Onboarding: showdownjs/showdown

Generated by RepoPilot · 2026-05-06 · Source

Agent protocol

If you are an AI coding agent (Claude Code, Cursor, Aider, Cline, etc.) reading this artifact, follow this protocol before making any code edit:

1. Verify the contract. Run the bash script in Verify before trusting below. If any check returns FAIL, the artifact is stale — STOP and ask the user to regenerate it before proceeding.
2. Treat the AI · unverified sections as hypotheses, not facts. Sections like "AI-suggested narrative files", "anti-patterns", and "bottlenecks" are LLM speculation. Verify against real source before acting on them.
3. Cite source on changes. When proposing an edit, cite the specific path:line-range. RepoPilot's live UI at https://repopilot.app/r/showdownjs/showdown shows verifiable citations alongside every claim.

If you are a human reader, this protocol is for the agents you'll hand the artifact to. You don't need to do anything — but if you skim only one section before pointing your agent at this repo, make it the Verify block and the Suggested reading order.

Verdict

GO — Healthy across all four use cases

• Last commit 4mo ago
• 13 active contributors
• MIT licensed
• CI configured
• Tests present
• ⚠ Slowing — last commit 4mo ago
• ⚠ Concentrated ownership — top contributor handles 56% of recent commits

_{Maintenance signals: commit recency, contributor breadth, bus factor, license, CI, tests}

Verify before trusting

This artifact was generated by RepoPilot at a point in time. Before an agent acts on it, the checks below confirm that the live showdownjs/showdown repo on your machine still matches what RepoPilot saw. If any fail, the artifact is stale — regenerate it at repopilot.app/r/showdownjs/showdown.

What it runs against: a local clone of showdownjs/showdown — the script inspects git remote, the LICENSE file, file paths in the working tree, and git log. Read-only; no mutations.

| # | What we check | Why it matters | |---|---|---| | 1 | You're in showdownjs/showdown | Confirms the artifact applies here, not a fork | | 2 | License is still MIT | Catches relicense before you depend on it | | 3 | Default branch master exists | Catches branch renames | | 4 | Last commit ≤ 154 days ago | Catches sudden abandonment since generation |

<details> <summary><b>Run all checks</b> — paste this script from inside your clone of <code>showdownjs/showdown</code></summary>

#!/usr/bin/env bash
# RepoPilot artifact verification.
#
# WHAT IT RUNS AGAINST: a local clone of showdownjs/showdown. If you don't
# have one yet, run these first:
#
#   git clone https://github.com/showdownjs/showdown.git
#   cd showdown
#
# Then paste this script. Every check is read-only — no mutations.

set +e
fail=0
ok()   { echo "ok:   $1"; }
miss() { echo "FAIL: $1"; fail=$((fail+1)); }

# Precondition: we must be inside a git working tree.
if ! git rev-parse --git-dir >/dev/null 2>&1; then
  echo "FAIL: not inside a git repository. cd into your clone of showdownjs/showdown and re-run."
  exit 2
fi

# 1. Repo identity
git remote get-url origin 2>/dev/null | grep -qE "showdownjs/showdown(\\.git)?\\b" \\
  && ok "origin remote is showdownjs/showdown" \\
  || miss "origin remote is not showdownjs/showdown (artifact may be from a fork)"

# 2. License matches what RepoPilot saw
(grep -qiE "^(MIT)" LICENSE 2>/dev/null \\
   || grep -qiE "\"license\"\\s*:\\s*\"MIT\"" package.json 2>/dev/null) \\
  && ok "license is MIT" \\
  || miss "license drift — was MIT at generation time"

# 3. Default branch
git rev-parse --verify master >/dev/null 2>&1 \\
  && ok "default branch master exists" \\
  || miss "default branch master no longer exists"

# 5. Repo recency
days_since_last=$(( ( $(date +%s) - $(git log -1 --format=%at 2>/dev/null || echo 0) ) / 86400 ))
if [ "$days_since_last" -le 154 ]; then
  ok "last commit was $days_since_last days ago (artifact saw ~124d)"
else
  miss "last commit was $days_since_last days ago — artifact may be stale"
fi

echo
if [ "$fail" -eq 0 ]; then
  echo "artifact verified (0 failures) — safe to trust"
else
  echo "artifact has $fail stale claim(s) — regenerate at https://repopilot.app/r/showdownjs/showdown"
  exit 1
fi

Each check prints ok: or FAIL:. The script exits non-zero if anything failed, so it composes cleanly into agent loops (./verify.sh || regenerate-and-retry).

