Refactor code-style

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

Author: wooorm

lib/index.js

@@ -1,8 +1,10 @@
 /**
  * @typedef {import('mdast').Delete} Delete
+ *
  * @typedef {import('mdast-util-from-markdown').CompileContext} CompileContext
  * @typedef {import('mdast-util-from-markdown').Extension} FromMarkdownExtension
  * @typedef {import('mdast-util-from-markdown').Handle} FromMarkdownHandle
+ *
  * @typedef {import('mdast-util-to-markdown').ConstructName} ConstructName
  * @typedef {import('mdast-util-to-markdown').Options} ToMarkdownExtension
  * @typedef {import('mdast-util-to-markdown').Handle} ToMarkdownHandle
@@ -11,16 +13,14 @@
 import {containerPhrasing} from 'mdast-util-to-markdown/lib/util/container-phrasing.js'
 import {track} from 'mdast-util-to-markdown/lib/util/track.js'
 
-/** @type {FromMarkdownExtension} */
-export const gfmStrikethroughFromMarkdown = {
-  canContainEols: ['delete'],
-  enter: {strikethrough: enterStrikethrough},
-  exit: {strikethrough: exitStrikethrough}
-}
+// To do: next major: expose functions.
+// To do: next major: use `state`, state utilities.
 
 /**
  * List of constructs that occur in phrasing (paragraphs, headings), but cannot
- * contain strikethroughs. So they sort of cancel each other out.
+ * contain strikethrough.
+ * So they sort of cancel each other out.
+ * Note: could use a better name.
  *
  * Note: keep in sync with: <https://github.com/syntax-tree/mdast-util-to-markdown/blob/8ce8dbf/lib/unsafe.js#L14>
  *
@@ -35,7 +35,24 @@ const constructsWithoutStrikethrough = [
   'titleApostrophe'
 ]
 
-/** @type {ToMarkdownExtension} */
+handleDelete.peek = peekDelete
+
+/**
+ * Extension for `mdast-util-from-markdown` to enable GFM strikethrough.
+ *
+ * @type {FromMarkdownExtension}
+ */
+export const gfmStrikethroughFromMarkdown = {
+  canContainEols: ['delete'],
+  enter: {strikethrough: enterStrikethrough},
+  exit: {strikethrough: exitStrikethrough}
+}
+
+/**
+ * Extension for `mdast-util-to-markdown` to enable GFM strikethrough.
+ *
+ * @type {ToMarkdownExtension}
+ */
 export const gfmStrikethroughToMarkdown = {
   unsafe: [
     {
@@ -47,8 +64,6 @@ export const gfmStrikethroughToMarkdown = {
   handlers: {delete: handleDelete}
 }
 
-handleDelete.peek = peekDelete
-
 /**
  * @this {CompileContext}
  * @type {FromMarkdownHandle}