fix: recognize Definition node in no-missing-link-fragments

[lumirlumir]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

Which language are you using?

CommonMark and GFM.

What did you do?

I expected the [reference]: #non-existent definition to be reported by the no-missing-link-fragments rule, but it isn't.

The Invalid link reference is also considered valid link when using reference-style links.

What did you expect to happen?

I expected the [reference]: #non-existent definition to be reported by the no-missing-link-fragments rule.

Link to minimal reproducible Example

The following example would help identify the problem:

<!-- eslint markdown/no-missing-link-fragments: "error" -->

<!-- Valid -->

[Valid](#non-existent)

<!-- Invalid (False Negative) -->

[Invalid][reference]

[reference]: #non-existent

What changes did you make? (Give an overview)

In this PR, I've fixed a false negative caused by an unrecognized Definition node in no-missing-link-fragments.

This issue was discovered while working on #583, and to avoid making that PR too large, I split this into a separate PR.

---

Previously, the rule did not handle a Definition node like the following:

markdown/src/rules/no-missing-link-fragments.js

Lines 129 to 141 in 8f92161

	link(node) {
	const url = node.url;
	if (!url || !url.startsWith("#")) {
	return;
	}
	const fragment = url.slice(1);
	if (!fragment) {
	return;
	}
	linkNodes.push({ node, fragment });
	},

I've updated the logic to recognize Definition nodes accordingly.

---

I've also refactored the codebase. There was some unnecessary or duplicate logic in the rule, so I refactored it to make the rule logic clearer, and faster. I've left comments on the notable parts of this change.

Related Issues

Ref: #583

Is there anything you'd like reviewers to focus on?

N/A

[lumirlumir]

These test cases were added primarily to ensure that githubLineReferencePattern is case-insensitive.

Before adding this test, the suite passed even when I added the i flag to the following regex pattern:

markdown/src/rules/no-missing-link-fragments.js

Line 29 in 8f92161

const githubLineReferencePattern = /^L\d+(?:C\d+)?(?:-L\d+(?:C\d+)?)?$/u;

---

FYI: You can verify that the githubLineReferencePattern is case-insensitive using the following examples:

• OK

https://github.com/eslint/markdown/blob/main/package.json#L4
https://github.com/eslint/markdown/blob/main/package.json#L4-L6

• Not OK

https://github.com/eslint/markdown/blob/main/package.json#l4 // Unworking.
https://github.com/eslint/markdown/blob/main/package.json#l4-l6 // Unworking.

[lumirlumir]

This is a small improvement: I've simplified the logic to reduce a bit of memory usage and computation.

Previously, we created a new data structure type as follows:

markdown/src/rules/no-missing-link-fragments.js

Lines 88 to 89 in 8f92161

	/** @type {Array<{node: Link, fragment: string}>} */
	const linkNodes = [];

markdown/src/rules/no-missing-link-fragments.js

Line 140 in 8f92161

linkNodes.push({ node, fragment });

However, now we no longer create a new object and instead use an existing object reference; it's small, but it would reduce memory usage and computation.

[Copilot]

Pull request overview

This PR fixes a false negative in the no-missing-link-fragments rule where reference-style link definitions (e.g., [reference]: #fragment) were not being validated against existing headings. The rule now correctly reports when Definition nodes reference non-existent fragments.

Key changes:

• Added support for checking Definition nodes in addition to Link nodes
• Refactored redundant code, including removing unnecessary toLowerCase() calls (github-slugger already lowercases)
• Replaced htmlCommentPattern regex with stripHtmlComments() utility function for better maintainability

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/rules/no-missing-link-fragments.js	Updated to recognize Definition nodes using a combined "definition, link" handler; refactored to remove redundant toLowerCase operations since github-slugger already returns lowercase slugs; switched to using stripHtmlComments() utility function
tests/rules/no-missing-link-fragments.test.js	Added test cases for valid and invalid Definition nodes; added additional test cases for invalid line reference formats (#l20 and #l20-l30) to verify case-sensitive validation

---

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[snitin315]

LGTM, thanks! Leaving it open for @DMartens

[DMartens]

Changes LGTM, thanks.