Add support for compiling unknown nodes

This updates ensures mdast-html is able to deal, “somewhat”,
with future nodes and plugin-specific nodes.

All unknown nodes are compiled as `div` elements, but the in
e713382 introduced support for specifying how mdastnode(7)s are
compiled enables plug-in authors to introduce other HTML elements.

Closes GH-2.

Author: wooorm

index.js

@@ -43,9 +43,11 @@ function plugin(mdast, options) {
      * Extensible constructor.
      */
     function HTMLCompiler(file) {
-        file.move({
-            'extension': 'html'
-        });
+        if (file.extension) {
+            file.move({
+                'extension': 'html'
+            });
+        }
 
         MarkdownCompiler.apply(this, [file, options]);
     }

lib/compilers.js

@@ -132,6 +132,66 @@ function generateFootnotes() {
     }, null, true) + '\n';
 }
 
+/**
+ * Stringify an unknown node.
+ *
+ * @example
+ *   unknown({
+ *     data: {
+ *       htmlName: 'section'
+ *     },
+ *     children: [
+ *       {
+ *         type: 'text',
+ *         value: 'foo'
+ *       }
+ *     ]
+ *   }); // '<section>foo</section>'
+ *
+ * @param {Node} node - Node to compile.
+ * @return {string} - Compiled node.
+ * @this {HTMLCompiler}
+ */
+function unknown(node) {
+    var content = 'children' in node ? this.all(node) : node.value;
+
+    return h(this, node, {
+        'name': 'div',
+        'content': content || ''
+    }, node.data);
+}
+
+/**
+ * Visit a node.
+ *
+ * @example
+ *   var compiler = new Compiler();
+ *
+ *   compiler.visit({
+ *     type: 'strong',
+ *     children: [{
+ *       type: 'text',
+ *       value: 'Foo'
+ *     }]
+ *   });
+ *   // '**Foo**'
+ *
+ * @param {Object} node - Node.
+ * @param {Object?} [parent] - `node`s parent.
+ * @return {string} - Compiled `node`.
+ */
+function one(node, parent) {
+    var self = this;
+    var type = node && node.type;
+    var fn = typeof self[type] === 'function' ? type : 'unknown';
+
+    if (!type) {
+        self.file.fail('Expected node `' + node + '`');
+    }
+
+    return self[fn](node, parent);
+}
+
 /**
  * Stringify the children of `node`.
  *
@@ -800,7 +860,7 @@ function image(node) {
  *         value: 'foo'
  *       }
  *     ]
- *   }); // '~~foo~~'
+ *   }); // '<del>foo</del>'
  *
  * @param {Node} node - Node to compile.
  * @return {string} - Compiled node.
@@ -862,7 +922,9 @@ function ignore() {
  * Helpers.
  */
 
+visitors.visit = one;
 visitors.all = all;
+visitors.unknown = unknown;
 visitors.generateFootnotes = generateFootnotes;
 
 /*

mdast-html.js

@@ -44,9 +44,11 @@ function plugin(mdast, options) {
      * Extensible constructor.
      */
     function HTMLCompiler(file) {
-        file.move({
-            'extension': 'html'
-        });
+        if (file.extension) {
+            file.move({
+                'extension': 'html'
+            });
+        }
 
         MarkdownCompiler.apply(this, [file, options]);
     }
@@ -205,6 +207,66 @@ function generateFootnotes() {
     }, null, true) + '\n';
 }
 
+/**
+ * Stringify an unknown node.
+ *
+ * @example
+ *   unknown({
+ *     data: {
+ *       htmlName: 'section'
+ *     },
+ *     children: [
+ *       {
+ *         type: 'text',
+ *         value: 'foo'
+ *       }
+ *     ]
+ *   }); // '<section>foo</section>'
+ *
+ * @param {Node} node - Node to compile.
+ * @return {string} - Compiled node.
+ * @this {HTMLCompiler}
+ */
+function unknown(node) {
+    var content = 'children' in node ? this.all(node) : node.value;
+
+    return h(this, node, {
+        'name': 'div',
+        'content': content || ''
+    }, node.data);
+}
+
+/**
+ * Visit a node.
+ *
+ * @example
+ *   var compiler = new Compiler();
+ *
+ *   compiler.visit({
+ *     type: 'strong',
+ *     children: [{
+ *       type: 'text',
+ *       value: 'Foo'
+ *     }]
+ *   });
+ *   // '**Foo**'
+ *
+ * @param {Object} node - Node.
+ * @param {Object?} [parent] - `node`s parent.
+ * @return {string} - Compiled `node`.
+ */
+function one(node, parent) {
+    var self = this;
+    var type = node && node.type;
+    var fn = typeof self[type] === 'function' ? type : 'unknown';
+
+    if (!type) {
+        self.file.fail('Expected node `' + node + '`');
+    }
+
+    return self[fn](node, parent);
+}
+
 /**
  * Stringify the children of `node`.
  *
@@ -873,7 +935,7 @@ function image(node) {
  *         value: 'foo'
  *       }
  *     ]
- *   }); // '~~foo~~'
+ *   }); // '<del>foo</del>'
  *
  * @param {Node} node - Node to compile.
  * @return {string} - Compiled node.
@@ -935,7 +997,9 @@ function ignore() {
  * Helpers.
  */
 
+visitors.visit = one;
 visitors.all = all;
+visitors.unknown = unknown;
 visitors.generateFootnotes = generateFootnotes;
 
 /*