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Readme

html-to-markdown-wasm

Universal HTML to Markdown converter using WebAssembly.

Powered by the same Rust engine as the Node.js, Python, and Ruby bindings, so Markdown output stays identical regardless of runtime.

Runs anywhere: Node.js, Deno, Bun, browsers, and edge runtimes.

Performance

Universal WebAssembly bindings with excellent performance across all JavaScript runtimes.

Benchmark Results (Apple M4)

Document Type	ops/sec	Notes
Small (5 paragraphs)	70,300	Simple documents
Medium (25 paragraphs)	15,282	Nested formatting
Large (100 paragraphs)	3,836	Complex structures
Tables (20 tables)	3,748	Table processing
Lists (500 items)	1,391	Nested lists
Wikipedia (129KB)	1,022	Real-world content
Wikipedia (653KB)	147	Large documents

Average: ~15,536 ops/sec across varied workloads.

Comparison

vs Native NAPI: ~1.17× slower (WASM has minimal overhead)
vs Python: ~6.3× faster (no FFI overhead)
Best for: Universal deployment (browsers, Deno, edge runtimes, cross-platform apps)

Installation

npm / Yarn / pnpm

npm install html-to-markdown-wasm
# or
yarn add html-to-markdown-wasm
# or
pnpm add html-to-markdown-wasm

Deno

// Via npm specifier
import { convert } from "npm:html-to-markdown-wasm";

Usage

Basic Conversion

import { convert } from 'html-to-markdown-wasm';

const html = '<h1>Hello World</h1><p>This is <strong>fast</strong>!</p>';
const markdown = convert(html);
console.log(markdown);
// # Hello World
//
// This is **fast**!

With Options

import { convert } from 'html-to-markdown-wasm';

const markdown = convert(html, {
  headingStyle: 'atx',
  codeBlockStyle: 'backticks',
  listIndentWidth: 2,
  bullets: '-',
  wrap: true,
  wrapWidth: 80
});

Preserve Complex HTML (NEW in v2.5)

import { convert } from 'html-to-markdown-wasm';

const html = `
<h1>Report</h1>
<table>
  <tr><th>Name</th><th>Value</th></tr>
  <tr><td>Foo</td><td>Bar</td></tr>
</table>
`;

const markdown = convert(html, {
  preserveTags: ['table'] // Keep tables as HTML
});

Deno

import { convert } from "npm:html-to-markdown-wasm";

const html = await Deno.readTextFile("input.html");
const markdown = convert(html, { headingStyle: "atx" });
await Deno.writeTextFile("output.md", markdown);

Performance Tip: For Node.js/Bun, use html-to-markdown-node for 1.17× better performance with native bindings.

Browser (ESM)

<!DOCTYPE html>
<html>
<head>
  <title>HTML to Markdown</title>
</head>
<body>
  <script type="module">
    import init, { convert } from 'https://unpkg.com/html-to-markdown-wasm/dist-web/html_to_markdown_wasm.js';

    // Initialize WASM module
    await init();

    const html = '<h1>Hello World</h1><p>This runs in the <strong>browser</strong>!</p>';
    const markdown = convert(html, { headingStyle: 'atx' });

    console.log(markdown);
    document.body.innerHTML = `<pre>${markdown}</pre>`;
  </script>
</body>
</html>

Vite / Webpack / Bundlers

import { convert } from 'html-to-markdown-wasm';

const markdown = convert('<h1>Hello</h1>', {
  headingStyle: 'atx',
  codeBlockStyle: 'backticks'
});

Cloudflare Workers

import { convert } from 'html-to-markdown-wasm';

export default {
  async fetch(request: Request): Promise<Response> {
    const html = await request.text();
    const markdown = convert(html, { headingStyle: 'atx' });

    return new Response(markdown, {
      headers: { 'Content-Type': 'text/markdown' }
    });
  }
};

TypeScript

Full TypeScript support with type definitions:

import {
  convert,
  convertWithInlineImages,
  WasmInlineImageConfig,
  type WasmConversionOptions
} from 'html-to-markdown-wasm';

const options: WasmConversionOptions = {
  headingStyle: 'atx',
  codeBlockStyle: 'backticks',
  listIndentWidth: 2,
  wrap: true,
  wrapWidth: 80
};

const markdown = convert('<h1>Hello</h1>', options);

