Initial implementation of MD051/valid-link-fragments (refs #253, closes

#495).

README.md

@@ -107,6 +107,7 @@ playground for learning and exploring.
 * **[MD048](doc/Rules.md#md048)** *code-fence-style* - Code fence style
 * **[MD049](doc/Rules.md#md049)** *emphasis-style* - Emphasis style should be consistent
 * **[MD050](doc/Rules.md#md050)** *strong-style* - Strong style should be consistent
+* **[MD051](doc/Rules.md#md051)** *valid-link-fragments* - Link fragments should be valid
 
 <!-- markdownlint-restore -->
 
@@ -142,7 +143,7 @@ rules at once.
 * **indentation** - MD005, MD006, MD007, MD027
 * **language** - MD040
 * **line_length** - MD013
-* **links** - MD011, MD034, MD039, MD042
+* **links** - MD011, MD034, MD039, MD042, MD051
 * **ol** - MD029, MD030, MD032
 * **spaces** - MD018, MD019, MD020, MD021, MD023
 * **spelling** - MD044

demo/markdownlint-browser.js

@@ -4383,6 +4383,58 @@ module.exports = {
 };
 
 
+/***/ }),
+
+/***/ "../lib/md051.js":
+/*!***********************!*\
+  !*** ../lib/md051.js ***!
+  \***********************/
+/***/ ((module, __unused_webpack_exports, __webpack_require__) => {
+
+"use strict";
+// @ts-check
+
+var _a = __webpack_require__(/*! ../helpers */ "../helpers/helpers.js"), addError = _a.addError, forEachHeading = _a.forEachHeading, filterTokens = _a.filterTokens;
+/**
+ * Converts a Markdown heading into an HTML fragment
+ * according to the rules used by GitHub.
+ *
+ * @param {string} string The string to convert.
+ * @returns {string} The converted string.
+ */
+function convertHeadingToHTMLFragment(string) {
+    return "#" + string
+        .toLowerCase()
+        .replace(/ /g, "-")
+        .replace(/[^-_a-z0-9]/g, "");
+}
+module.exports = {
+    "names": ["MD051", "valid-link-fragments"],
+    "description": "Link fragments should be valid",
+    "tags": ["links"],
+    "function": function MD051(params, onError) {
+        var validLinkFragments = [];
+        forEachHeading(params, function (_heading, content) {
+            validLinkFragments.push(convertHeadingToHTMLFragment(content));
+        });
+        filterTokens(params, "inline", function (token) {
+            token.children.forEach(function (child) {
+                var lineNumber = child.lineNumber, type = child.type, attrs = child.attrs;
+                if (type === "link_open") {
+                    var href = attrs.find(function (attr) { return attr[0] === "href"; });
+                    if (href !== undefined &&
+                        href[1].startsWith("#") &&
+                        !validLinkFragments.includes(href[1])) {
+                        var detail = "Link Fragment is invalid";
+                        addError(onError, lineNumber, detail, href[1]);
+                    }
+                }
+            });
+        });
+    }
+};
+
+
 /***/ }),
 
 /***/ "../lib/rules.js":
@@ -4441,7 +4493,8 @@ var rules = [
     __webpack_require__(/*! ./md047 */ "../lib/md047.js"),
     __webpack_require__(/*! ./md048 */ "../lib/md048.js"),
     __webpack_require__(/*! ./md049 */ "../lib/md049.js"),
-    __webpack_require__(/*! ./md050 */ "../lib/md050.js")
+    __webpack_require__(/*! ./md050 */ "../lib/md050.js"),
+    __webpack_require__(/*! ./md051 */ "../lib/md051.js")
 ];
 rules.forEach(function (rule) {
     var name = rule.names[0].toLowerCase();

doc/Rules.md

@@ -56,7 +56,8 @@ Aliases: first-heading-h1, first-header-h1
 Parameters: level (number; default 1)
 
 > Note: *MD002 has been deprecated and is disabled by default.*
-> [MD041/first-line-heading](#md041) offers an improved implementation.
+> [MD041/first-line-heading](#md041---first-line-in-a-file-should-be-a-top-level-heading)
+offers an improved implementation.
 
 This rule is intended to ensure document headings start at the top level and
 is triggered when the first heading in the document isn't an h1 heading:
@@ -783,7 +784,8 @@ The `lines_above` and `lines_below` parameters can be used to specify a differen
 number of blank lines (including 0) above or below each heading.
 