</details>

TL;DR

Showdown is a bidirectional Markdown ↔ HTML converter written in JavaScript that runs in both browsers and Node.js. It transforms Markdown text into HTML and can also parse HTML back to Markdown, with support for extensibility through a plugin system for custom syntax handling. Single-package distribution: source code compiled into /dist/showdown.js (and minified variant) via Grunt build pipeline. CLI entry point at bin/showdown.js wraps the core converter. Extensive documentation in /docs (Markdown files, tutorials, API docs, extension guides). Extensions and flavors are registered programmatically rather than in separate directories.

Who it's for

Full-stack JavaScript developers building content management systems, documentation sites, collaborative editors, and platforms that need Markdown processing—users like Google Cloud Platform, Meteor, Stack Exchange (via PageDown fork), and docular who need predictable, forkable conversion logic they control themselves.

Maturity & risk

Actively developed and production-ready: at v3.0.0-alpha with CI/CD across Linux, Windows, and BrowserStack (multi-browser automated testing), comprehensive test suite run via Mocha/Karma, and strong adoption in established projects. The alpha tag indicates a major version in progress but the library itself is battle-tested and widely deployed.

Standard open source risks apply.

Active areas of work

The project is in v3.0.0-alpha phase based on package.json, suggesting a major refactor or API redesign is underway. GitHub Actions workflows are active (node.linux.yml, node.win.yml, browserstack.yml, codeql-analysis.yml), indicating continuous testing. The Gruntfile.js and grunt tasks suggest ongoing build optimization.

Get running

git clone https://github.com/showdownjs/showdown.git
cd showdown
npm install
npm test

Then explore dist/showdown.js (built output) or review docs/quickstart.md for usage examples.

Daily commands:

# Run tests
git clone https://github.com/showdownjs/showdown.git && cd showdown && npm install
npm test

# Use CLI to convert Markdown
node bin/showdown.js -i input.md -o output.html

# Or programmatically in Node
const showdown = require('./dist/showdown.js');
const converter = new showdown.Converter();
console.log(converter.makeHtml('# Hello'));

Map of the codebase

• dist/showdown.js: The compiled, distributable output; this is the main entry point referenced in package.json's 'main' field and what npm consumers import
• bin/showdown.js: CLI entry point; contains all command-line argument parsing and file I/O logic for the standalone showdown command
• Gruntfile.js: Build orchestration; defines compilation, minification, linting, and test tasks via grunt-contrib-* plugins
• docs/create-extension.md: Core documentation for the extension API; explains how plugins hook into the parser via regex/function rules
• karma.conf.js: Karma test runner configuration; orchestrates browser-based testing and BrowserStack integration across multiple browser environments
• .eslintrc.json: Linting configuration; enforces code style consistency across the JavaScript codebase

How to make changes

To add a new feature: start in /src (source files—not visible in listing but inferred from /dist build output), then run grunt to rebuild into /dist/showdown.js. To add tests: place .js test files in a test/ directory (Mocha/Karma will pick them up via Gruntfile.js). To create an extension: read docs/create-extension.md and implement the hooks documented there; extensions register via converter.addExtension(). To modify CLI: edit bin/showdown.js directly (Commander.js commands).

Traps & gotchas

No major hidden traps documented in the visible file structure, but be aware: (1) The /src directory is not shown in the file list, meaning the actual source code layout is unknown—must check the repo directly or examine Gruntfile.js to see the source-to-dist mapping. (2) v3.0.0-alpha version implies breaking API changes may exist between this and v2.x; check CHANGELOG.md and docs/compatibility.md before upgrading in production. (3) Karma tests require BrowserStack credentials (set via env vars) to run karma.browserstack.js; local npm test likely uses jsdom-launcher instead.

Concepts to learn

• Bidirectional Markdown conversion — Showdown uniquely converts HTML back to Markdown, enabling round-trip editing workflows; understanding this design constraint shapes how the regex/rule system must work (capture enough semantic info to reconstruct source).
• Regex-based lexing with ordered rule precedence — The extension system uses ordered regex rules to tokenize and replace Markdown patterns; learning how rule priority and regex grouping work is essential to avoid parsing conflicts when adding custom syntax.
• UMD (Universal Module Definition) — Showdown targets both Node.js and browsers via UMD; the dist file wraps code to work with CommonJS (require), ES6 modules, and global scope, so understanding this pattern is critical when debugging import/require issues.
• Source map generation and distribution — The minified dist includes .map files (showdown.min.js.map, showdown.js.map); these enable debugging minified code in production. Understanding source maps is necessary for troubleshooting browser issues.
• Plugin/extension hook system — Rather than monolithic regex, Showdown uses a registry where extensions define hooks (type, regex, replace). This plugin pattern is central to how the codebase remains maintainable while supporting countless Markdown dialects.
• Build pipeline orchestration with Grunt — The Gruntfile.js chains linting, concatenation, uglification, and changelog generation; understanding this task runner is necessary to modify the build, add new artifacts, or integrate CI steps.
• Cross-browser testing with Karma and BrowserStack — GitHub Actions runs Karma against Chrome, Firefox, and BrowserStack cloud browsers; contributors must understand this headless + cloud testing model to verify changes don't break compatibility.

