test: separate cases, so it's easier to know what fails

Similar to https://github.com/DavidAnson/markdownlint/blob/main/doc/md051.md

Fixes #2

BREAKING CHANGE: Validate links fragments in relative links

.github/ISSUE_TEMPLATE/BUG.md

@@ -1,8 +1,8 @@
 ---
-name: '🐛 Bug Report'
+name: "🐛 Bug Report"
-about: 'Report an unexpected problem or unintended behavior.'
+about: "Report an unexpected problem or unintended behavior."
-title: '[Bug]'
+title: "[Bug]"
-labels: 'bug'
+labels: "bug"
 ---
 
 <!--

.github/ISSUE_TEMPLATE/DOCUMENTATION.md

@@ -1,8 +1,8 @@
 ---
-name: '📜 Documentation'
+name: "📜 Documentation"
-about: 'Correct spelling errors, improvements or additions to documentation files (README, CONTRIBUTING...).'
+about: "Correct spelling errors, improvements or additions to documentation files (README, CONTRIBUTING...)."
-title: '[Documentation]'
+title: "[Documentation]"
-labels: 'documentation'
+labels: "documentation"
 ---
 
 <!-- Please make sure your issue has not already been fixed. -->

.github/ISSUE_TEMPLATE/FEATURE_REQUEST.md

@@ -1,8 +1,8 @@
 ---
-name: '✨ Feature Request'
+name: "✨ Feature Request"
-about: 'Suggest a new feature idea.'
+about: "Suggest a new feature idea."
-title: '[Feature]'
+title: "[Feature]"
-labels: 'feature request'
+labels: "feature request"
 ---
 
 <!-- Please make sure your issue has not already been fixed. -->

.github/ISSUE_TEMPLATE/IMPROVEMENT.md

@@ -1,8 +1,8 @@
 ---
-name: '🔧 Improvement'
+name: "🔧 Improvement"
-about: 'Improve structure/format/performance/refactor/tests of the code.'
+about: "Improve structure/format/performance/refactor/tests of the code."
-title: '[Improvement]'
+title: "[Improvement]"
-labels: 'improvement'
+labels: "improvement"
 ---
 
 <!-- Please make sure your issue has not already been fixed. -->

.github/ISSUE_TEMPLATE/QUESTION.md

@@ -1,8 +1,8 @@
 ---
-name: '🙋 Question'
+name: "🙋 Question"
-about: 'Further information is requested.'
+about: "Further information is requested."
-title: '[Question]'
+title: "[Question]"
-labels: 'question'
+labels: "question"
 ---
 
 ### Question

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,6 @@
 <!-- Please first discuss the change you wish to make via issue before making a change. It might avoid a waste of your time. -->
 
-## What changes this PR introduce?
+# What changes this PR introduce?
 
 ## List any relevant issue numbers
 

.github/workflows/lint.yml

@@ -1,4 +1,4 @@
-name: 'Lint'
+name: "Lint"
 
 on:
   push:
@@ -8,21 +8,22 @@ on:
 
 jobs:
   lint:
-    runs-on: 'ubuntu-latest'
+    runs-on: "ubuntu-latest"
     steps:
-      - uses: 'actions/checkout@v3.5.0'
+      - uses: "actions/checkout@v4.1.1"
 
-      - name: 'Use Node.js'
+      - name: "Setup Node.js"
-        uses: 'actions/setup-node@v3.6.0'
+        uses: "actions/setup-node@v4.0.1"
         with:
-          node-version: 'lts/*'
+          node-version: "lts/*"
-          cache: 'npm'
+          cache: "npm"
 
-      - name: 'Install'
+      - name: "Install dependencies"
-        run: 'npm install'
+        run: "npm clean-install"
 
       - run: 'npm run lint:commit -- --to "${{ github.sha }}"'
-      - run: 'npm run lint:editorconfig'
+      - run: "npm run lint:editorconfig"
-      - run: 'npm run lint:markdown'
+      - run: "npm run lint:markdown"
-      - run: 'npm run lint:eslint'
+      - run: "npm run lint:eslint"
-      - run: 'npm run lint:prettier'
+      - run: "npm run lint:prettier"
+      - run: "npm run lint:javascript"

.github/workflows/release.yml

@@ -1,4 +1,4 @@
-name: 'Release'
+name: "Release"
 
 on:
   push:
@@ -6,24 +6,32 @@ on:
 
 jobs:
   release:
-    runs-on: 'ubuntu-latest'
+    runs-on: "ubuntu-latest"
+    permissions:
+      contents: "write"
+      issues: "write"
+      pull-requests: "write"
+      id-token: "write"
     steps:
-      - uses: 'actions/checkout@v3.5.0'
+      - uses: "actions/checkout@v4.1.1"
         with:
           fetch-depth: 0
           persist-credentials: false
 
