add doc

doc/themes/next/scripts/helpers/engine.js

@@ -0,0 +1,26 @@
+/* global hexo */
+
+'use strict';
+
+hexo.extend.helper.register('hexo_env', function(type) {
+  return this.env[type];
+});
+
+hexo.extend.helper.register('next_env', function(type) {
+  var path = require('path');
+  var env = require(path.normalize('../../package.json'));
+  return env[type];
+});
+
+hexo.extend.helper.register('item_active', function(path, className) {
+  var canonical = this.page.canonical_path;
+  var current = this.url_for(canonical).replace('index.html', '', 'g');
+  var result = '';
+
+  if (current.indexOf(path) !== -1) {
+    if (path !== '/' || path === current) {
+      result = ' ' + className;
+    }
+  }
+  return result;
+});