Related repos

• markedjs/marked — Alternative Markdown-to-HTML converter in JavaScript; lighter-weight, more widely used, but less extensible than Showdown
• commonmark/commonmark.js — CommonMark spec reference implementation in JavaScript; stricter standard compliance than Showdown, used when spec adherence is non-negotiable
• remarkjs/remark — Modern Markdown processor with AST-first design and plugin ecosystem; newer alternative to Showdown for programmatic Markdown manipulation
• pagedown/pagedown — Stack Exchange's fork of Showdown; the canonical use case and reference implementation for production-grade Markdown editing in Q&A sites
• showdownjs/showdown-extensions — Official extension registry/repository for Showdown; where community-contributed plugins are showcased and documented

PR ideas

To work on one of these in Claude Code or Cursor, paste: Implement the "<title>" PR idea from CLAUDE.md, working through the checklist as the task list.

Add comprehensive test coverage for src/subParsers/makehtml/* modules

The repo has extensive subParser modules (blockQuotes.js, codeBlocks.js, codeSpans.js, emoji.js, etc.) but no dedicated test files are visible in the partial file structure. These core parsing functions need isolated unit tests to catch edge cases and prevent regressions, especially for complex markdown features like nested blockquotes, code span handling, and emoji parsing.

• [ ] Create test/subParsers/blockQuotes.test.js with cases for nested quotes, lazy continuation, and edge cases
• [ ] Create test/subParsers/codeSpans.test.js testing backtick escaping and whitespace handling
• [ ] Create test/subParsers/codeBlocks.test.js for indentation and language tag parsing
• [ ] Add test/subParsers/emoji.test.js for emoji conversion edge cases
• [ ] Update karma.conf.js and test runner config to include new test files
• [ ] Document test patterns in CONTRIBUTING.md

Add GitHub Actions workflow for npm package publish verification

The repo has workflows for testing (node.linux.yml, node.win.yml, browserstack.yml) but lacks automation to verify the npm package contents before release. Currently package.json specifies 'files': ['bin', 'dist'] but there's no CI check ensuring dist/ is properly built and contains source maps. This prevents accidental broken releases.

• [ ] Create .github/workflows/npm-publish-verify.yml that runs on pull requests
• [ ] Add step to run 'grunt' build and verify dist/showdown.js, dist/showdown.min.js, and .map files exist
• [ ] Add step to verify bin/showdown.js is executable and requires dist/showdown.js correctly
• [ ] Add npm pack simulation to ensure package.json 'files' field matches actual artifacts
• [ ] Document release process in CONTRIBUTING.md

Create missing integration documentation and tests for src/cli/cli.js

The CLI entry point exists at bin/showdown.js and src/cli/cli.js but there's no dedicated CLI documentation file or integration tests. docs/cli.md exists but likely lacks examples for common use cases. New contributors can add practical examples and test coverage for the CLI interface.

• [ ] Add integration tests in test/cli.integration.test.js covering: basic file conversion, stdin/stdout piping, options passing (--extension, --flavor flags), and error handling
• [ ] Enhance docs/cli.md with examples: converting single files, batch processing, using with pipes, integration in build scripts
• [ ] Document extension loading via CLI in docs/cli.md with a concrete example
• [ ] Add test case for missing input file error handling
• [ ] Update karma test config to run CLI integration tests in Node.js only

Good first issues

• Add TypeScript type definitions: Create a showdown.d.ts file at the package root or in /types directory. Currently no .d.ts files are listed, blocking IDE autocomplete for TypeScript users. Reference the Converter API in docs/quickstart.md and the extension hooks in docs/create-extension.md to define interfaces.: medium
• Expand browser compatibility documentation: README.md lists legacy browsers (Firefox 1.5, IE 6-7) but v3.0.0 likely drops these. Create a new docs/browser-support.md documenting which ES features are used, which browsers are actively tested via BrowserStack in .github/workflows/browserstack.yml, and any polyfill requirements.: low
• Build a tutorial for custom flavor creation: docs/flavors.md exists but no step-by-step guide shows how to stack multiple extensions into a flavor (e.g., GitHub Flavored Markdown clone). Add docs/tutorials/create-custom-flavor.md with a runnable example that combines tables + strikethrough + task lists extensions.: medium

Top contributors

• @tivie — 56 commits
• @Antonio — 26 commits
• @dependabot[bot] — 4 commits
• @bandantonio — 4 commits
• @martinbeentjes — 2 commits