-      - name: 'Use Node.js'
+      - name: "Setup Node.js"
-        uses: 'actions/setup-node@v3.6.0'
+        uses: "actions/setup-node@v4.0.1"
         with:
-          node-version: 'lts/*'
+          node-version: "lts/*"
-          cache: 'npm'
+          cache: "npm"
 
-      - name: 'Install'
+      - name: "Install dependencies"
-        run: 'npm install'
+        run: "npm clean-install"
 
-      - name: 'Release'
+      - name: "Verify the integrity of provenance attestations and registry signatures for installed dependencies"
-        run: 'npm run release'
+        run: "npm audit signatures"
+
+      - name: "Release"
+        run: "npm run release"
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           NPM_TOKEN: ${{ secrets.NPM_TOKEN }}

.github/workflows/test.yml

@@ -1,4 +1,4 @@
-name: 'Test'
+name: "Test"
 
 on:
   push:
@@ -8,18 +8,18 @@ on:
 
 jobs:
   test:
-    runs-on: 'ubuntu-latest'
+    runs-on: "ubuntu-latest"
     steps:
-      - uses: 'actions/checkout@v3.5.0'
+      - uses: "actions/checkout@v4.1.1"
 
-      - name: 'Use Node.js'
+      - name: "Setup Node.js"
-        uses: 'actions/setup-node@v3.6.0'
+        uses: "actions/setup-node@v4.0.1"
         with:
-          node-version: 'lts/*'
+          node-version: "lts/*"
-          cache: 'npm'
+          cache: "npm"
 
-      - name: 'Install'
+      - name: "Install dependencies"
-        run: 'npm install'
+        run: "npm clean-install"
 
-      - name: 'Test'
+      - name: "Test"
-        run: 'npm run test'
+        run: "npm run test"

.husky/pre-commit

@@ -2,4 +2,5 @@
 . "$(dirname "$0")/_/husky.sh"
 
 npm run lint:staged
+npm run lint:javascript
 npm run test

.markdownlint-cli2.jsonc

@@ -1,5 +1,11 @@
 {
+  "config": {
+    "extends": "markdownlint/style/prettier",
+    "relative-links": true,
+    "default": true,
+    "MD033": false
+  },
   "globs": ["**/*.{md,mdx}"],
-  "ignores": ["**/node_modules", "**/test/fixtures"],
+  "ignores": ["**/node_modules", "**/test/fixtures/**"],
   "customRules": ["./src/index.js"]
 }

.markdownlint.json

@@ -1,8 +0,0 @@
-{
-  "default": true,
-  "relative-links": true,
-  "extends": "markdownlint/style/prettier",
-  "MD024": false,
-  "MD033": false,
-  "MD041": false
-}

.npmrc

@@ -1 +1,2 @@
 save-exact=true
+provenance=true

.prettierrc.json

@@ -1,6 +1,3 @@
 {
-  "singleQuote": true,
+  "semi": false
-  "jsxSingleQuote": true,
-  "semi": false,
-  "trailingComma": "none"
 }

.taprc

@@ -1,8 +0,0 @@
-ts: false
-jsx: false
-flow: false
-check-coverage: false
-coverage: false
-
-files:
-  - 'test/**/*.test.js'

.vscode/settings.json

@@ -3,6 +3,6 @@
   "prettier.configPath": ".prettierrc.json",
   "editor.formatOnSave": true,
   "editor.codeActionsOnSave": {
-    "source.fixAll": true
+    "source.fixAll": "explicit"
   }
 }

CODE_OF_CONDUCT.md

@@ -60,7 +60,7 @@ representative at an online or offline event.
 
 Instances of abusive, harassing, or otherwise unacceptable behavior may be
 reported to the community leaders responsible for enforcement at
-contact@divlo.fr.
+<contact@theoludwig.fr>.
 All complaints will be reviewed and investigated promptly and fairly.
 
 All community leaders are obligated to respect the privacy and security of the

CONTRIBUTING.md

@@ -2,6 +2,14 @@
 
 Thanks a lot for your interest in contributing to **markdownlint-rule-relative-links**! 🎉
 
+## Code of Conduct
+
+**markdownlint-rule-relative-links** adopted the [Contributor Covenant](https://www.contributor-covenant.org/) as its Code of Conduct, and we expect project participants to adhere to it. Please read [the full text](./CODE_OF_CONDUCT.md) so that you can understand what actions will and will not be tolerated.
+
+## Open Development
+
+All work on **markdownlint-rule-relative-links** happens directly on this repository. Both core team members and external contributors send pull requests which go through the same review process.
+
 ## Types of contributions
 
 - Reporting a bug.
@@ -11,7 +19,7 @@ Thanks a lot for your interest in contributing to **markdownlint-rule-relative-l
 
 ## Pull Requests
 
-- **Please first discuss** the change you wish to make via [issue](https://github.com/Divlo/markdownlint-rule-relative-links/issues) before making a change. It might avoid a waste of your time.
+- **Please first discuss** the change you wish to make via [issue](https://github.com/theoludwig/markdownlint-rule-relative-links/issues) before making a change. It might avoid a waste of your time.
 
