeb(stream).pipe(res);
fill('header', '<h1>Hello</h1>');
close();
Returns { stream, fill, append, close, abort, isClosed }.
If you need more control, the building blocks are exported too:
import {
renderSlotPlaceholder,
renderSlotFill,
renderSlotAppend,
createSlot,
generateShell
} from 'stream-slots';
renderSlotPlaceholder('header')
// => '<?start name="header"><?end>'
renderSlotPlaceholder('header', 'Loading...')
// => '<?start name="header">Loading...<?end>'
renderSlotFill('header', '<h1>Hello</h1>')
// => '<template for="header"><h1>Hello</h1></template>'
renderSlotAppend('results', '<li>Item 1</li>')
// => '<template for="results"><li>Item 1</li><?marker name="results"></template>'
const slot = createSlot('sidebar');
slot.placeholder('Loading nav...')
slot.fill('<nav>Links</nav>')
slot.append('<li>More</li>')
The spec's <?marker> processing instruction lets you stream content incrementally into the same slot. This is perfect for search results, feeds, or any list that builds up over time:
const page = createPage({
title: 'Search',
slots: [{ name: 'results', fallback: 'Searching...' }]
});
const stream = page.stream();
// Stream results as they come in
page.append('results', '<li>Result 1</li>'); // keeps slot open
page.append('results', '<li>Result 2</li>'); // keeps slot open
page.append('results', '<li>Result 3</li>', false); // final append
page.close();
Each append() sends a <template for="results"> with a <?marker> that tells the browser where to insert the next chunk.
import http from 'node:http';
import { Readable } from 'node:stream';
import { createPage } from 'stream-slots';
http.createServer(async (req, res) => {
const page = createPage({
title: 'My App',
slots: [
'header',
{ name: 'content', fallback: 'Loading...' },
'sidebar'
]
});
res.writeHead(200, { 'Content-Type': 'text/html' });
Readable.fromWeb(page.stream()).pipe(res);
page.fill('header', '<h1>Hello World</h1>');
setTimeout(() => {
page.fill('sidebar', '<nav>Navigation</nav>');
}, 500);
setTimeout(() => {
page.fill('content', '<p>Main content loaded!</p>');
page.close();
}, 1000);
}).listen(3000);
You can wrap the slots in your own markup using {{slots}}:
const page = createPage({
title: 'Dashboard',
slots: ['nav', 'main', 'aside'],
body: `
<div style="display:grid; grid-template-columns:250px 1fr 300px; min-height:100vh;">
{{slots}}
</div>
`
});
try {
const data = await fetchData();
page.fill('widget', renderWidget(data));
} catch (err) {
page.error('widget', err); // Shows a styled error in the slot
}
// Or abort the whole stream:
page.abort(new Error('Critical failure'));
| Feature | Status |
|---|---|
| DPU (native) | Chrome 148+ with enable-experimental-web-platform-features flag |
| Fallback | Slots show fallback content if DPU is not supported (progressive enhancement) |
git clone https://github.com/hemanth/stream-slots
cd stream-slots
node demo/server.js
# Open http://localhost:3000
The playground lets you add slots, set delays, and watch the chunks arrive in real time.
MIT