Add function to generate TOC from document headings (#474)

lib/core/getTOC.js

@@ -0,0 +1,52 @@
+const Remarkable = require('remarkable');
+const toSlug = require('./toSlug');
+
+const tagToLevel = tag => Number(tag.slice(1));
+
+/**
+ * Returns a table of content from the headings
+ *
+ * @return array
+ * Array of heading objects with `hashLink`, `text` and `children` fields
+ *
+ */
+module.exports = (content, headingTags = 'h2', subHeadingTags = 'h3') => {
+  const headingLevels = [].concat(headingTags).map(tagToLevel);
+  const subHeadingLevels = subHeadingTags
+    ? [].concat(subHeadingTags).map(tagToLevel)
+    : [];
+
+  const md = new Remarkable();
+  const tokens = md.parse(content, {});
+  const headings = [];
+  for (let i = 0; i < tokens.length; i++) {
+    if (
+      tokens[i].type == 'heading_open' &&
+      headingLevels.concat(subHeadingLevels).includes(tokens[i].hLevel)
+    ) {
+      headings.push({
+        hLevel: tokens[i].hLevel,
+        text: tokens[i + 1].content,
+      });
+    }
+  }
+
+  const toc = [];
+  let current;
+  headings.forEach(heading => {
+    const entry = {
+      hashLink: toSlug(heading.text),
+      text: heading.text,
+      children: [],
+    };
+
+    if (headingLevels.includes(heading.hLevel)) {
+      toc.push(entry);
+      current = entry;
+    } else {
+      current && current.children.push(entry);
+    }
+  });
+
+  return toc;
+};