 - Ensure your code respect linting.
 
@@ -21,26 +29,4 @@ If you're adding new features to **markdownlint-rule-relative-links**, please in
 
 ## Commits
 
-The commit message guidelines respect [@commitlint/config-conventional](https://github.com/conventional-changelog/commitlint/tree/master/%40commitlint/config-conventional) and [Semantic Versioning](https://semver.org/) for releases.
+The commit message guidelines adheres to [Conventional Commits](https://www.conventionalcommits.org/) and [Semantic Versioning](https://semver.org/) for releases.
-
-### Types
-
-Types define which kind of changes you made to the project.
-
-| Types    | Description                                                                                                  |
-| -------- | ------------------------------------------------------------------------------------------------------------ |
-| feat     | A new feature.                                                                                               |
-| fix      | A bug fix.                                                                                                   |
-| docs     | Documentation only changes.                                                                                  |
-| style    | Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc).      |
-| refactor | A code change that neither fixes a bug nor adds a feature.                                                   |
-| perf     | A code change that improves performance.                                                                     |
-| test     | Adding missing tests or correcting existing tests.                                                           |
-| build    | Changes that affect the build system or external dependencies (example scopes: gulp, broccoli, npm).         |
-| ci       | Changes to our CI configuration files and scripts (example scopes: Travis, Circle, BrowserStack, SauceLabs). |
-| chore    | Other changes that don't modify src or test files.                                                           |
-| revert   | Reverts a previous commit.                                                                                   |
-
-### Scopes
-
-Scopes define what part of the code changed.

LICENSE

@@ -1,6 +1,6 @@
-MIT License
+# MIT License
 
-Copyright (c) Divlo
+Copyright (c) Théo LUDWIG
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

README.md

@@ -5,12 +5,12 @@
 </p>
 
 <p align="center">
-  <a href="./CONTRIBUTING.md"><img src="https://img.shields.io/badge/PRs-welcome-brightgreen.svg?style=flat" /></a>
+  <a href="./CONTRIBUTING.md"><img src="https://img.shields.io/badge/PRs-welcome-brightgreen.svg?style=flat" alt="CONTRIBUTING" /></a>
   <a href="./LICENSE"><img src="https://img.shields.io/badge/licence-MIT-blue.svg" alt="Licence MIT"/></a>
   <a href="./CODE_OF_CONDUCT.md"><img src="https://img.shields.io/badge/Contributor%20Covenant-v2.0%20adopted-ff69b4.svg" alt="Contributor Covenant" /></a>
   <br />
-  <a href="https://github.com/Divlo/markdownlint-rule-relative-links/actions/workflows/lint.yml"><img src="https://github.com/Divlo/markdownlint-rule-relative-links/actions/workflows/lint.yml/badge.svg?branch=develop" /></a>
+  <a href="https://github.com/theoludwig/markdownlint-rule-relative-links/actions/workflows/lint.yml"><img src="https://github.com/theoludwig/markdownlint-rule-relative-links/actions/workflows/lint.yml/badge.svg?branch=develop" alt="Lint" /></a>
-  <a href="https://github.com/Divlo/markdownlint-rule-relative-linksactions/workflows/test.yml"><img src="https://github.com/Divlo/markdownlint-rule-relative-links/actions/workflows/test.yml/badge.svg?branch=develop" /></a>
+  <a href="https://github.com/theoludwig/markdownlint-rule-relative-linksactions/workflows/test.yml"><img src="https://github.com/theoludwig/markdownlint-rule-relative-links/actions/workflows/test.yml/badge.svg?branch=develop" alt="Test" /></a>
   <br />
   <a href="https://conventionalcommits.org"><img src="https://img.shields.io/badge/Conventional%20Commits-1.0.0-yellow.svg" alt="Conventional Commits" /></a>
   <a href="https://github.com/semantic-release/semantic-release"><img src="https://img.shields.io/badge/%20%20%F0%9F%93%A6%F0%9F%9A%80-semantic--release-e10079.svg" alt="semantic-release" /></a>
@@ -21,7 +21,7 @@
 
 **markdownlint-rule-relative-links** is a [markdownlint](https://github.com/DavidAnson/markdownlint) custom rule to validate relative links.
 
-It ensures that relative links (using `file:` protocol) are working and not "dead" which means that it exists in the file system of the project that uses `markdownlint`.
+It ensures that relative links (using `file:` protocol) are working and exists in the file system of the project that uses [markdownlint](https://github.com/DavidAnson/markdownlint).
 
