React Express Proxy

Question

Reactでpostを行い、Expressでそれを受け取るところでcorsの対策でプロキシを設定しようとしました。
localhost:8080/api/userをlocalhost:3000/api/userで受け取る
下記の内容だとエラーが出ないのですがコメントアウトしているところを足すとエラーが発生します。
改善策をご存知でしたら、よろしくお願いします。

const merge = require("webpack-merge");
const webpackConfig = require("./webpack.config.js");

module.exports = merge(webpackConfig, {
  mode: "development",
  contentBase: "http://localhost",
  //下記に二つをたすとエラーが発生
  //port: 8080,
  //devServer: {
    proxy: {
      "/api": {
        target: "http://localhost:3000",
      },
    },
  },
});

 Example app listening at http://:::3000
[0] ✖ ｢wds｣: Invalid configuration object. Webpack has been initialised using a configuration object that does not match the API schema.
[0]  - configuration has an unknown property 'port'. These properties are valid:
[0]    object { amd?, bail?, cache?, context?, dependencies?, devServer?, devtool?, entry?, externals?, infrastructureLogging?, loader?, mode?, module?, name?, node?, optimization?, output?, parallelism?, performance?, plugins?, profile?, recordsInputPath?, recordsOutputPath?, recordsPath?, resolve?, resolveLoader?, serve?, stats?, target?, watch?, watchOptions? }
[0]    For typos: please correct them.
[0]    For loader options: webpack >= v2.0.0 no longer allows custom properties in configuration.
[0]      Loaders should be updated to allow passing options via loader options in module.rules.
[0]      Until loaders are updated one can use the LoaderOptionsPlugin to pass these options to the loader:
[0]      plugins: [
[0]        new webpack.LoaderOptionsPlugin({
[0]          // test: /.xxx$/, // may apply this only for some modules
[0]          options: {
[0]            port: ...
[0]          }
[0]        })
[0]      ]
[0] npm ERR! code ELIFECYCLE
[0] npm ERR! errno 1
[0] npm ERR! react-hogehoge-heroku@1.0.0 client: `webpack-dev-server --config ./config/webpack.config.dev.js --open --mode development`
[0] npm ERR! Exit status 1
[0] npm ERR!
[0] npm ERR! Failed at the react-hogehoge-heroku@1.0.0 client script.
[0] npm ERR! This is probably not a problem with npm. There is likely additional logging output above.