circular refs don't crash it now

lib/traverse.js

@@ -1,16 +1,34 @@
 module.exports = Traverse;
 module.exports.Traverse = Traverse;
 
+var sys = require('sys');
+
 function Traverse (refObj) {
     if (!(this instanceof Traverse)) return new Traverse(refObj);
 
     // clone refObj for a properly immutable interface:
+    clone.refs = [];
+    clone.nodes = [];
     function clone(ref) {
         if (typeof ref == 'object' && ref !== null) {
             var node = ref instanceof Array ? [] : {};
+            clone.refs.push(ref);
+            clone.nodes.push(node);
+
             Object.keys(ref).forEach(function (key) {
-                node[key] = clone(ref[key]);
+                var i = clone.refs.indexOf(ref[key]);
+                if (i >= 0) {
+                    node[key] = clone.nodes[i];
+                }
+                else {
+                    node[key] = clone(ref[key]);
+                }
+
             });
+
+            clone.refs.pop();
+            clone.nodes.pop();
+
             // To make instanceof work:
             node.__proto__ = ref.__proto__;
             // Probably there are other attributes worth copying
@@ -47,9 +65,15 @@ function Traverse (refObj) {
                     state.node = x;
                 },
                 level : path.length,
+                circular : null
             };
+
             if (typeof node == 'object' && node !== null) {
                 state.isLeaf = Object.keys(node).length == 0
+                var circs = parents.filter(function (p) {
+                    return node == p.node
+                });
+                if (circs.length) state.circular = circs[0];
             }
             else {
                 state.isLeaf = true;
@@ -59,7 +83,8 @@ function Traverse (refObj) {
             state.notRoot = !state.isRoot;
 
             f.call(state, node);
-            if (typeof state.node == 'object' && state.node !== null) {
+            if (typeof state.node == 'object'
+            && state.node !== null && !state.circular) {
                 parents.push(state);
                 Object.keys(state.node).forEach(function (key) {
                     path.push(key);