 ### Example
 
@@ -37,15 +37,28 @@ With `awesome.md` content:
 ```md
 [abc](./abc.txt)
 
-[Dead link](./dead.txt)
+[Invalid link](./invalid.txt)
 ```
 
-Running `markdownlint-cli2` with `markdownlint-rule-relative-links` will output:
+Running [markdownlint-cli2](https://github.com/DavidAnson/markdownlint-cli2) with `markdownlint-rule-relative-links` will output:
 
 ```sh
-awesome.md:3 relative-links Relative links should be valid [Link "./dead.txt" is dead]
+awesome.md:3 relative-links Relative links should be valid ["./invalid.txt" should exist in the file system]
 ```
 
+### Additional features
+
+- Support images (e.g: `![Image](./image.png)`).
+- Support anchors (heading fragment links) (e.g: `[Link](./awesome.md#existing-heading)`).
+- Ignore external links and absolute paths as it only checks relative links (e.g: `https://example.com/` or `/absolute/path.png`).
+
+### Limitations
+
+- Only images and links defined using markdown syntax are supported, html syntax is not supported (e.g: `<a href="./link.txt" />` or `<img src="./image.png" />`).
+- Anchors checking is limited to headings, other elements are not supported (e.g: with a "id", `<div id="anchor" />`).
+
+Contributions are welcome to improve the rule, and to alleviate these limitations. See [CONTRIBUTING.md](./CONTRIBUTING.md) for more information.
+
 ### Related links
 
 - [DavidAnson/markdownlint#253](https://github.com/DavidAnson/markdownlint/issues/253)
@@ -64,9 +77,9 @@ npm install --save-dev markdownlint-rule-relative-links
 
 ## Configuration
 
-There are various ways `markdownlint` can be configured using objects, config files etc. For more information on `markdownlint` configuration refer [options.config](https://github.com/DavidAnson/markdownlint#optionsconfig).
+There are various ways [markdownlint](https://github.com/DavidAnson/markdownlint) can be configured using objects, config files etc. For more information on configuration refer to [options.config](https://github.com/DavidAnson/markdownlint#optionsconfig).
 
-We recommend configuring `markdownlint-cli2` over `markdownlint-cli` for compatibility with the `vscode-markdownlint` plugin.
+We recommend configuring [markdownlint-cli2](https://github.com/DavidAnson/markdownlint-cli2) over [markdownlint-cli](https://github.com/igorshubovych/markdownlint-cli) for compatibility with the [vscode-markdownlint](https://github.com/DavidAnson/vscode-markdownlint) extension.
 
 `.markdownlint-cli2.jsonc`
 

jsconfig.json

@@ -0,0 +1,24 @@
+{
+  "compilerOptions": {
+    "lib": ["ESNext"],
+    "target": "ESNext",
+    "module": "CommonJS",
+    "moduleResolution": "Node",
+    "checkJs": true,
+    "allowJs": true,
+    "noEmit": true,
+    "rootDir": ".",
+    "baseUrl": ".",
+    "strict": true,
+    "allowUnusedLabels": false,
+    "allowUnreachableCode": false,
+    "noFallthroughCasesInSwitch": true,
+    "noImplicitAny": true,
+    "noImplicitOverride": true,
+    "noImplicitReturns": true,
+    "noUncheckedIndexedAccess": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "forceConsistentCasingInFileNames": true
+  }
+}

