Refactor code-style

*   Add more docs to JSDoc
*   Add support for `null` in input of API types

Author: wooorm

lib/index.js

@@ -1,17 +1,30 @@
 /**
  * @typedef {import('mdast').Heading} Heading
- * @typedef {'atx'|'atx-closed'|'setext'} Style
  */
 
 /**
+ * @typedef {'atx' | 'atx-closed' | 'setext'} Style
+ *   Style of heading.
+ */
+
+/**
+ * Get the heading style of a heading, optionally `relative` to a preferred
+ * style.
+ *
+ * This is because ATX headings with a depth of three or more could be
+ * considered setext.
+ *
  * @param {Heading} node
- * @param {Style} [relative]
- * @returns {Style|null}
+ *   Heading node to check.
+ * @param {Style | null | undefined} [relative]
+ *   Relative style.
+ * @returns {Style | null}
+ *   Style, if it can be inferred.
  */
 export function headingStyle(node, relative) {
   const last = node.children[node.children.length - 1]
   const depth = node.depth
-  const pos = node && node.position && node.position.end
+  const pos = node.position && node.position.end
   const final = last && last.position && last.position.end
 
   if (!pos) {
@@ -44,8 +57,8 @@ export function headingStyle(node, relative) {
  * Get the probable style of an atx-heading, depending on preferred style.
  *
  * @param {number} depth
- * @param {Style|undefined} relative
- * @returns {Style|null}
+ * @param {Style | null | undefined} relative
+ * @returns {Style | null}
  */
 function consolidate(depth, relative) {
   return depth < 3