 Note: If `lines_above` or `lines_below` are configured to require more than one
-blank line, [MD012/no-multiple-blanks](#md012) should also be customized.
+blank line, [MD012/no-multiple-blanks](#md012---multiple-consecutive-blank-lines)
+should also be customized.
 
 Rationale: Aside from aesthetic reasons, some parsers, including `kramdown`, will
 not parse headings that don't have a blank line before, and will parse them as
@@ -1984,3 +1986,26 @@ The configured strong style can be a specific symbol to use ("asterisk",
 "underscore"), or can require that usage be consistent within the document.
 
 Rationale: Consistent formatting makes it easier to understand a document.
+
+<a name="md051"></a>
+
+## MD051 - Link fragments should be valid
+
+Tags: links
+
+Aliases: valid-link-fragments
+
+This rule is triggered if a link fragment does not correspond to a
+heading within the document:
+
+```markdown
+# Title
+
+[Link](#invalid-fragment)
+```
+
+To fix this issue, ensure that the heading exists,
+here you could replace `#invalid-fragment` by `#title`.
+
+It's not part of the CommonMark specification,
+for example [GitHub turn headings into links](https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#section-links).

lib/md051.js

@@ -0,0 +1,46 @@
+// @ts-check
+
+"use strict";
+
+const { addError, forEachHeading, filterTokens } = require("../helpers");
+
+/**
+ * Converts a Markdown heading into an HTML fragment
+ * according to the rules used by GitHub.
+ *
+ * @param {string} string The string to convert.
+ * @returns {string} The converted string.
+ */
+function convertHeadingToHTMLFragment(string) {
+  return "#" + string
+    .toLowerCase()
+    .replace(/ /g, "-")
+    .replace(/[^-_a-z0-9]/g, "");
+}
+
+module.exports = {
+  "names": [ "MD051", "valid-link-fragments" ],
+  "description": "Link fragments should be valid",
+  "tags": [ "links" ],
+  "function": function MD051(params, onError) {
+    const validLinkFragments = [];
+    forEachHeading(params, (_heading, content) => {
+      validLinkFragments.push(convertHeadingToHTMLFragment(content));
+    });
+    filterTokens(params, "inline", (token) => {
+      token.children.forEach((child) => {
+        const { lineNumber, type, attrs } = child;
+        if (type === "link_open") {
+          const href = attrs.find((attr) => attr[0] === "href");
+          if (href !== undefined &&
+              href[1].startsWith("#") &&
+              !validLinkFragments.includes(href[1])
+          ) {
+            const detail = "Link Fragment is invalid";
+            addError(onError, lineNumber, detail, href[1]);
+          }
+        }
+      });
+    });
+  }
+};

lib/rules.js

@@ -50,7 +50,8 @@ const rules = [
   require("./md047"),
   require("./md048"),
   require("./md049"),
-  require("./md050")
+  require("./md050"),
+  require("./md051")
 ];
 rules.forEach((rule) => {
   const name = rule.names[0].toLowerCase();

schema/.markdownlint.jsonc

@@ -262,5 +262,8 @@
   "MD050": {
     // Strong style should be consistent
     "style": "consistent"
-  }
+  },
+
+  // MD051/valid-link-fragments - Link fragments should be valid
+  "MD051": true
 }

schema/.markdownlint.yaml

@@ -236,4 +236,7 @@ MD049:
 # MD050/strong-style - Strong style should be consistent
 MD050:
   # Strong style should be consistent
-  style: "consistent"
+  style: "consistent"
+
+# MD051/valid-link-fragments - Link fragments should be valid
+MD051: true

schema/markdownlint-config-schema.json

@@ -878,6 +878,16 @@
     "strong-style": {
       "$ref": "#/properties/MD050"
     },
+    "MD051": {
+      "description": "MD051/valid-link-fragments - Link fragments should be valid",
+      "type": "boolean",
+      "default": true
+    },
+    "valid-link-fragments": {
+      "description": "MD051/valid-link-fragments - Link fragments should be valid",
+      "type": "boolean",
+      "default": true
+    },
     "headings": {
       "description": "headings - MD001, MD002, MD003, MD018, MD019, MD020, MD021, MD022, MD023, MD024, MD025, MD026, MD036, MD041, MD043",
       "type": "boolean",
@@ -914,7 +924,7 @@
       "default": true
     },
     "links": {
-      "description": "links - MD011, MD034, MD039, MD042",
+      "description": "links - MD011, MD034, MD039, MD042, MD051",
       "type": "boolean",
       "default": true
     },