package.json

@@ -3,16 +3,16 @@
   "version": "0.0.0-development",
   "public": true,
   "description": "Custom rule for markdownlint to validate relative links.",
-  "author": "Divlo <contact@divlo.fr>",
+  "author": "Théo LUDWIG <contact@theoludwig.fr>",
   "license": "MIT",
   "repository": {
     "type": "git",
-    "url": "https://github.com/Divlo/markdownlint-rule-relative-links.git"
+    "url": "https://github.com/theoludwig/markdownlint-rule-relative-links.git"
   },
   "bugs": {
-    "url": "https://github.com/Divlo/markdownlint-rule-relative-links/issues"
+    "url": "https://github.com/theoludwig/markdownlint-rule-relative-links/issues"
   },
-  "homepage": "https://github.com/Divlo/markdownlint-rule-relative-links#readme",
+  "homepage": "https://github.com/theoludwig/markdownlint-rule-relative-links#readme",
   "keywords": [
     "markdownlint",
     "markdownlint-rule"
@@ -21,42 +21,51 @@
   "files": [
     "src"
   ],
+  "publishConfig": {
+    "access": "public",
+    "provenance": true
+  },
   "engines": {
     "node": ">=16.0.0",
-    "npm": ">=8.0.0"
+    "npm": ">=9.0.0"
   },
   "scripts": {
     "lint:commit": "commitlint",
     "lint:editorconfig": "editorconfig-checker",
     "lint:markdown": "markdownlint-cli2",
-    "lint:eslint": "eslint \".\" --ignore-path \".gitignore\"",
+    "lint:eslint": "eslint . --max-warnings 0 --report-unused-disable-directives --ignore-path .gitignore",
-    "lint:prettier": "prettier \".\" --check --ignore-path \".gitignore\"",
+    "lint:prettier": "prettier . --check --ignore-path .gitignore",
+    "lint:javascript": "tsc --project jsconfig.json --noEmit",
     "lint:staged": "lint-staged",
-    "test": "tap",
+    "test": "node --test --experimental-test-coverage ./test",
     "release": "semantic-release",
     "postinstall": "husky install",
     "prepublishOnly": "pinst --disable",
     "postpublish": "pinst --enable"
   },
+  "dependencies": {
+    "markdown-it": "14.0.0"
+  },
   "devDependencies": {
-    "@commitlint/cli": "17.5.1",
+    "@commitlint/cli": "18.4.4",
-    "@commitlint/config-conventional": "17.4.4",
+    "@commitlint/config-conventional": "18.4.4",
-    "@types/tap": "15.0.8",
+    "@types/markdown-it": "13.0.7",
-    "editorconfig-checker": "5.0.1",
+    "@types/node": "20.10.8",
-    "eslint": "8.37.0",
+    "editorconfig-checker": "5.1.2",
-    "eslint-config-conventions": "8.0.0",
+    "eslint": "8.56.0",
-    "eslint-config-prettier": "8.8.0",
+    "eslint-config-conventions": "13.1.0",
-    "eslint-plugin-import": "2.27.5",
+    "eslint-config-prettier": "9.1.0",
-    "eslint-plugin-prettier": "4.2.1",
+    "eslint-plugin-import": "2.29.1",
+    "eslint-plugin-prettier": "5.1.2",
     "eslint-plugin-promise": "6.1.1",
-    "eslint-plugin-unicorn": "46.0.0",
+    "eslint-plugin-unicorn": "50.0.1",
     "husky": "8.0.3",
-    "lint-staged": "13.2.0",
+    "lint-staged": "15.2.0",
-    "markdownlint": "0.28.0",
+    "markdownlint": "0.33.0",
-    "markdownlint-cli2": "0.6.0",
+    "markdownlint-cli2": "0.11.0",
     "pinst": "3.0.0",
-    "prettier": "2.8.7",
+    "prettier": "3.1.1",
-    "semantic-release": "21.0.1",
+    "semantic-release": "22.0.12",
-    "tap": "16.3.4"
+    "typescript": "5.3.3"
   }
 }

src/index.js

@@ -1,86 +1,94 @@
-'use strict'
+"use strict"
 
-const { pathToFileURL } = require('node:url')
+const { pathToFileURL } = require("node:url")
-const fs = require('node:fs')
+const fs = require("node:fs")
+
+const {
+  filterTokens,
+  convertHeadingToHTMLFragment,
+  getMarkdownHeadings,
+} = require("./utils.js")
+
+/** @typedef {import('markdownlint').Rule} MarkdownLintRule */
 
 /**
- * Calls the provided function for each matching token.
+ * @type {MarkdownLintRule}
- *
- * @param {Object} params RuleParams instance.
- * @param {string} type Token type identifier.
- * @param {Function} handler Callback function.
- * @returns {void}
  */
-const filterTokens = (params, type, handler) => {
-  for (const token of params.tokens) {
-    if (token.type === type) {
-      handler(token)
-    }
-  }
-}
-
-/**
- * Adds a generic error object via the onError callback.
- *
- * @param {Object} onError RuleOnError instance.
- * @param {number} lineNumber Line number.
- * @param {string} [detail] Error details.
- * @param {string} [context] Error context.
- * @param {number[]} [range] Column and length of error.
- * @param {Object} [fixInfo] RuleOnErrorFixInfo instance.
- * @returns {void}
- */
-const addError = (onError, lineNumber, detail, context, range, fixInfo) => {
-  onError({
-    lineNumber,
-    detail,
-    context,
-    range,
-    fixInfo
-  })
-}
-
 const customRule = {
-  names: ['relative-links'],
+  names: ["relative-links"],
-  description: 'Relative links should be valid',
+  description: "Relative links should be valid",
-  tags: ['links'],
+  tags: ["links"],
   function: (params, onError) => {
-    filterTokens(params, 'inline', (token) => {
+    filterTokens(params, "inline", (token) => {
-      token.children.forEach((child) => {
+      const children = token.children ?? []
-        const { lineNumber, type, attrs } = child
+      for (const child of children) {
+        const { type, attrs, lineNumber } = child
 
