Refactor code-style

*   Add support for `null` in API input types
*   Only pass `undefined` around internally, not `null`
*   Add more docs to JSDoc
*   Refactor a *ton* of code
*   Rename `entities` option to `characterReferences`
*   Add exports of useful types (`CharacterReferences`, `Quote`,
    `Space`)

Author: wooorm

index.js

@@ -1,5 +1,8 @@
 /**
+ * @typedef {import('./lib/types.js').CharacterReferences} CharacterReferences
  * @typedef {import('./lib/types.js').Options} Options
+ * @typedef {import('./lib/types.js').Quote} Quote
+ * @typedef {import('./lib/types.js').Space} Space
  */
 
 export {toHtml} from './lib/index.js'

lib/comment.js

@@ -0,0 +1,45 @@
+/**
+ * @typedef {import('../types.js').Comment} Comment
+ * @typedef {import('../types.js').Parent} Parent
+ * @typedef {import('../types.js').State} State
+ */
+
+import {stringifyEntities} from 'stringify-entities'
+
+/**
+ * Serialize a comment.
+ *
+ * @param {Comment} node
+ *   Node to handle.
+ * @param {number | undefined} _1
+ *   Index of `node` in `parent.
+ * @param {Parent | undefined} _2
+ *   Parent of `node`.
+ * @param {State} state
+ *   Info passed around about the current state.
+ * @returns {string}
+ *   Serialized node.
+ */
+export function comment(node, _1, _2, state) {
+  // See: <https://html.spec.whatwg.org/multipage/syntax.html#comments>
+  return state.settings.bogusComments
+    ? '<?' +
+        stringifyEntities(
+          node.value,
+          Object.assign({}, state.settings.characterReferences, {subset: ['>']})
+        ) +
+        '>'
+    : '<!--' + node.value.replace(/^>|^->|<!--|-->|--!>|<!-$/g, encode) + '-->'
+
+  /**
+   * @param {string} $0
+   */
+  function encode($0) {
+    return stringifyEntities(
+      $0,
+      Object.assign({}, state.settings.characterReferences, {
+        subset: ['<', '>']
+      })
+    )
+  }
+}

lib/constants.js

@@ -0,0 +1,28 @@
+/**
+ * @typedef {import('../types.js').DocType} DocType
+ * @typedef {import('../types.js').Parent} Parent
+ * @typedef {import('../types.js').State} State
+ */
+
+/**
+ * Serialize a doctype.
+ *
+ * @param {DocType} _1
+ *   Node to handle.
+ * @param {number | undefined} _2
+ *   Index of `node` in `parent.
+ * @param {Parent | undefined} _3
+ *   Parent of `node`.
+ * @param {State} state
+ *   Info passed around about the current state.
+ * @returns {string}
+ *   Serialized node.
+ */
+export function doctype(_1, _2, _3, state) {
+  return (
+    '<!' +
+    (state.settings.upperDoctype ? 'DOCTYPE' : 'doctype') +
+    (state.settings.tightDoctype ? '' : ' ') +
+    'html>'
+  )
+}