diff --git a/Gruntfile.js b/Gruntfile.js
index 6639f3f..947c7c6 100644
--- a/Gruntfile.js
+++ b/Gruntfile.js
@@ -67,6 +67,21 @@ module.exports = function (grunt) {
                 files: {
                     'tmp/assignToVar.js': ['test/fixtures/default.js']
                 }
+            },
+            dynamicReplacement: {
+                options: {
+                    vars: {
+                        FOO_BAR: {
+                            key: 'foo.bar',
+                            callback : function(newV, oldV) {
+                                return oldV + ' ' + newV;
+                            }
+                        }
+                    }
+                },
+                files: {
+                    'tmp/dynamicReplacement.js': ['test/fixtures/default.js']
+                }
             }
         },
 
@@ -90,7 +105,7 @@ module.exports = function (grunt) {
 
     // Whenever the "test" task is run, first clean the "tmp" dir, then run this
     // plugin's task(s), then test the result.
-    grunt.registerTask('test', ['env:default', 'clean', 'envtojson', 'nodeunit', ]);
+    grunt.registerTask('test', ['env:default', 'clean', 'envtojson', 'nodeunit', 'clean']);
 
     // By default, lint and run all tests.
     grunt.registerTask('default', ['jshint', 'test']);
diff --git a/tasks/envToJson.js b/tasks/envToJson.js
index 10b9ab6..a3665e2 100644
--- a/tasks/envToJson.js
+++ b/tasks/envToJson.js
@@ -21,7 +21,7 @@ module.exports = function (grunt) {
         // Merge task-specific and/or target-specific options with these defaults.
         options = this.options({
             vars: {},
-            assignToVar: 'module.exports',
+            assignToVar: false,
             whiteSpace: 4
         });
 
@@ -38,43 +38,53 @@ module.exports = function (grunt) {
             }).map(function (filepath) {
                 // Read file source.
                 var modulePath = path.resolve(filepath);
-                var js = require(modulePath);
-                writeFile(f, JSON.stringify(updateValues(js, options.vars), null, (options.whiteSpace)), options.assignToVar);
+                var js = _.clone(require(modulePath), true);
+                writeFile(f.dest, JSON.stringify(updateValues(js, options.vars), null, (options.whiteSpace)), options.assignToVar);
             });
         });
     });
 
     var updateValues = function (js, vars) {
-        _.each(vars, function (path, key) {
+        _.each(vars, function (replace, key) {
             var val = process.env[key];
             if (val) {
-                js = updateValue(js, path.split('.'), val) || js;
+                var keyPath, cb;
+                if (typeof replace === 'string') {
+                    keyPath = replace;
+                    cb = function(newV, oldV) {
+                        return newV;
+                    };
+                } else {
+                    keyPath = replace.key;
+                    cb = replace.callback;
+                }
+                js = updateValue(js, keyPath.split('.'), _.bind(cb, this, val)) || js;
             }
         });
         return js;
     };
 
-    var updateValue = function (object, segments, val) {
+    var updateValue = function (object, segments, cb) {
         if (object && segments.length) {
             var segment = segments.shift();
             if (segments.length === 0) {
-                object[segment] = val;
+                object[segment] = cb(object[segment]);
                 return object;
             } else {
-                object[segment] = updateValue((object[segment] || {}), segments, val);
+                object[segment] = updateValue((object[segment] || {}), segments, cb);
                 return object;
             }
         }
     };
 
-    var writeFile = function(file, content, assignToVar) {
+    var writeFile = function(dest, content, assignToVar) {
         if (assignToVar) {
             content = _.template("<%=assignToVar%>=<%=content%>;")({
                 content: content,
                 assignToVar: assignToVar
             });
         }
-        grunt.file.write(file.dest, content);
+        grunt.file.write(dest, content);
     };
 
 };
diff --git a/test/envToJson_test.js b/test/envToJson_test.js
index 2fca94b..187eb35 100644
--- a/test/envToJson_test.js
+++ b/test/envToJson_test.js
@@ -55,6 +55,16 @@ exports.envtojson = {
         var expected = grunt.file.read('test/expected/noWhiteSpace.js');
         test.equal(actual, expected, 'plain javascript object with altered/created keys no whitespace output');
 
+        test.done();
+    },
+
+    dynamic: function(test) {
+        test.expect(1);
+
+        var actual = grunt.file.read('tmp/dynamicReplacement.js');
+        var expected = grunt.file.read('test/expected/dynamicReplacement.js');
+        test.equal(actual, expected, 'plain javascript object with first key dynamically altered');
+
         test.done();
     }
 };
diff --git a/test/expected/default.js b/test/expected/default.js
index 4097741..47d9825 100644
--- a/test/expected/default.js
+++ b/test/expected/default.js
@@ -1,4 +1,4 @@
-module.exports={
+{
     "foo": {
         "bar": "altered"
     },
@@ -13,4 +13,4 @@ module.exports={
             "stones": "newProp"
         }
     }
-};
\ No newline at end of file
+}
\ No newline at end of file
diff --git a/test/expected/dynamicReplacement.js b/test/expected/dynamicReplacement.js
new file mode 100644
index 0000000..c114b3b
--- /dev/null
+++ b/test/expected/dynamicReplacement.js
@@ -0,0 +1,11 @@
+{
+    "foo": {
+        "bar": "howdy altered"
+    },
+    "spam": {
+        "eggs": {
+            "parrot": "grail"
+        }
+    },
+    "beer": "good"
+}
\ No newline at end of file
diff --git a/test/expected/noWhiteSpace.js b/test/expected/noWhiteSpace.js
index b9b191f..206e0e5 100644
--- a/test/expected/noWhiteSpace.js
+++ b/test/expected/noWhiteSpace.js
@@ -1 +1 @@
-module.exports={"foo":{"bar":"altered"},"spam":{"eggs":{"parrot":"altered"}},"beer":"altered","one":{"hundred":{"stones":"newProp"}}};
\ No newline at end of file
+{"foo":{"bar":"altered"},"spam":{"eggs":{"parrot":"altered"}},"beer":"altered","one":{"hundred":{"stones":"newProp"}}}
\ No newline at end of file