use webpack-merge

build/webpack.client.config.js

 const webpack = require('webpack')
+const merge = require('webpack-merge')
 const base = require('./webpack.base.config')
 const vueConfig = require('./vue-loader.config')
 const HTMLPlugin = require('html-webpack-plugin')
 const SWPrecachePlugin = require('sw-precache-webpack-plugin')
 
-const config = Object.assign({}, base, {
+const config = merge(base, {
   resolve: {
-    alias: Object.assign({}, base.resolve.alias, {
+    alias: {
       'create-api': './create-api-client.js'
-    })
+    }
   },
-  plugins: (base.plugins || []).concat([
+  plugins: [
     // strip dev-only code in Vue source
     new webpack.DefinePlugin({
       'process.env.NODE_ENV': JSON.stringify(process.env.NODE_ENV || 'development'),
@@ -24,7 +25,7 @@ const config = Object.assign({}, base, {
     new HTMLPlugin({
       template: 'src/index.template.html'
     })
-  ])
+  ]
 })
 
 if (process.env.NODE_ENV === 'production') {

build/webpack.server.config.js

 const webpack = require('webpack')
+const merge = require('webpack-merge')
 const base = require('./webpack.base.config')
 const VueSSRPlugin = require('vue-ssr-webpack-plugin')
 
-module.exports = Object.assign({}, base, {
+module.exports = merge(base, {
   target: 'node',
   entry: './src/entry-server.js',
-  output: Object.assign({}, base.output, {
+  output: {
     filename: 'server-bundle.js',
     libraryTarget: 'commonjs2'
-  }),
+  },
   resolve: {
-    alias: Object.assign({}, base.resolve.alias, {
+    alias: {
       'create-api': './create-api-server.js'
-    })
+    }
   },
   externals: Object.keys(require('../package.json').dependencies),
   plugins: [

package.json

@@ -28,7 +28,8 @@
     "vue-server-renderer": "^2.2.4",
     "vue-style-loader": "^2.0.4",
     "vuex": "^2.2.1",
-    "vuex-router-sync": "^4.1.2"
+    "vuex-router-sync": "^4.1.2",
+    "webpack-merge": "^4.0.0"
   },
   "devDependencies": {
     "autoprefixer": "^6.7.7",

yarn.lock

@@ -1913,7 +1913,7 @@ loader-runner@^2.3.0:
   version "2.3.0"
   resolved "https://registry.yarnpkg.com/loader-runner/-/loader-runner-2.3.0.tgz#f482aea82d543e07921700d5a46ef26fdac6b8a2"
 
-loader-utils@^0.2.15, loader-utils@^0.2.16, loader-utils@^0.2.7:
+loader-utils@^0.2.15, loader-utils@^0.2.16:
   version "0.2.16"
   resolved "https://registry.yarnpkg.com/loader-utils/-/loader-utils-0.2.16.tgz#f08632066ed8282835dff88dfb52704765adee6d"
   dependencies:
@@ -1975,7 +1975,7 @@ lodash.uniq@^4.3.0:
   version "4.5.0"
   resolved "https://registry.yarnpkg.com/lodash.uniq/-/lodash.uniq-4.5.0.tgz#d0225373aeb652adc1bc82e4945339a842754773"
 
-lodash@^4.14.0, lodash@^4.17.3:
+lodash@^4.14.0, lodash@^4.17.3, lodash@^4.17.4:
   version "4.17.4"
   resolved "https://registry.yarnpkg.com/lodash/-/lodash-4.17.4.tgz#78203a4d1c328ae1d86dca6460e369b57f4055ae"
 
@@ -3619,14 +3619,7 @@ vue-ssr-webpack-plugin@^1.0.2:
   dependencies:
     chalk "^1.1.3"
 
-vue-style-loader@^2.0.0:
-  version "2.0.0"
-  resolved "https://registry.yarnpkg.com/vue-style-loader/-/vue-style-loader-2.0.0.tgz#1a3bb55239ac541ee3af0301d66f16fc86786543"
-  dependencies:
-    hash-sum "^1.0.2"
-    loader-utils "^0.2.7"
-
-vue-style-loader@^2.0.4:
+vue-style-loader@^2.0.0, vue-style-loader@^2.0.4:
   version "2.0.4"
   resolved "https://registry.yarnpkg.com/vue-style-loader/-/vue-style-loader-2.0.4.tgz#4fc19e18bf162225338aac3b17d0fd0e1cfb7439"
   dependencies:
@@ -3682,6 +3675,12 @@ webpack-hot-middleware@^2.17.1:
     querystring "^0.2.0"
     strip-ansi "^3.0.0"
 
+webpack-merge@^4.0.0:
+  version "4.0.0"
+  resolved "https://registry.yarnpkg.com/webpack-merge/-/webpack-merge-4.0.0.tgz#4347ebeb9e71aaa413baef66be74d4eb2656b66a"
+  dependencies:
+    lodash "^4.17.4"
+
 webpack-sources@^0.1.4:
   version "0.1.4"
   resolved "https://registry.yarnpkg.com/webpack-sources/-/webpack-sources-0.1.4.tgz#ccc2c817e08e5fa393239412690bb481821393cd"