Inline Images

Extract and decode inline images (data URIs, SVG):

import { convertWithInlineImages, WasmInlineImageConfig } from 'html-to-markdown-wasm';

const html = '<img src="data:image/png;base64,iVBORw0..." alt="Logo">';

const config = new WasmInlineImageConfig(5 * 1024 * 1024); // 5MB max
config.inferDimensions = true;
config.filenamePrefix = 'img_';
config.captureSvg = true;

const result = convertWithInlineImages(html, null, config);

console.log(result.markdown);
console.log(`Extracted ${result.inlineImages.length} images`);

for (const img of result.inlineImages) {
  console.log(`${img.filename}: ${img.format}, ${img.data.length} bytes`);
  // img.data is a Uint8Array - save to file or upload
}

Build Targets

Three build targets are provided for different environments:

Target	Path	Use Case
Bundler	`html-to-markdown-wasm`	Webpack, Vite, Rollup, esbuild
Node.js	`html-to-markdown-wasm/dist-node`	Node.js, Bun (CommonJS/ESM)
Web	`html-to-markdown-wasm/dist-web`	Direct browser ESM imports

Runtime Compatibility

Runtime	Support	Package
✅ Node.js 18+	Full support	`dist-node`
✅ Deno	Full support	npm: specifier
✅ Bun	Full support (prefer native)	Default export
✅ Browsers	Full support	`dist-web`
✅ Cloudflare Workers	Full support	Default export
✅ Deno Deploy	Full support	npm: specifier

When to Use

Choose html-to-markdown-wasm when:

🌐 Running in browsers or edge runtimes
🦕 Using Deno
☁️ Deploying to Cloudflare Workers, Deno Deploy
📦 Building universal libraries
🔄 Need consistent behavior across all platforms

Use html-to-markdown-node for:

⚡ Maximum performance in Node.js/Bun (~3× faster)
🖥️ Server-side only applications

Configuration Options

See the TypeScript definitions for all available options:

Heading styles (atx, underlined, atxClosed)
Code block styles (indented, backticks, tildes)
List formatting (indent width, bullet characters)
Text escaping and formatting
Tag preservation (preserveTags) and stripping (stripTags)
Preprocessing for web scraping
hOCR table extraction
And more...

Examples

Preserving HTML Tags

Keep specific HTML tags in their original form:

import { convert } from '@html-to-markdown/wasm';

const html = `
<p>Before table</p>
<table class="data">
    <tr><th>Name</th><th>Value</th></tr>
    <tr><td>Item 1</td><td>100</td></tr>
</table>
<p>After table</p>
`;

const markdown = convert(html, {
  preserveTags: ['table']
});

// Result includes the table as HTML

Combine with stripTags:

const markdown = convert(html, {
  preserveTags: ['table', 'form'],  // Keep as HTML
  stripTags: ['script', 'style']    // Remove entirely
});

Deno Web Server

import { convert } from "npm:html-to-markdown-wasm";

Deno.serve((req) => {
  const url = new URL(req.url);

  if (url.pathname === "/convert" && req.method === "POST") {
    const html = await req.text();
    const markdown = convert(html, { headingStyle: "atx" });

    return new Response(markdown, {
      headers: { "Content-Type": "text/markdown" }
    });
  }

  return new Response("Not found", { status: 404 });
});

Browser File Conversion

<input type="file" id="htmlFile" accept=".html">
<button onclick="convertFile()">Convert to Markdown</button>
<pre id="output"></pre>

<script type="module">
  import init, { convert } from 'https://unpkg.com/html-to-markdown-wasm/dist-web/html_to_markdown_wasm.js';

  await init();

  window.convertFile = async () => {
    const file = document.getElementById('htmlFile').files[0];
    const html = await file.text();
    const markdown = convert(html, { headingStyle: 'atx' });
    document.getElementById('output').textContent = markdown;
  };
</script>

Web Scraping (Deno)

import { convert } from "npm:html-to-markdown-wasm";

const response = await fetch("https://example.com");
const html = await response.text();

const markdown = convert(html, {
  preprocessing: {
    enabled: true,
    preset: "aggressive",
    removeNavigation: true,
    removeForms: true
  },
  headingStyle: "atx",
  codeBlockStyle: "backticks"
});

console.log(markdown);

Other Runtimes

The same Rust engine ships as native bindings for other ecosystems:

🖥️ Node.js / Bun: html-to-markdown-node
🐍 Python: html-to-markdown
💎 Ruby: html-to-markdown
🦀 Rust crate & CLI: html-to-markdown-rs

License

MIT