diff --git a/test/mutability.js b/test/mutability.js
new file mode 100644
index 0000000..95b1f7a
--- /dev/null
+++ b/test/mutability.js
@@ -0,0 +1,68 @@
+var assert = require('assert');
+var Traverse = require('traverse');
+
+exports.mutate = function () {
+    var obj = { a : 1, b : 2, c : [ 3, 4 ] };
+    var res = Traverse(obj).forEach(function (x) {
+        if (typeof x === 'number' && x % 2 === 0) {
+            this.update(x * 10);
+        }
+    });
+    assert.eql(obj, res);
+    assert.eql(obj, { a : 1, b : 20, c : [ 3, 40 ] });
+};
+
+exports.mutateT = function () {
+    var obj = { a : 1, b : 2, c : [ 3, 4 ] };
+    var res = Traverse.forEach(obj, function (x) {
+        if (typeof x === 'number' && x % 2 === 0) {
+            this.update(x * 10);
+        }
+    });
+    assert.eql(obj, res);
+    assert.eql(obj, { a : 1, b : 20, c : [ 3, 40 ] });
+};
+
+exports.map = function () {
+    var obj = { a : 1, b : 2, c : [ 3, 4 ] };
+    var res = Traverse(obj).map(function (x) {
+        if (typeof x === 'number' && x % 2 === 0) {
+            this.update(x * 10);
+        }
+    });
+    assert.eql(obj, { a : 1, b : 2, c : [ 3, 4 ] });
+    assert.eql(res, { a : 1, b : 20, c : [ 3, 40 ] });
+};
+
+exports.mapT = function () {
+    var obj = { a : 1, b : 2, c : [ 3, 4 ] };
+    var res = Traverse.map(obj, function (x) {
+        if (typeof x === 'number' && x % 2 === 0) {
+            this.update(x * 10);
+        }
+    });
+    assert.eql(obj, { a : 1, b : 2, c : [ 3, 4 ] });
+    assert.eql(res, { a : 1, b : 20, c : [ 3, 40 ] });
+};
+
+exports.clone = function () {
+    var obj = { a : 1, b : 2, c : [ 3, 4 ] };
+    var res = Traverse(obj).clone();
+    assert.eql(obj, res);
+    assert.ok(obj !== res);
+    obj.a ++;
+    assert.eql(res.a, 1);
+    obj.c.push(5);
+    assert.eql(res.c, [ 3, 4 ]);
+};
+
+exports.cloneT = function () {
+    var obj = { a : 1, b : 2, c : [ 3, 4 ] };
+    var res = Traverse.clone(obj);
+    assert.eql(obj, res);
+    assert.ok(obj !== res);
+    obj.a ++;
+    assert.eql(res.a, 1);
+    obj.c.push(5);
+    assert.eql(res.c, [ 3, 4 ]);
+};