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diff --git a/jstests/mr_mutable_properties.js b/jstests/mr_mutable_properties.js
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+// See SERVER-9448
+// Test argument and receiver (aka 'this') objects and their children can be mutated
+// in Map, Reduce and Finalize functions
+
+var collection = db.mrMutableReceiver;
+collection.drop();
+collection.insert({a:1});
+
+var map = function() {
+    // set property on receiver
+    this.feed = {beef:1};
+
+    // modify property on receiever
+    this.a = {cake:1};
+    emit(this._id, this.feed);
+    emit(this._id, this.a);
+}
+
+var reduce = function(key, values) {
+    // set property on receiver
+    this.feed = {beat:1};
+
+    // set property on key arg
+    key.fed = {mochi:1};
+
+    // push properties onto values array arg
+    values.push(this.feed);
+    values.push(key.fed);
+
+    // modify each value in the (modified) array arg
+    values.forEach(function(val) { val.mod = 1; });
+    return {food:values};
+}
+
+var finalize = function(key, values) {
+    // set property on receiver
+    this.feed = {ice:1};
+
+    // set property on key arg
+    key.fed = {cream:1};
+
+    // push properties onto values array arg
+    printjson(values);
+    values.food.push(this.feed);
+    values.food.push(key.fed);
+
+    // modify each value in the (modified) array arg
+    values.food.forEach(function(val) { val.mod = 1; });
+    return values;
+}
+
+var mr = collection.mapReduce(map, reduce, {finalize: finalize, out: {inline: 1}});
+printjson(mr);
+
+// verify mutated properties exist (order dictated by emit sequence and properties added)
+assert.eq(mr.results[0].value.food[0].beef, 1);
+assert.eq(mr.results[0].value.food[1].cake, 1);
+assert.eq(mr.results[0].value.food[2].beat, 1);
+assert.eq(mr.results[0].value.food[3].mochi, 1);
+assert.eq(mr.results[0].value.food[4].ice, 1);
+assert.eq(mr.results[0].value.food[5].cream, 1);
+mr.results[0].value.food.forEach(function(val) { assert.eq(val.mod, 1); });