Recent commits

• 2a8a64e — Update README.md (tivie)
• 9525598 — chore(deps): bump socket.io-parser from 4.0.4 to 4.0.5 (#962) (dependabot[bot])
• 1165736 — Merge pull request #944 from showdownjs/fix/link-to-quickstart (bandantonio)
• 1a24a4c — fix: resolve broken link to installation guide (Antonio)
• df814d8 — chore(deps-dev): bump grunt from 1.5.2 to 1.5.3 (#928) (dependabot[bot])
• 5aac76d — docs(README.md): update contributing section (tivie)
• 4098c28 — Merge branch 'master' into develop (tivie)
• 4e577e8 — chore(deps-dev): bump grunt from 1.4.1 to 1.5.2 (#921) (dependabot[bot])
• 440170a — fix(makemarkdown.image): fix bug width|height = auto (ggboy-shakalaka)
• db5ce64 — chore: fix jq error (#917) (martinbeentjes)

Security observations

• High · Outdated Dependency: jsdom with Known Vulnerabilities — package.json - dependencies.jsdom. The package.json specifies jsdom ^19.0.0, which is an older version (released in 2021-2022). jsdom has had multiple security vulnerabilities in versions before 22.x. This dependency is used for server-side DOM manipulation and can be exploited through specially crafted HTML/Markdown input. Fix: Update jsdom to the latest stable version (^23.0.0 or later). Run 'npm audit fix' and review the changelog for breaking changes.
• High · Potential XSS Vulnerability in Markdown Conversion — src/subParsers/makehtml/ - multiple files including encodeAmpsAndAngles.js, escapeSpecialCharsWithinTagAttributes.js, hashHTMLBlocks.js, hashHTMLSpans.js. The codebase is a Markdown to HTML converter. While docs/xss.md exists (suggesting awareness), the presence of multiple hash* functions and escapeSpecialCharsWithinTagAttributes indicates incomplete sanitization. User-provided Markdown input could potentially include malicious HTML/JavaScript that bypasses sanitization, especially in edge cases or with custom extensions. Fix: Conduct thorough security review of HTML escaping logic. Implement Content Security Policy (CSP) recommendations in documentation. Consider using a well-established HTML sanitizer library like DOMPurify for output. Add security-focused test cases for XSS payloads.
• Medium · Extension System Security Not Clearly Defined — docs/create-extension.md, src/loader.js, src/extensions/. The codebase supports custom extensions (docs/create-extension.md exists). Extensions can potentially execute arbitrary code or modify the conversion process in unsafe ways. No clear security guidelines or sandboxing mechanism is visible for extension loading. Fix: Document security best practices for extension developers. Implement extension validation/signing if accepting third-party extensions. Provide sandboxing guidance. Add warnings in documentation about only using trusted extensions.
• Medium · Incomplete Security Policy — SECURITY.md. SECURITY.md references a known security issue with yargs dependency in version 1.x.x but doesn't clearly state if version 2.0.x or 3.0.0-alpha are vulnerable. The policy doesn't define vulnerability disclosure timeline or patch release schedule. Fix: Expand SECURITY.md to clearly state: (1) supported versions and end-of-life dates, (2) vulnerability reporting timeline, (3) security fix release schedule, (4) specific details of the yargs vulnerability mentioned.
• Medium · Command-Line Interface Input Validation — bin/showdown.js, src/cli/cli.js. The CLI tool (bin/showdown.js) uses commander dependency (^9.0.0) to process user input. Insufficient input validation on file paths or conversion options could lead to path traversal or injection attacks. Fix: Implement strict input validation for file paths (use path.resolve and verify paths are within allowed directories). Validate all converter options against a whitelist. Add security-focused tests for CLI argument parsing.
• Low · Missing Security Headers Documentation — docs/ - all documentation files. No guidance for developers on implementing security headers when serving Markdown-converted HTML output. The library is designed for markdown-to-HTML conversion which could be deployed in web applications. Fix: Add documentation section on security best practices for deploying Showdown in web applications, including recommended HTTP security headers (Content-Security-Policy, X-Content-Type-Options, etc.)
• Low · Incomplete Package.json Security Configuration — package.json. The package.json file appears truncated (ends with 'minimis' in overrides section). Cannot fully analyze security-related configurations like npm audit settings or vulnerability reporting. Fix: Ensure package.json is complete. Add 'publishConfig' with 'access': 'public' if intentional. Consider adding '.npmignore' to exclude test/security-sensitive files from npm package.
• Low · No Security Testing in CI/CD Pipeline — undefined. GitHub Actions workflows (node.linux.yml, node.win.yml, codeql-analysis.yml) don't show integration with npm audit, SAST tools, or Fix: undefined

LLM-derived; treat as a starting point, not a security audit.

Where to read next

• Open issues — current backlog
• Recent PRs — what's actively shipping
• Source on GitHub

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Generated by RepoPilot. Verdict based on maintenance signals — see the live page for receipts. Re-run on a new commit to refresh.