md4w

A Markdown renderer written in Zig & C, compiled to WebAssymbly.

• Compliance: powered by md4c that is fully compliant to CommonMark 0.31, and partially supports GFM like task list, table, etc.
• Fast: written in Zig & C, compiled to WebAssembly (it's about 2.5x faster than markdown-it, see benchmark).
• Small: ~28KB gzipped.
• Simple: zero dependencies, easy to use.
• Streaming: supports web streaming API for reducing memory usage.
• Universal: works in any JavaScript runtime (Node.js, Deno, Bun, Browsers, Cloudflare Workers, etc).

Usage

// npm i md4w (Node.js, Bun, Cloudflare Workers, etc.)
import { init, mdToHtml, mdToJSON, mdToReadableHtml } from "md4w";
// or use the CDN url (Deno, Browsers)
import { init, mdToHtml, mdToJSON, mdToReadableHtml } from "https://esm.sh/md4w";

// waiting for md4w.wasm...
await init();

// markdown -> HTML
const html = mdToHtml("Stay _foolish_, stay **hungry**!");

// markdown -> HTML (ReadableStream)
const readable = mdToReadableHtml("Stay _foolish_, stay **hungry**!");
const response = new Response(readable, {
  headers: { "Content-Type": "text/html" },
});

// markdown -> JSON
const tree = mdToJSON("Stay _foolish_, stay **hungry**!");

Wasm Mode

md4w provides two webassembly binary files:

• md4w-fast.wasm: Faster but larger binary file. (270KB gzipped)
• md4w-small.wasm: Tiny but slower binary file. (28KB gzipped)

By default, md4w uses the md4w-fast.wasm binary from file system, uses the md4w-small.wasm binary from CDN. You can also specify the wasm file by adding the wasmMode option.

import { init } from "md4w";

await init("fast"); // or "small"

If you are using a bundler like vite, you need to configure the wasm input manually.

import { init } from "md4w";
import wasmUrl from "md4w/js/md4w-fast.wasm?url";

await init(wasmUrl);

Parse Flags

By default, md4w uses the following parse flags:

• COLLAPSE_WHITESPACE: Collapse non-trivial whitespace into single space.
• PERMISSIVE_ATX_HEADERS: Do not require space in ATX headers (###header).
• PERMISSIVE_URL_AUTO_LINKS: Recognize URLs as links.
• STRIKETHROUGH: Text enclosed in tilde marks, e.g. ~foo bar~.
• TABLES: Support GitHub-style tables.
• TASK_LISTS: Support GitHub-style task lists.

You can use the parseFlags option to change the renderer behavior:

mdToHtml("Stay _foolish_, stay **hungry**!", {
  parseFlags: [
    "DEFAULT",
    "NO_HTML",
    "LATEX_MATH_SPANS",
    // ... other parse flags
  ],
});

All available parse flags are:

export enum ParseFlags {
  /** Collapse non-trivial whitespace into single space. */
  COLLAPSE_WHITESPACE,
  /** Do not require space in ATX headers ( ###header ) */
  PERMISSIVE_ATX_HEADERS,
  /** Recognize URLs as links. */
  PERMISSIVE_URL_AUTO_LINKS,
  /** Recognize e-mails as links.*/
  PERMISSIVE_EMAIL_AUTO_LINKS,
  /** Disable indented code blocks. (Only fenced code works.) */
  NO_INDENTED_CODE_BLOCKS,
  /** Disable raw HTML blocks. */
  NO_HTML_BLOCKS,
  /** Disable raw HTML (inline). */
  NO_HTML_SPANS,
  /** Support GitHub-style tables. */
  TABLES,
  /** Support strike-through spans (text enclosed in tilde marks, e.g. ~foo bar~). */
  STRIKETHROUGH,
  /** Support WWW autolinks (without proto; just 'www.') */
  PERMISSIVE_WWW_AUTO_LINKS,
  /** Support GitHub-style task lists. */
  TASKLISTS,
  /** Support LaTeX math spans ($...$) and LaTeX display math spans ($$...$$) are supported. (Note though that the HTML renderer outputs them verbatim in a custom tag <x-equation>.) */
  LATEX_MATH_SPANS,
  /** Support wiki-style links ([[link label]] and [[target article|link label]]) are supported. (Note that the HTML renderer outputs them in a custom tag <x-wikilink>.) */
  WIKI_LINKS,
  /** Denotes an underline instead of an ordinary emphasis or strong emphasis. */
  UNDERLINE,
  /** Using hard line breaks. */
  HARD_SOFT_BREAKS,
  /** Shorthand for NO_HTML_BLOCKS | NO_HTML_SPANS */
  NO_HTML,
  /** Default flags COLLAPSE_WHITESPACE | PERMISSIVE_ATX_HEADERS | PERMISSIVE_URL_AUTO_LINKS | STRIKETHROUGH | TABLES | TASK_LISTS */
  DEFAULT,
}

Code Highlighter

md4w would not add colors to the code blocks by default, however, we provide a setCodeHighlighter function to allow you to add any code highlighter you like.

import { setCodeHighlighter } from "md4w";

setCodeHighlighter((code, lang) => {
  return `<pre><code class="language-${lang}">${hl(code)}</code></pre>`;
});

Caveats

• The returned code will be inserted into the html directly, without html escaping. You should take care of the html escaping by yourself.
• Although we don't send back the highlighted code to the wasm module, the performance is still impacted by the code highlighter.

Web Streaming API

md4w provides the web streaming API, that is useful for a http server to stream the outputed html.

import { mdToReadableHtml } from "md4w";

const readable = mdToReadableHtml(readFile("large.md"));

// write to file
const file = await Deno.open("/foo/bar.html", { write: true, create: true });
readable.pipeTo(file.writable);

// or send to browser
const response = new Response(readable, {
  headers: { "Content-Type": "text/html" },
});

Buffer Size

By default, md4w uses a buffer size of 4KB for streaming, you can change it by adding the bufferSize option.

mdToReadableHtml(largeMarkdown, {
  bufferSize: 16 * 1024,
});

Caveats

The streaming API currently only uses the buffer for output, you still need to load the whole markdown data into memory.

Rendering to JSON

md4w also provides a mdToJSON function to render the markdown to JSON.

const traverse = (node) => {
  // text node
  if (typeof node === "string") {
    console.log(node);
    return;
  }

  // element type
  console.log(node.type);

  // element attributes (may be undefined)
  console.log(node.props);

  // element children (may be undefined)
  node.children?.forEach(traverse);
};

const tree = mdToJSON("Stay _foolish_, stay **hungry**!");
traverse(tree);

Node Type

The node type is a number that represents the type of the node. You can import the NodeType enum to get the human-readable node type.

import { NodeType } from "md4w";

console.log(NodeType.P); // 9
console.log(NodeType.IMG); // 33

if (node.type === NodeType.IMG) {
  console.log("This is an image node, `src` is", node.props.src);
}

All available node types are defined in the NodeType enum.

Development

The renderer is written in Zig, ensure you have it (0.11.0) installed.

zig build && deno test -A

Benchmark

zig build && deno bench -A test/benchmark.js

Prior Art

• md4c - C Markdown parser. Fast. SAX-like interface. Compliant to CommonMark specification.
• markdown-wasm - Very fast Markdown parser and HTML generator implemented in WebAssembly, based on md4c.

License

MIT