Change build.extend hook to return new webpack config

lib/builder/webpack/client.config.js

@@ -220,10 +220,14 @@ export default function webpackClientConfig () {
 
   // Extend config
   if (typeof this.options.build.extend === 'function') {
-    this.options.build.extend.call(this, config, {
+    const extendedConfig = this.options.build.extend.call(this, config, {
       dev: this.options.dev,
       isClient: true
     })
+    // Only overwrite config when something is returned for backwards compatibility
+    if (extendedConfig !== undefined) {
+      config = extendedConfig
+    }
   }
 
   return config

lib/builder/webpack/server.config.js

@@ -74,10 +74,14 @@ export default function webpackServerConfig () {
 
   // Extend config
   if (typeof this.options.build.extend === 'function') {
-    this.options.build.extend.call(this, config, {
+    const extendedConfig = this.options.build.extend.call(this, config, {
       dev: this.options.dev,
       isServer: true
     })
+    // Only overwrite config when something is returned for backwards compatibility
+    if (extendedConfig !== undefined) {
+      config = extendedConfig
+    }
   }
 
   return config

test/fixtures/with-config/nuxt.config.js

@@ -34,7 +34,9 @@ module.exports = {
       generateStatsFile: true
     },
     extend (config, options) {
-      config.devtool = 'nosources-source-map'
+      return Object.assign({}, config, {
+        devtool: 'nosources-source-map'
+      })
     }
   },
   css: [