markdown-wasm

Very fast Markdown parser & HTML renderer implemented in WebAssembly

• Zero dependencies (31 kB gzipped)
• Portable & safe (WASM executes in isolated memory and can run almost anywhere)
• Simple API
• Very fast
• Based on md4c — compliant to the CommonMark specification

Examples

In NodeJS, single file with embedded compressed WASM

const markdown = require("./dist/markdown.node.js")
console.log(markdown.parse("# hello\n*world*"))

ES module with WASM loaded separately

import * as markdown from "./dist/markdown.es.js"
await markdown.ready
console.log(markdown.parse("# hello\n*world*"))

Web browser

<script src="markdown.js"></script>
<script>
window["markdown"].ready.then(markdown => {
  console.log(markdown.parse("# hello\n*world*"))
})
</script>

Install

npm install markdown-wasm

Benchmarks

The test/benchmark directory contain a benchmark suite which you can run yourself. It tests a few popular markdown parser-renderers by parsing & rendering a bunch of different sample markdown files.

The following results were samples on a 2.9 GHz MacBook running macOS 10.15, NodeJS v14.11.0

Average ops/second

Ops/second represents how many times a library is able to parse markdown and render HTML during a second, on average across all sample files.

Average throughput

Throughput is the average amount of markdown data processed during a second while both parsing and rendering to HTML. The statistics does not include HTML generated but only bytes of markdown source text parsed.

Min–max parse time

This graph shows the spread between the fastest and slowest parse-and-render operations for each library. Lower numbers are better.

See test/benchmark for more information.

API

/**
 * parse reads markdown source at s and converts it to HTML.
 * When output is a byte array, it will be a reference.
 */
export function parse(s :Source, o? :ParseOptions & { bytes? :never|false }) :string
export function parse(s :Source, o? :ParseOptions & { bytes :true }) :Uint8Array

/** Markdown source code can be provided as a JavaScript string or UTF8 encoded data */
type Source = string | ArrayLike<number>

/** Options for the parse function */
export interface ParseOptions {
  /** Customize parsing. Defaults to ParseFlags.DEFAULT */
  parseFlags? :ParseFlags

  /** Select output format. Defaults to "html" */
  format? : "html" | "xhtml"

  /**
   * bytes=true causes parse() to return the result as a Uint8Array instead of a string.
   *
   * The returned Uint8Array is only valid until the next call to parse().
   * If you need to keep the returned data around, call Uint8Array.slice() to make a copy,
   * as each call to parse() uses the same underlying memory.
   *
   * This only provides a performance benefit when you never need to convert the output
   * to a string. In most cases you're better off leaving this unset or false.
   */
  bytes? :boolean

  /** Allow "javascript:" in links */
  allowJSURIs? :boolean

  /**
   * Optional callback which if provided is called for each code block.
   * langname holds the "language tag", if any, of the block.
   *
   * The returned value is inserted into the resulting HTML verbatim, without HTML escaping.
   * Thus, you should take care of properly escaping any special HTML characters.
   *
   * If the function returns null or undefined, or an exception occurs, the body will be
   * included as-is after going through HTML escaping.
   *
   * Note that use of this callback has an adverse impact on performance as it casues
   * calls and data to be bridged between WASM and JS on every invocation.
   */
  onCodeBlock? :(langname :string, body :UTF8Bytes) => Uint8Array|string|null|undefined

  /** @depreceated use "bytes" instead (v1.1.1) */
  asMemoryView? :boolean
}

/** UTF8Bytes is a Uint8Array representing UTF8 text  */
export interface UTF8Bytes extends Uint8Array {
  /** toString returns a UTF8 decoded string (lazily decoded and cached) */
  toString() :string
}

/** Flags that customize Markdown parsing */
export enum ParseFlags {
  /** In TEXT, collapse non-trivial whitespace into single ' ' */ COLLAPSE_WHITESPACE,
  /** Enable $ and $$ containing LaTeX equations. */              LATEX_MATH_SPANS,
  /** Disable raw HTML blocks. */                                 NO_HTML_BLOCKS,
  /** Disable raw HTML (inline). */                               NO_HTML_SPANS,
  /** Disable indented code blocks. (Only fenced code works.) */  NO_INDENTED_CODE_BLOCKS,
  /** Do not require space in ATX headers ( ###header ) */        PERMISSIVE_ATX_HEADERS,
  /** Recognize e-mails as links even without <...> */            PERMISSIVE_EMAIL_AUTO_LINKS,
  /** Recognize URLs as links even without <...> */               PERMISSIVE_URL_AUTO_LINKS,
  /** Enable WWW autolinks (without proto; just 'www.') */        PERMISSIVE_WWW_AUTOLINKS,
  /** Enable strikethrough extension. */                          STRIKETHROUGH,
  /** Enable tables extension. */                                 TABLES,
  /** Enable task list extension. */                              TASK_LISTS,
  /** Enable wiki links extension. */                             WIKI_LINKS,
  /** Enable underline extension (disables '_' for emphasis) */   UNDERLINE,

  /** Default flags are:
   *    COLLAPSE_WHITESPACE |
   *    PERMISSIVE_ATX_HEADERS |
   *    PERMISSIVE_URL_AUTO_LINKS |
   *    STRIKETHROUGH |
   *    TABLES |
   *    TASK_LISTS
   */
  DEFAULT,

  /** Shorthand for NO_HTML_BLOCKS | NO_HTML_SPANS */
  NO_HTML,
}

See markdown.d.ts

Building from source

npm install
npx wasmc

Build debug version of markdown into ./build/debug and watch source files:

npx wasmc -g -w

If you need special builds, like for example an ES module with embedded WASM, edit the wasmc.js file and add module({...}) directives.

Example:

module({ ...m,
  name:   "markdown-custom",
  out:    outdir + "/markdown.custom.js",
  embed:  true,
  format: "es",
})