look at the example of vuex, there is a configuration is not clear, in vuex examples server.js a configuration publicPath:"/ _ build__/","/ _ build__/" is in which directory, there is no such directory to run?
configuration:

//server.js


const express = require("express")
const webpack = require("webpack")
const webpackDevMiddleware = require("webpack-dev-middleware")
const webpackHotMiddleware = require("webpack-hot-middleware")
const WebpackConfig = require("./webpack.config")

const app = express()
const compiler = webpack(WebpackConfig)

app.use(webpackDevMiddleware(compiler, {
  publicPath: "/__build__/",//
  stats: {
    colors: true,
    chunks: false
  }
}))

app.use(webpackHotMiddleware(compiler))

app.use(express.static(__dirname))

const port = process.env.PORT || 8080
module.exports = app.listen(port, () => {
  console.log(`Server listening on http://localhost:${port}, Ctrl+C to stop`)
})

vuex source code:
https://github.com/vuejs/vuex.