fix(merge): Clone arrays correctly if only one exists

Closes #106.

src/join-arrays.js

@@ -1,5 +1,5 @@
 import {
-  cloneDeep, isFunction, isPlainObject, mergeWith
+  clone, cloneDeep, isFunction, isPlainObject, mergeWith
 } from 'lodash';
 
 const isArray = Array.isArray;
@@ -35,6 +35,10 @@ export default function joinArrays({
       return cloneDeep(b);
     }
 
+    if (isArray(b)) {
+      return [...b];
+    }
+
     return b;
   };
 }

tests/test-merge.js

@@ -291,6 +291,47 @@ function customizeMergeTests(merge) {
 
     assert.equal(receivedKey, 'plugins');
   });
+
+  it('should not mutate plugins #106', function () {
+    const config1 = {
+      entry: {
+        page1: 'src/page1',
+        page2: 'src/page2'
+      },
+      output: {
+        path: 'dist',
+        publicPath: '/'
+      }
+    };
+    const config2 = {
+      entry: {
+        page3: 'src/page3',
+        page4: 'src/page4'
+      },
+      output: {
+        path: 'dist',
+        publicPath: '/'
+      }
+    };
+    const enhance = {
+      plugins: [
+        new webpack.IgnorePlugin(/^\.\/locale$/, /moment$/)
+      ]
+    };
+
+    const result1 = merge(config1, enhance);
+    const result2 = merge(config2, enhance);
+
+    assert.equal(result1.plugins.length, 1);
+    assert.equal(result2.plugins.length, 1);
+
+    result1.plugins.push(
+      new webpack.HotModuleReplacementPlugin()
+    );
+
+    assert.equal(result1.plugins.length, 2);
+    assert.equal(result2.plugins.length, 1);
+  });
 }
 
 module.exports = normalMergeTests;