02. streamHTML into an element

A static page loads first. A button kicks off a fetch() whose body is piped into a target element via streamHTMLUnsafe(). Tokens appear as they arrive on the wire.

Target

click the button to stream content here.

The minimal JS

const res = await fetch('/02/stream');
res.body
  .pipeThrough(new TextDecoderStream())
  .pipeTo(document.getElementById('target').streamHTMLUnsafe());

learn more

Chrome blog: Declarative Partial Updates — Includes the streaming-html walkthrough.
WICG explainer: dynamic markup revamped — Why streamHTMLUnsafe() and friends exist; the full set of new HTML insertion methods.
html-setters polyfill — streamHTMLUnsafe / streamAppendHTMLUnsafe etc. for unsupported browsers.
Chrome Status: Out of order streaming — DevTrial in 148 (behind flag), shipping target 150.
Chrome Status: setHTMLUnsafe and parseHTMLUnsafe — The Chrome 124 building blocks the streaming methods extend.
WHATWG HTML PR #11818 — The actual spec change.
WHATWG HTML issue #2142 — The original 2017 thread that started it all.

source

Everything the server runs for this example. Each file links to GitHub so you can copy or fork it.

handler.ts/app/src/examples/02-streaming-fetch/handler.ts

import { sleep, streamingResponse } from "../../lib/streaming.ts";
import { loadShell, tryStatic } from "../../lib/files.ts";

const SHELL = loadShell(import.meta.url);

const TOKENS = [
  "<h2>streamed paragraph</h2>",
  "<p>this content is being emitted ",
  "one chunk at a time, ",
  "with a 200ms pause between each piece. ",
  "Each chunk is parsed and reflected ",
  "in the DOM as it arrives, ",
  "no buffering, no full replace.</p>",
  "<ul>",
  "<li>first item</li>",
  "<li>second item</li>",
  "<li>third item</li>",
  "</ul>",
  '<p style="color:var(--muted);font-size:.85rem;">stream ended.</p>',
];

export default function handle(_req: Request, path: string): Response | Promise<Response> {
  if (path === "/" || path === "/index.html") {
    return new Response(SHELL, { headers: { "content-type": "text/html; charset=utf-8" } });
  }
  if (path === "/stream") {
    return streamingResponse(async (write) => {
      for (const chunk of TOKENS) {
        await write(chunk);
        await sleep(220);
      }
    });
  }
  return tryStatic(import.meta.url, path) ?? new Response("Not found", { status: 404 });
}

open on GitHub →

client.js/app/src/examples/02-streaming-fetch/client.js

const target = document.getElementById("target");

document.getElementById("go").addEventListener("click", async () => {
  target.replaceChildren();
  const res = await fetch("/02/stream");
  if (!("streamHTMLUnsafe" in target)) {
    target.textContent =
      "streamHTMLUnsafe not supported. Enable chrome://flags/#enable-experimental-web-platform-features";
    return;
  }
  await res.body
    .pipeThrough(new TextDecoderStream())
    .pipeTo(target.streamHTMLUnsafe());
});

document.getElementById("clear").addEventListener("click", () => {
  target.innerHTML = '<p style="color:var(--muted);margin:0;">cleared.</p>';
});

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index.html/app/src/examples/02-streaming-fetch/index.html

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>02 · Streaming fetch</title>
  <link rel="stylesheet" href="/public/styles.css">
</head>
<body>
<main>
  <p class="crumbs"><a href="/">&larr; back to index</a></p>
  <h1>02. streamHTML into an element</h1>
  <p class="lede">A static page loads first. A button kicks off a <code>fetch()</code> whose body is piped into a target element via <code>streamHTMLUnsafe()</code>. Tokens appear as they arrive on the wire.</p>

  <button id="go">stream into target</button>
  <button id="clear">clear</button>

  <h3>Target</h3>
  <section class="demo-area" id="target">
    <p style="color:var(--muted);margin:0;">click the button to stream content here.</p>
  </section>

  <h3>The minimal JS</h3>
  <pre><code>const res = await fetch('/02/stream');
res.body
  .pipeThrough(new TextDecoderStream())
  .pipeTo(document.getElementById('target').streamHTMLUnsafe());</code></pre>

