feat: validate relative links fragments

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

Fixes #2

BREAKING CHANGE: Validate links fragments in relative links

test/basic.test.js

@@ -5,16 +5,17 @@ const { markdownlint } = require('markdownlint').promises
 
 const relativeLinks = require('../src/index.js')
 
-test('ensure we validate correctly', async () => {
+test('ensure the rule validate correctly', async () => {
   const lintResults = await markdownlint({
     files: ['test/fixtures/Valid.md', 'test/fixtures/Invalid.md'],
     config: {
+      default: false,
       'relative-links': true
     },
     customRules: [relativeLinks]
   })
   assert.equal(lintResults['test/fixtures/Valid.md'].length, 0)
-  assert.equal(lintResults['test/fixtures/Invalid.md'].length, 2)
+  assert.equal(lintResults['test/fixtures/Invalid.md'].length, 3)
 
   assert.equal(
     lintResults['test/fixtures/Invalid.md'][0]?.ruleDescription,
@@ -22,7 +23,7 @@ test('ensure we validate correctly', async () => {
   )
   assert.equal(
     lintResults['test/fixtures/Invalid.md'][0]?.errorDetail,
-    'Link "./basic.test.js" is dead'
+    'Link "./basic.test.js" is not valid'
   )
 
   assert.equal(
@@ -31,6 +32,15 @@ test('ensure we validate correctly', async () => {
   )
   assert.equal(
     lintResults['test/fixtures/Invalid.md'][1]?.errorDetail,
-    'Link "../image.png" is dead'
+    'Link "../image.png" is not valid'
+  )
+
+  assert.equal(
+    lintResults['test/fixtures/Invalid.md'][2]?.ruleDescription,
+    'Relative links should be valid'
+  )
+  assert.equal(
+    lintResults['test/fixtures/Invalid.md'][2]?.errorDetail,
+    'Link "./Valid.md#not-existing-heading" is not valid'
   )
 })

test/fixtures/Invalid.md

@@ -3,3 +3,17 @@
 [basic.js](./basic.test.js)
 
 ![Image](../image.png)
+
+[Link fragment](./Valid.md#not-existing-heading)
+
+## Existing Heading
+
+### Repeated Heading
+
+Text
+
+### Repeated Heading
+
+Text
+
+### Repeated Heading

test/fixtures/Valid.md

@@ -11,3 +11,11 @@
 [External https link 2](https:./external.https)
 
 [External ftp link](ftp:./external.ftp)
+
+[Link fragment](./Invalid.md#existing-heading)
+
+[Link fragment Repeated 0](./Invalid.md#repeated-heading)
+
+[Link fragment Repeated 1](./Invalid.md#repeated-heading-1)
+
+[Link fragment Repeated 2](./Invalid.md#repeated-heading-2)

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!']
+    )
+  })
+})