fix(merge) - Merge should not mutate inputs with mismatches keys

It retained a reference if you passed an object that didn't end up
matching a rule within the logic. Now it checks for that case and clones
instead so you get a new object.

CHANGELOG.md

@@ -1,3 +1,8 @@
+1.0.2 / 2016-11-29
+==================
+
+  * Bug fix - `merge` should not mutate inputs with mismatched keys.
+
 1.0.0 / 2016-11-28
 ==================
 

package.json

@@ -15,6 +15,7 @@
     "lib"
   ],
   "dependencies": {
+    "lodash.clonedeep": "^4.5.0",
     "lodash.differencewith": "^4.5.0",
     "lodash.isequal": "^4.4.0",
     "lodash.isfunction": "^3.0.8",

src/join-arrays.js

@@ -1,3 +1,4 @@
+const cloneDeep = require('lodash.clonedeep');
 const isFunction = require('lodash.isfunction');
 const isPlainObject = require('lodash.isplainobject');
 const mergeWith = require('lodash.mergewith');
@@ -30,6 +31,10 @@ module.exports = function joinArrays({
       }));
     }
 
+    if (isPlainObject(b)) {
+      return cloneDeep(b);
+    }
+
     return b;
   };
 };

tests/merge-tests.js

@@ -214,6 +214,21 @@ function mergeTests(merge) {
 
     assert.deepEqual(merge(a, b).postcss('a'), expected);
   });
+
+  it('should not mutate inputs with mismatched keys', function () {
+    const a = {
+      entry: {}
+    };
+
+    const b = {};
+
+    const aClone = JSON.parse(JSON.stringify(a));
+    const config = merge({}, a, b);
+
+    config.entry.main = 'src/index.js';
+
+    assert.deepEqual(a, aClone);
+  });
 }
 
 module.exports = mergeTests;