-        /** @type {string | null} */
+        /** @type {string | undefined} */
-        let hrefSrc = null
+        let hrefSrc
 
-        if (type === 'link_open') {
+        if (type === "link_open") {
-          attrs.forEach((attr) => {
+          for (const attr of attrs) {
-            if (attr[0] === 'href') {
+            if (attr[0] === "href") {
               hrefSrc = attr[1]
+              break
             }
-          })
+          }
         }
 
-        if (type === 'image') {
+        if (type === "image") {
-          attrs.forEach((attr) => {
+          for (const attr of attrs) {
-            if (attr[0] === 'src') {
+            if (attr[0] === "src") {
               hrefSrc = attr[1]
+              break
             }
-          })
+          }
         }
 
         if (hrefSrc != null) {
           const url = new URL(hrefSrc, pathToFileURL(params.name))
-          url.hash = ''
           const isRelative =
-            url.protocol === 'file:' && !hrefSrc.startsWith('/')
+            url.protocol === "file:" && !hrefSrc.startsWith("/")
-          if (isRelative && !fs.existsSync(url)) {
+          if (isRelative) {
-            const detail = `Link "${hrefSrc}" is dead`
+            const detail = `"${hrefSrc}"`
-            addError(onError, lineNumber, detail)
+
+            if (!fs.existsSync(url)) {
+              onError({
+                lineNumber,
+                detail: `${detail} should exist in the file system`,
+              })
+              continue
+            }
+
+            if (type === "link_open" && url.hash !== "") {
+              const fileContent = fs.readFileSync(url, { encoding: "utf8" })
+              const headings = getMarkdownHeadings(fileContent)
+
+              /** @type {Map<string, number>} */
+              const fragments = new Map()
+
+              const headingsHTMLFragments = headings.map((heading) => {
+                const fragment = convertHeadingToHTMLFragment(heading)
+                const count = fragments.get(fragment) ?? 0
+                fragments.set(fragment, count + 1)
+                if (count !== 0) {
+                  return `${fragment}-${count}`
+                }
+                return fragment
+              })
+
+              if (!headingsHTMLFragments.includes(url.hash)) {
+                onError({
+                  lineNumber,
+                  detail: `${detail} should have a valid fragment identifier`,
+                })
+              }
+            }
           }
         }
-      })
+      }
     })
-  }
+  },
 }
 
 module.exports = customRule

src/utils.js

@@ -0,0 +1,101 @@
+const MarkdownIt = require("markdown-it")
+
+/** @typedef {import('markdownlint').RuleParams} MarkdownLintRuleParams */
+/** @typedef {import('markdownlint').MarkdownItToken} MarkdownItToken */
+
+/**
+ * Calls the provided function for each matching token.
+ *
+ * @param {MarkdownLintRuleParams} params RuleParams instance.
+ * @param {string} type Token type identifier.
+ * @param {(token: MarkdownItToken) => void} handler Callback function.
+ * @returns {void}
+ */
+const filterTokens = (params, type, handler) => {
+  for (const token of params.tokens) {
+    if (token.type === type) {
+      handler(token)
+    }
+  }
+}
+
+/**
+ * Converts a Markdown heading into an HTML fragment according to the rules
+ * used by GitHub.
+ *
+ * @see https://github.com/DavidAnson/markdownlint/blob/d01180ec5a014083ee9d574b693a8d7fbc1e566d/lib/md051.js#L1
+ * @param {string} inlineText Inline token for heading.
+ * @returns {string} Fragment string for heading.
+ */
+const convertHeadingToHTMLFragment = (inlineText) => {
+  return (
+    "#" +
+    encodeURIComponent(
+      inlineText
+        .toLowerCase()
+        // RegExp source with Ruby's \p{Word} expanded into its General Categories
+        // https://github.com/gjtorikian/html-pipeline/blob/main/lib/html/pipeline/toc_filter.rb
+        // https://ruby-doc.org/core-3.0.2/Regexp.html
+        .replace(
+          /[^\p{Letter}\p{Mark}\p{Number}\p{Connector_Punctuation}\- ]/gu,
+          "",
+        )
+        .replace(/ /gu, "-"),
+    )
+  )
+}
+
+const headingTags = new Set(["h1", "h2", "h3", "h4", "h5", "h6"])
+const ignoredTokens = new Set(["heading_open", "heading_close"])
+
+/**
+ * Gets the headings from a Markdown string.
+ * @param {string} content
+ * @returns {string[]}
+ */
+const getMarkdownHeadings = (content) => {
+  const markdownIt = new MarkdownIt({ html: true })
+  const tokens = markdownIt.parse(content, {})
+
+  /** @type {string[]} */
+  const headings = []
+
+  /** @type {string | null} */
+  let headingToken = null
+
+  for (const token of tokens) {
+    if (headingTags.has(token.tag)) {
+      if (token.type === "heading_open") {
+        headingToken = token.markup
+      } else if (token.type === "heading_close") {
+        headingToken = null
+      }
+    }
+
+    if (ignoredTokens.has(token.type)) {
+      continue
+    }
+
+    if (headingToken === null) {
+      continue
+    }
+
+    const children = token.children ?? []
+
+    headings.push(
+      `${children
+        .map((token) => {
+          return token.content
+        })
+        .join("")}`,
+    )
+  }
+
+  return headings
+}
+
+module.exports = {
+  filterTokens,
+  convertHeadingToHTMLFragment,
+  getMarkdownHeadings,
+}

