showdown

Showdown is a JavaScript library that converts Markdown into HTML and can also run the conversion in reverse, turning HTML back into Markdown. It works both server-side under Node.js and client-side in the browser, with no required dependency on any particular framework. The converter is built around a flavor system (vanilla, github/GFM, commonmark, original) that toggles preset bundles of options, so a project can opt into GitHub-flavored Markdown behaviors like tables, task lists, strikethrough, and auto-linking without hand-configuring each one.

Beyond the core makeHtml/makeMarkdown API, Showdown ships a documented extension system (lang, output, and listener-style extensions), an event bus for hooking into stages of the parse pipeline, and a bundled CLI (showdown makehtml / showdown makemarkdown) for converting files from the command line. It has been a stable dependency in the Node ecosystem for well over a decade, with a large user base including projects like Ghost’s markdown handling.

What You Get

• A Converter class exposing makeHtml(markdown) and makeMarkdown(html) for bidirectional conversion
• Flavor presets (vanilla, github/gfm, commonmark, original) that bundle common option combinations
• GitHub-flavored Markdown extras: tables, task lists, strikethrough, fenced code blocks, GFM mentions, and simplified autolinking
• An extension system (listener, lang, and output extensions) plus an event bus for hooking into the parse pipeline
• A bundled CLI (showdown makehtml / showdown makemarkdown) for converting files or stdin without writing any JS
• Configurable option system (setOption/getOption/setFlavor) at both the global and per-instance level

Common Use Cases

• Rendering user-authored Markdown (comments, docs, README previews) to HTML in a Node backend or in-browser editor
• Building a live Markdown preview pane that re-renders HTML as the user types
• Converting existing HTML content into Markdown for migration into a Markdown-based CMS or static site generator
• Enabling GitHub-flavored Markdown features like task lists and tables in an internal tool without building a parser from scratch
• Batch-converting a directory of Markdown files to HTML (or vice versa) from the command line via the bundled CLI

Under The Hood

Architecture Showdown’s core is src/converter.js, which builds a per-instance Converter by merging global and local options (src/options.js) and running text through an ordered pipeline of subparsers under src/subParsers/makehtml/ for Markdown-to-HTML, or walking a DOM (produced via helper.parseHTML) through the makeMarkdown.node dispatcher in src/subParsers/makemarkdown/ for the reverse direction. src/showdown.js defines the shared static namespace — global options, the subparser/extension registries, and the flavor presets (vanilla/gfm/commonmark/original) that are just bundles of option overrides layered onto the same pipeline. Each pipeline stage emits onStart/onPreParse/onEnd-style events through src/event.js, and extensions (in lang, output, or listener form) hook into that event bus rather than patching the pipeline directly, which keeps the core conversion path linear and inspectable even as third-party behavior is added. src/helpers/ factors out shared low-level concerns — HTML block hashing, entity/escape handling, regex tables — used by multiple subparsers, and src/cli/cli.js is a thin wrapper around the same public Converter API rather than a separate code path.