Add support for 'preserve' rules.

src/rules.js

@@ -6,12 +6,14 @@ export default function Rules (options) {
   this.options = options
   this._keep = []
   this._remove = []
+  this._preserve = []
 
   this.blankRule = {
     replacement: options.blankReplacement
   }
 
   this.keepReplacement = options.keepReplacement
+  this.preserveReplacement = options.preserveReplacement
 
   this.defaultRule = {
     replacement: options.defaultReplacement
@@ -42,12 +44,20 @@ Rules.prototype = {
     })
   },
 
+  preserve: function (filter) {
+    this._preserve.unshift({
+      filter: filter,
+      replacement: this.preserveReplacement
+    })
+  },
+
   forNode: function (node) {
     if (node.isBlank) return this.blankRule
     var rule
 
     if ((rule = findRule(this.array, node, this.options))) return rule
     if ((rule = findRule(this._keep, node, this.options))) return rule
+    if ((rule = findRule(this._preserve, node, this.options))) return rule
     if ((rule = findRule(this._remove, node, this.options))) return rule
 
     return this.defaultRule

src/turndown.js

@@ -1,6 +1,6 @@
 import COMMONMARK_RULES from './commonmark-rules'
 import Rules from './rules'
-import { extend, trimLeadingNewlines, trimTrailingNewlines } from './utilities'
+import { extend, trimLeadingNewlines, trimTrailingNewlines, getOpenTag, getCloseTag } from './utilities'
 import RootNode from './root-node'
 import Node from './node'
 var reduce = Array.prototype.reduce
@@ -44,6 +44,15 @@ export default function TurndownService (options) {
     },
     defaultReplacement: function (content, node) {
       return node.isBlock ? '\n\n' + content + '\n\n' : content
+    },
+    preserveReplacement: function (innerContent, node) {
+      const html = node.cloneNode()
+      html.setAttribute('markdown', '1')
+      const openTag = getOpenTag(html)
+      const closeTag = getCloseTag(html)
+
+      const outerContent = openTag + innerContent + closeTag
+      return node.isBlock ? '\n\n' + outerContent + '\n\n' : outerContent
     }
   }
   this.options = extend({}, defaults, options)
@@ -131,6 +140,19 @@ TurndownService.prototype = {
     return this
   },
 
+  /**
+   * Preserves (using markdown="1") a node that matches the filter
+   * @public
+   * @param {String|Array|Function} filter The unique key of the rule
+   * @returns The Turndown instance for chaining
+   * @type Object
+   */
+
+  preserve: function (filter) {
+    this.rules.preserve(filter)
+    return this
+  },
+
   /**
    * Escapes Markdown syntax
    * @public

src/utilities.js

@@ -74,3 +74,20 @@ function has (node, tagNames) {
     })
   )
 }
+
+export function getOpenTag (element) {
+  // Credit: https://stackoverflow.com/questions/9604235
+  const outerHtml = element.outerHTML
+  const len = outerHtml.length
+
+  if (outerHtml[len - 2] === '/') { // Is self-closing tag?
+    return outerHtml.slice(0, len - 2) + '>'
+  } else {
+    const openTagLength = len - element.innerHTML.length - (element.tagName.length + 3)
+    return outerHtml.slice(0, openTagLength)
+  }
+}
+
+export function getCloseTag (element) {
+  return `</${element.tagName.toLowerCase()}>`
+}