test/basic.test.js

@@ -1,34 +0,0 @@
-const tap = require('tap')
-const { markdownlint } = require('markdownlint').promises
-
-const relativeLinks = require('../src/index.js')
-
-tap.test('ensure we validate correctly', async (t) => {
-  const lintResults = await markdownlint({
-    files: ['test/fixtures/Valid.md', 'test/fixtures/Invalid.md'],
-    config: {
-      'relative-links': true
-    },
-    customRules: [relativeLinks]
-  })
-  t.equal(lintResults['test/fixtures/Valid.md'].length, 0)
-  t.equal(lintResults['test/fixtures/Invalid.md'].length, 2)
-
-  t.equal(
-    lintResults['test/fixtures/Invalid.md'][0]?.ruleDescription,
-    'Relative links should be valid'
-  )
-  t.equal(
-    lintResults['test/fixtures/Invalid.md'][0]?.errorDetail,
-    'Link "./basic.test.js" is dead'
-  )
-
-  t.equal(
-    lintResults['test/fixtures/Invalid.md'][1]?.ruleDescription,
-    'Relative links should be valid'
-  )
-  t.equal(
-    lintResults['test/fixtures/Invalid.md'][1]?.errorDetail,
-    'Link "../image.png" is dead'
-  )
-})

test/fixtures/Invalid.md

@@ -1,5 +0,0 @@
-# Invalid
-
-[basic.js](./basic.test.js)
-
-![Image](../image.png)

test/fixtures/invalid/non-existing-file.md

@@ -0,0 +1,3 @@
+# Invalid
+
+[File](./index.test.js)

test/fixtures/invalid/non-existing-heading-fragment/awesome.md

@@ -0,0 +1,3 @@
+# Valid
+
+## Existing Heading

test/fixtures/invalid/non-existing-heading-fragment/non-existing-heading-fragment.md

@@ -0,0 +1,3 @@
+# Invalid
+
+[Link fragment](./awesome.md#non-existing-heading)

test/fixtures/invalid/non-existing-image.md

@@ -0,0 +1,3 @@
+# Invalid
+
+![Image](./image.png)

test/fixtures/valid/existing-file.md

@@ -0,0 +1,3 @@
+# Valid
+
+[File](../../index.test.js)

test/fixtures/valid/existing-heading-fragment/awesome.md

@@ -0,0 +1,13 @@
+# Valid
+
+## Existing Heading
+
+### Repeated Heading
+
+Text
+
+### Repeated Heading
+
+Text
+
+### Repeated Heading

test/fixtures/valid/existing-heading-fragment/existing-heading-fragment.md

@@ -0,0 +1,9 @@
+# Valid
+
+[Link fragment](./awesome.md#existing-heading)
+
+[Link fragment Repeated 0](./awesome.md#repeated-heading)
+
+[Link fragment Repeated 1](./awesome.md#repeated-heading-1)
+
+[Link fragment Repeated 2](./awesome.md#repeated-heading-2)

test/fixtures/valid/existing-image.md

@@ -0,0 +1,3 @@
+# Valid
+
+![Image](./image.png)

test/fixtures/valid/ignore-absolute-paths.md

@@ -0,0 +1,3 @@
+# Valid
+
+![Absolute Path](/absolute/path.png)

test/fixtures/valid/ignore-external-links.md

@@ -1,11 +1,5 @@
 # Valid
 
-[basic.js](../basic.test.js)
-
-![Image](./image.png)
-
-![Absolute Path](/absolute/path.png)
-
 [External https link](https://example.com/)
 
