Extract alternate ids for headings

fixes #1824

Author: dontcallmedom

schemas/browserlib/extract-headings.json

@@ -12,7 +12,8 @@
       "href": { "$ref": "../common.json#/$defs/url" },
       "title": { "type": "string" },
       "level": { "type": "integer" },
-      "number": { "$ref": "../common.json#/$defs/headingNumber" }
+      "number": { "$ref": "../common.json#/$defs/headingNumber" },
+      "alternateIds": { "type": "array", "items": { "$ref": "../common.json#/$defs/id"} }
     }
   }
 }

src/browserlib/extract-headings.mjs

@@ -41,13 +41,15 @@ export default function (spec, idToHeading) {
       href,
       title: n.textContent.trim()
     };
-
     const res = {
       id: heading.id,
       href: heading.href,
       level: parseInt(headingEl.tagName.slice(1), 10),
       title: heading.title
     };
+    if (heading.alternateIds?.length) {
+      res.alternateIds = heading.alternateIds;
+    }
     if (heading.number) {
       res.number = heading.number;
     }

src/browserlib/map-ids-to-headings.mjs

@@ -86,18 +86,21 @@ export default function () {
     let href = nodeid;
 
     if (parentSection) {
+      const alternateIds = [];
       let id;
 
       const heading = parentSection.heading;
       if (heading.id) {
         id = heading.id;
         href = getAbsoluteUrl(heading, { singlePage });
+	alternateIds.push(id);
       }
       else {
         const anchor = heading.querySelector('a[name]');
         if (anchor) {
           id = anchor.getAttribute('name');
           href = getAbsoluteUrl(anchor, { singlePage, attribute: 'name' });
+	  alternateIds.push(id);
         }
       }
 
@@ -116,6 +119,7 @@ export default function () {
       }
       mapping.href = href;
       mapping.title = trimmedText.replace(reNumber, '');
+      mapping.alternateIds = alternateIds.filter(id => id !== mapping.id);
       mappingTable[nodeid] = mapping;
 
       if (number) {

test/extract-headings.js

@@ -58,6 +58,11 @@ const testHeadings = [
     html: "<section id><h1 id=title>Heading in a section with empty id</h1>",
     res: [{id: "title", "href": "about:blank#title", title: "Heading in a section with empty id", level: 1}]
   },
+  {
+    title: "documents alternate ids when they exist",
+    html: "<section id=title-0><h1 id=title>Heading in a section with its own id</h1>",
+    res: [{id: "title-0", "href": "about:blank#title-0", title: "Heading in a section with its own id", level: 1, alternateIds: ["title"]}]
+  },
 ];
 
 describe("Test headings extraction", function () {