test/detailed-results-MD041-MD050.results.json

@@ -314,5 +314,31 @@
       "MD050",
       "strong-style"
     ]
+  },
+  {
+    "errorContext": "#",
+    "errorDetail": "Link Fragment is invalid",
+    "errorRange": null,
+    "fixInfo": null,
+    "lineNumber": 5,
+    "ruleDescription": "Link fragments should be valid",
+    "ruleInformation": "https://github.com/DavidAnson/markdownlint/blob/v0.0.0/doc/Rules.md#md051",
+    "ruleNames": [
+      "MD051",
+      "valid-link-fragments"
+    ]
+  },
+  {
+    "errorContext": "#one",
+    "errorDetail": "Link Fragment is invalid",
+    "errorRange": null,
+    "fixInfo": null,
+    "lineNumber": 17,
+    "ruleDescription": "Link fragments should be valid",
+    "ruleInformation": "https://github.com/DavidAnson/markdownlint/blob/v0.0.0/doc/Rules.md#md051",
+    "ruleNames": [
+      "MD051",
+      "valid-link-fragments"
+    ]
   }
 ]

test/empty-links.md

@@ -12,22 +12,24 @@
 
 [text]( <> "title" ) {MD042}
 
-[text](#) {MD042}
+[text](#) {MD042} {MD051}
 
-[text]( # ) {MD042}
+[text]( # ) {MD042} {MD051}
 
-[text](# "title") {MD042}
+[text](# "title") {MD042} {MD051}
 
-[text]( # "title" ) {MD042}
+[text]( # "title" ) {MD042} {MD051}
 
-[text][frag] {MD042}
+[text][frag] {MD042} {MD051}
 
-[text][ frag ] {MD042}
+[text][ frag ] {MD042} {MD051}
 
 [frag]: #
 
 ## Non-empty links
 
+### frag
+
 [text](link)
 
 [text]( link )

test/markdownlint-test.js

@@ -841,7 +841,7 @@ test.cb("customFileSystemAsync", (t) => {
 });
 
 test.cb("readme", (t) => {
-  t.plan(119);
+  t.plan(121);
   const tagToRules = {};
   rules.forEach(function forRule(rule) {
     rule.tags.forEach(function forTag(tag) {
@@ -917,7 +917,7 @@ test.cb("readme", (t) => {
 });
 
 test.cb("rules", (t) => {
-  t.plan(352);
+  t.plan(359);
   fs.readFile("doc/Rules.md", "utf8",
     (err, contents) => {
       t.falsy(err);
@@ -1094,7 +1094,7 @@ test("validateConfigExampleJson", async(t) => {
 });
 
 test("allBuiltInRulesHaveValidUrl", (t) => {
-  t.plan(138);
+  t.plan(141);
   rules.forEach(function forRule(rule) {
     t.truthy(rule.information);
     t.true(Object.getPrototypeOf(rule.information) === URL.prototype);

test/spaces_inside_codespan_elements.md

@@ -78,21 +78,21 @@ Text [link](https://example.com/link`link`link`link) text `code`.
 
 Text [link](https://example.com/link "title`title") text `code`.
 
-Text [link](#link`link) text `code`.
+Text [link](#link`link) text `code`. {MD051}
 
 Text [link] (#link`link) text `code`. {MD038}
 
-Text [link[link](#link`link) text `code`.
+Text [link[link](#link`link) text `code`. {MD051}
 
-Text [link(link](#link`link) text `code`.
+Text [link(link](#link`link) text `code`. {MD051}
 
-Text [link)link](#link`link) text `code`.
+Text [link)link](#link`link) text `code`. {MD051}
 
-Text [link](#link[link`link) text `code`.
+Text [link](#link[linklink) text `code`. {MD051}
 
-Text [link](#link]link`link) text `code`.
+Text [link](#link[linklink) text `code`. {MD051}
 
-Text [link](#link(link`link) text `code`. {MD038}
+Text [link](#link[linklink) text `code`. {MD051}
 
 Text [`link`](xref:custom.link`1) text `code`.