Plugin to remove elements in <defs> without id. Resolves svg#310

.svgo.yml

@@ -23,6 +23,7 @@ plugins:
   - convertStyleToAttrs
   - cleanupIDs
   - removeRasterImages
+  - removeUselessDefs
   - cleanupNumericValues
   - cleanupListOfValues
   - convertColors

README.md

@@ -25,6 +25,7 @@ Today we have:
 * [ [>](https://github.com/svg/svgo/blob/master/plugins/removeMetadata.js) ] remove `<metadata>`
 * [ [>](https://github.com/svg/svgo/blob/master/plugins/removeTitle.js) ] remove `<title>` (disabled by default)
 * [ [>](https://github.com/svg/svgo/blob/master/plugins/removeDesc.js) ] remove `<desc>` (disabled by default)
+* [ [>](https://github.com/svg/svgo/blob/master/plugins/removeUselessDefs.js) ] remove elements of `<defs>` without `id`
 * [ [>](https://github.com/svg/svgo/blob/master/plugins/removeEditorsNSData.js) ] remove editors namespaces, elements and attributes
 * [ [>](https://github.com/svg/svgo/blob/master/plugins/removeEmptyAttrs.js) ] remove empty attributes
 * [ [>](https://github.com/svg/svgo/blob/master/plugins/removeHiddenElems.js) ] remove hidden elements

README.ru.md

@@ -25,6 +25,7 @@ SVGO имеет расширяемую архитектуру, в которой
 * [ [>](https://github.com/svg/svgo/blob/master/plugins/removeMetadata.js) ] удаление `<metadata>`
 * [ [>](https://github.com/svg/svgo/blob/master/plugins/removeTitle.js) ] удаление `<title>` (отключена по умолчанию)
 * [ [>](https://github.com/svg/svgo/blob/master/plugins/removeDesc.js) ] удаление `<desc>` (отключена по умолчанию)
+* [ [>](https://github.com/svg/svgo/blob/master/plugins/removeUselessDefs.js) ] удаление элементов в `<defs>` без `id`
 * [ [>](https://github.com/svg/svgo/blob/master/plugins/removeEditorsNSData.js) ] удаление пространств имён различных редакторов, их элементов и атрибутов
 * [ [>](https://github.com/svg/svgo/blob/master/plugins/removeEmptyAttrs.js) ] удаление пустых атрибутов
 * [ [>](https://github.com/svg/svgo/blob/master/plugins/removeHiddenElems.js) ] удаление скрытых элементов

plugins/removeUselessDefs.js

@@ -0,0 +1,41 @@
+"use strict";
+
+exports.type = 'perItem';
+
+exports.active = true;
+
+/**
+ * Removes content of defs without ids and thus useless.
+ *
+ * @param {Object} item current iteration item
+ * @return {Boolean} if false, item will be filtered out
+ *
+ * @author Lev Solntsev
+ */
+exports.fn = function(item) {
+
+    if (item.isElem('defs')) {
+
+        if (!item.isEmpty())
+            item.content = item.content.reduce(getUsefulItems, []);
+
+        if (item.isEmpty()) return false;
+
+    }
+
+};
+
+function getUsefulItems(usefulItems, item) {
+
+    if (item.hasAttr('id')) {
+
+        usefulItems.push(item);
+
+    } else if (!item.isEmpty()) {
+
+        item.content.reduce(getUsefulItems, usefulItems);
+
+    }
+
+    return usefulItems;
+}