 [External https link 2](https:./external.https)

test/index.test.js

@@ -0,0 +1,112 @@
+const { test } = require("node:test")
+const assert = require("node:assert/strict")
+
+const { markdownlint } = require("markdownlint").promises
+
+const relativeLinksRule = require("../src/index.js")
+
+/**
+ *
+ * @param {string} fixtureFile
+ * @returns
+ */
+const validateMarkdownLint = async (fixtureFile) => {
+  const lintResults = await markdownlint({
+    files: [fixtureFile],
+    config: {
+      default: false,
+      "relative-links": true,
+    },
+    customRules: [relativeLinksRule],
+  })
+  return lintResults[fixtureFile]
+}
+
+test("ensure the rule validates correctly", async (t) => {
+  await t.test("should be valid", async (t) => {
+    await t.test("with an existing heading fragment", async () => {
+      const lintResults = await validateMarkdownLint(
+        "test/fixtures/valid/existing-heading-fragment/existing-heading-fragment.md",
+      )
+      assert.equal(lintResults?.length, 0)
+    })
+
+    await t.test("with an existing file", async () => {
+      const lintResults = await validateMarkdownLint(
+        "test/fixtures/valid/existing-file.md",
+      )
+      assert.equal(lintResults?.length, 0)
+    })
+
+    await t.test("with an existing image", async () => {
+      const lintResults = await validateMarkdownLint(
+        "test/fixtures/valid/existing-image.md",
+      )
+      assert.equal(lintResults?.length, 0)
+    })
+
+    await t.test("should ignore absolute paths", async () => {
+      const lintResults = await validateMarkdownLint(
+        "test/fixtures/valid/ignore-absolute-paths.md",
+      )
+      assert.equal(lintResults?.length, 0)
+    })
+
+    await t.test("should ignore external links", async () => {
+      const lintResults = await validateMarkdownLint(
+        "test/fixtures/valid/ignore-external-links.md",
+      )
+      assert.equal(lintResults?.length, 0)
+    })
+  })
+
+  await t.test("should be invalid", async (t) => {
+    await t.test("with a non-existing heading fragment", async () => {
+      const lintResults = await validateMarkdownLint(
+        "test/fixtures/invalid/non-existing-heading-fragment/non-existing-heading-fragment.md",
+      )
+      assert.equal(lintResults?.length, 1)
+      assert.deepEqual(lintResults?.[0]?.ruleNames, relativeLinksRule.names)
+      assert.equal(
+        lintResults?.[0]?.ruleDescription,
+        relativeLinksRule.description,
+      )
+      assert.equal(
+        lintResults?.[0]?.errorDetail,
+        '"./awesome.md#non-existing-heading" should have a valid fragment identifier',
+      )
+    })
+
+    await t.test("with a non-existing file", async () => {
+      const lintResults = await validateMarkdownLint(
+        "test/fixtures/invalid/non-existing-file.md",
+      )
+      assert.equal(lintResults?.length, 1)
+      assert.deepEqual(lintResults?.[0]?.ruleNames, relativeLinksRule.names)
+      assert.equal(
+        lintResults?.[0]?.ruleDescription,
+        relativeLinksRule.description,
+      )
+      assert.equal(
+        lintResults?.[0]?.errorDetail,
+        '"./index.test.js" should exist in the file system',
+      )
+    })
+
+    await t.test("with a non-existing image", async () => {
+      const lintResults = await validateMarkdownLint(
+        "test/fixtures/invalid/non-existing-image.md",
+      )
+      assert.equal(lintResults?.length, 1)
+      assert.deepEqual(lintResults?.[0]?.ruleNames, relativeLinksRule.names)
+      assert.equal(
+        lintResults?.[0]?.ruleDescription,
+        relativeLinksRule.description,
+      )
+      assert.equal(
+        lintResults?.[0]?.errorDetail,
+        '"./image.png" should exist in the file system',
+      )
+    })
+  })
+})

test/utils.test.js

@@ -0,0 +1,37 @@
+const { test } = require("node:test")
+const assert = require("node:assert/strict")
+
+const {
+  convertHeadingToHTMLFragment,
+  getMarkdownHeadings,
+} = require("../src/utils.js")
+
+test("utils", async (t) => {
+  await t.test("convertHeadingToHTMLFragment", async () => {
+    assert.strictEqual(
+      convertHeadingToHTMLFragment("Valid Fragments"),
+      "#valid-fragments",
+    )
+    assert.strictEqual(
+      convertHeadingToHTMLFragment("Valid Heading With Underscores _"),
+      "#valid-heading-with-underscores-_",
+    )
+    assert.strictEqual(
+      convertHeadingToHTMLFragment(
+        `Valid Heading With Quotes ' And Double Quotes "`,
+      ),
+      "#valid-heading-with-quotes--and-double-quotes-",
+    )
+    assert.strictEqual(
+      convertHeadingToHTMLFragment("🚀 Valid Heading With Emoji"),
+      "#-valid-heading-with-emoji",
+    )
+  })
+
+  await t.test("getMarkdownHeadings", async () => {
+    assert.deepStrictEqual(
+      getMarkdownHeadings("# Hello\n\n## World\n\n## Hello, world!\n"),
+      ["Hello", "World", "Hello, world!"],
+    )
+  })
+})