  <script src="/02/client.js"></script>

  <section class="refs">
    <h3>learn more</h3>
    <ul>
      <li><a href="https://developer.chrome.com/blog/declarative-partial-updates?hl=en" target="_blank" rel="noopener">Chrome blog: Declarative Partial Updates</a> <span class="note">— Includes the streaming-html walkthrough.</span></li>
      <li><a href="https://github.com/WICG/declarative-partial-updates/blob/main/dynamic-markup-revamped-explainer.md" target="_blank" rel="noopener">WICG explainer: dynamic markup revamped</a> <span class="note">— Why <code>streamHTMLUnsafe()</code> and friends exist; the full set of new HTML insertion methods.</span></li>
      <li><a href="https://github.com/GoogleChromeLabs/html-setters-polyfill" target="_blank" rel="noopener">html-setters polyfill</a> <span class="note">— streamHTMLUnsafe / streamAppendHTMLUnsafe etc. for unsupported browsers.</span></li>
      <li><a href="https://chromestatus.com/feature/5111042975465472" target="_blank" rel="noopener">Chrome Status: Out of order streaming</a> <span class="note">— DevTrial in 148 (behind flag), shipping target 150.</span></li>
      <li><a href="https://chromestatus.com/feature/6560361081995264" target="_blank" rel="noopener">Chrome Status: setHTMLUnsafe and parseHTMLUnsafe</a> <span class="note">— The Chrome 124 building blocks the streaming methods extend.</span></li>
      <li><a href="https://github.com/whatwg/html/pull/11818" target="_blank" rel="noopener">WHATWG HTML PR #11818</a> <span class="note">— The actual spec change.</span></li>
      <li><a href="https://github.com/whatwg/html/issues/2142" target="_blank" rel="noopener">WHATWG HTML issue #2142</a> <span class="note">— The original 2017 thread that started it all.</span></li>
    </ul>
  </section>

  <!-- source-viewer goes here -->

  <footer class="byline">made by <a href="https://paul.kinlan.me/" target="_blank" rel="noopener">Paul Kinlan</a></footer>
</main>
</body>
</html>

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lib/streaming.ts/app/src/lib/streaming.ts

export class StreamAborted extends Error {
  constructor() {
    super("stream aborted by client");
    this.name = "StreamAborted";
  }
}

export type ChunkSource = (write: (chunk: string) => Promise<void>) => Promise<void>;

const encoder = new TextEncoder();

export function streamingResponse(
  source: ChunkSource,
  contentType = "text/html; charset=utf-8",
): Response {
  let aborted = false;
  const body = new ReadableStream<Uint8Array>({
    async start(controller) {
      const write = async (chunk: string) => {
        if (aborted) throw new StreamAborted();
        try {
          controller.enqueue(encoder.encode(chunk));
        } catch {
          aborted = true;
          throw new StreamAborted();
        }
      };
      try {
        await source(write);
      } catch (err) {
        if (!(err instanceof StreamAborted)) console.error("stream source error", err);
      } finally {
        if (!aborted) {
          try {
            controller.close();
          } catch {
            // already closed
          }
        }
      }
    },
    cancel() {
      aborted = true;
    },
  });
  return new Response(body, {
    headers: {
      "content-type": contentType,
      "cache-control": "no-store",
      "x-content-type-options": "nosniff",
    },
  });
}

export function sleep(ms: number): Promise<void> {
  return new Promise((resolve) => setTimeout(resolve, ms));
}

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lib/files.ts/app/src/lib/files.ts

// File helpers for examples: read sibling files, render the page shell with the
// source viewer injected, and serve sibling static assets (CSS/JS/HTML) on demand.

import { sourceBlock } from "./source.ts";

const MIME: Record<string, string> = {
  css: "text/css; charset=utf-8",
  js: "application/javascript; charset=utf-8",
  html: "text/html; charset=utf-8",
  svg: "image/svg+xml",
  json: "application/json; charset=utf-8",
};

function dirFor(handlerUrl: string): string {
  return new URL(".", handlerUrl).pathname;
}

export function readSibling(handlerUrl: string, name: string): string {
  return Deno.readTextFileSync(dirFor(handlerUrl) + name);
}

// Reads a sibling HTML file. The marker `<!-- source-viewer goes here -->` is replaced
// with the source-viewer block. Extra named placeholders ({{name}}) can be supplied via `vars`.
const SOURCE_MARKER = "<!-- source-viewer goes here -->";

export function loadShell(
  handlerUrl: string,
  name = "index.html",
  vars: Record<string, string> = {},
): string {
  let raw = readSibling(handlerUrl, name);
  raw = raw.replace(SOURCE_MARKER, sourceBlock(handlerUrl));
  for (const [k, v] of Object.entries(vars)) {
    raw = raw.replaceAll(`{{${k}}}`, v);
  }
  return raw;
}

// Try to serve a sibling static file from the example folder. Returns null when the
// file is missing, the path escapes the folder, or the path is empty.
export function tryStatic(handlerUrl: string, requestPath: string): Response | null {
  const safe = requestPath.replace(/^\/+/, "");
  if (!safe || safe.includes("..")) return null;
  // Never let the browser fetch handler.ts or README.md through this fallback.
  if (safe === "handler.ts" || safe === "README.md") return null;
  try {
    const content = Deno.readFileSync(dirFor(handlerUrl) + safe);
    const ext = safe.split(".").pop() ?? "";
    return new Response(content, {
      headers: {
        "content-type": MIME[ext] ?? "application/octet-stream",
        "cache-control": "no-store",
      },
    });
  } catch {
    return null;
  }
}

open on GitHub →