enable deep copy via $.extend(true, target, source)

Closes #356

src/zepto.js

@@ -177,14 +177,24 @@ var Zepto = (function() {
     return zepto.init(selector, context)
   }
 
+  function extend(target, source, deep) {
+    for (key in source)
+      if (deep && isPlainObject(source[key])) {
+        if (!isPlainObject(target[key])) target[key] = {}
+        extend(target[key], source[key], deep)
+      }
+      else if (source[key] !== undefined) target[key] = source[key]
+  }
+
   // Copy all but undefined properties from one or more
   // objects to the `target` object.
   $.extend = function(target){
-    slice.call(arguments, 1).forEach(function(source) {
-      for (key in source)
-        if (source[key] !== undefined)
-          target[key] = source[key]
-    })
+    var deep, args = slice.call(arguments, 1)
+    if (typeof target == 'boolean') {
+      deep = target
+      target = args.shift()
+    }
+    args.forEach(function(arg){ extend(target, arg, deep) })
     return target
   }
 

test/zepto.html

@@ -368,6 +368,39 @@ <h1>Zepto DOM unit tests</h1>
 
         // undefined values are not copied over
         t.assertSame({a:1}, $.extend({a:1}, {b:undefined}))
+
+        // shallow by default
+        obj = $.extend({ a:{b:"c"} }, { a:{d:"e"} })
+        t.assertSame({d:"e"}, obj.a)
+      },
+
+      testExtendDeep: function(t){
+        var obj = { a:{b:"c", x:{y:"z"}} }
+        $.extend(true, obj, { a:{d:"e"} }, { a:{b:"B", f:"g", x:{q:"x"}} })
+
+        t.assertEqual('a', Object.keys(obj).join(','))
+        t.assertEqual('b,d,f,x', Object.keys(obj.a).sort().join(','))
+        t.assertEqual('B', obj.a.b)
+        t.assertEqual('e', obj.a.d)
+        t.assertEqual('g', obj.a.f)
+        t.assertEqual('z', obj.a.x.y)
+        t.assertEqual('x', obj.a.x.q)
+
+        // creates non-existing keys on target object
+        obj = {}
+        $.extend(true, obj, { a:{b:"c"} })
+        t.assertEqual('a', Object.keys(obj).join(','))
+        t.assertEqual('c', obj.a.b)
+
+        // skips iterating over DOM elements
+        obj = {}
+        var dom = $('#some_element').get(0)
+        $.extend(true, obj, { element: dom })
+        t.assertIdentical(dom, obj.element)
+
+        // can override DOM element
+        $.extend(true, obj, { element:{a:'b'} })
+        t.assertEqual('b', obj.element.a)
       },
 
       testExtensionAPI: function(t) {