webpack -dev-server 启动服务时，提示unshift未定义，请问为什么？

Question

1、项目目录如下：

2、webpack.config.js配置如下：**

// JavaScript Document
var path = require('path');
var ExtractPlugin = require('extract-text-webpack-plugin');
let webpack = require("webpack");
module.exports = {
    devServer: {
        publicPath: '/',
        inline: true,
        hot: true
    },
    //配置入口
    entry: {
        '/js/common':  './origin/common.js', 
        '/css/main': './scss/main.scss',
        '/css/config': './scss/config.scss'
    },
    //编译后的文件路径
    output: {
        path: path.resolve(__dirname, './'),//文件路径
        filename: '[name].js', //文件名
    },
    module: {
        //编译规则
        loaders: [
            //配置scss编译规则
            {
                test: /\.scss$/,
                loader:ExtractPlugin.extract({fallback: 'style-loader', use: 'css-loader!sass-loader'}) 
            },
            {
                // 让webpack去验证文件是否是.js结尾将其转换
                test: /\.js$/,
                // 通过babel转换
                loader: 'babel-loader',
                // 不用转换的node_modules文件夹
                exclude: /node_modules/,
                query: {
                   'presets': ['es2016', 'stage-0'],
                   'plugins': ['transform-runtime']
                 }
            },
            {
                test: /\.(png|jpg)$/, 
                loader: 'url-loader?limit=8192&name=/images/[name].[ext]'
            }
        ]
    },
    plugins: [
        new ExtractPlugin('[name].css')
    ]
}

3、server.js 配置如下：

// JavaScript Document
var webpack = require("webpack");
var WebpackDevServer = require("webpack-dev-server");
var config = require("./webpack.config.js");

config.entry.app.unshift("webpack-dev-server/client?http://localhost:8099/");
var compiler = webpack(config);
var server = new WebpackDevServer(compiler, {
    publicPath: '/'
});
server.listen(8099);

当启动服务时，报错，请问为什么？我是按官网描述配置的

Answer 1

不是unshift未定义，而是config.entry.app未定义，你自己看看配置，哪来的app