3 files changed, 26 insertions, 15 deletions

diff --git a/README.md b/README.md
index a079e04..bd81b84 100644
--- a/README.md
+++ b/README.md
@@ -461,16 +461,25 @@ You can use the `transformDocument` argument to modify the document appropriatel
 ```javascript
 function transformElement(element) {
     if (element.children) {
-        element.children.forEach(transformElement);
+        var children = _.map(element.children, transformElement);
+        element = extend(element, {children: children});
     }
+    
     if (element.type === "paragraph") {
-        if (element.alignment === "center" && !element.styleId) {
-            element.styleId = "Heading2";
-        }
+        element = transformParagraph(element);
     }
+    
     return element;
 }
 
+function transformParagraph(element) {
+    if (element.alignment === "center" && !element.styleId) {
+        return extend(element, {styleId: "Heading2"});
+    } else {
+        return element;
+    }
+}
+
 var options = {
     transformDocument: transformElement
 };
@@ -481,12 +490,12 @@ The return value of `transformDocument` is used during HTML generation.
 The above can be written more succinctly using the helper `mammoth.transforms.paragraph`:
 
 ```javascript
-
 function transformParagraph(element) {
     if (element.alignment === "center" && !element.styleId) {
-        element.styleId = "Heading2";
+        return extend(element, {styleId: "Heading2"});
+    } else {
+        return element;
     }
-    return element;
 }
 
 var options = {
diff --git a/lib/transforms.js b/lib/transforms.js
index 988b456..5078387 100644
--- a/lib/transforms.js
+++ b/lib/transforms.js
@@ -1,15 +1,17 @@
+var _ = require("underscore");
+
 exports.paragraph = function(transform) {
     function transformElement(element) {
         if (element.children) {
-            for (var i = 0; i < element.children.length; i++) {
-                element.children[i] = transformElement(element.children[i]);
-            }
+            var children = _.map(element.children, transformElement);
+            element = _.extend(element, {children: children});
         }
+        
         if (element.type === "paragraph") {
-            return transform(element);
-        } else {
-            return element;
+            element = transform(element);
         }
+        
+        return element;
     }
     return transformElement;
 };
diff --git a/tests/mammoth.tests.js b/tests/mammoth.tests.js
index 801bb02..ad71539 100644
--- a/tests/mammoth.tests.js
+++ b/tests/mammoth.tests.js
@@ -1,6 +1,7 @@
 var assert = require("assert");
 var path = require("path");
 var fs = require("fs");
+var _ = require("underscore");
 
 var mammoth = require("../");
 var promises = require("../lib/promises");
@@ -156,8 +157,7 @@ describe('mammoth', function() {
         });
         var options = {
             transformDocument: mammoth.transforms.paragraph(function(paragraph) {
-                paragraph.styleId = "Heading1";
-                return paragraph;
+                return _.extend(paragraph, {styleId: "Heading1"});
             })
         };
         return mammoth.convertToHtml({file: docxFile}, options).then(function(result) {