change export object of vue.config.js to a function

vue.config.js

@@ -16,7 +16,8 @@ const name = defaultSettings.title || 'vue Element Admin' // page title
 const port = process.env.port || process.env.npm_config_port || 9527 // dev port
 
 // All configuration item explanations can be find in https://cli.vuejs.org/config/
-module.exports = {
+module.exports = () => {
+  return {
     /**
      * You will need to set publicPath if you plan to deploy your site under a sub path,
      * for example GitHub Pages. If you plan to deploy your site to https://foo.github.io/bar/,
@@ -36,7 +37,7 @@ module.exports = {
         warnings: false,
         errors: true
       },
-    before: require('./mock/mock-server.js')
+      before: require('./mock/mock-server.js').default
     },
     configureWebpack: {
       // provide the app's title in webpack's name field, so that
@@ -125,4 +126,5 @@ module.exports = {
           }
         )
     }
+  }
 }