Webpack config not accepting to config mode option

**getting error when trying to add mode option to the webpack config i need to configure {mode:'developement' } to enable hmp by looking at this answer github.com/webpack-contrib/webpack-hot-middleware/issues/… **

WebpackOptionsValidationError: Invalid configuration object. Webpack has been initialised using a configuration object that does not match the API schema. - configuration has an unknown property 'mode'. These properties are valid: object { amd?, bail?, cache?, context?, dependencies?, devServer?, devtool?, entry, externals?, loader?, module?, name?, node?, output?, performance?, plugins?, profile?, recordsInputPath?, recordsOutputPath?, recordsPath?, resolve?, resolveLoader?, stats?, target?, watch?, watchOptions? } For typos: please correct them. For loader options: webpack 2 no longer allows custom properties in configuration. Loaders should be updated to allow passing options via loader options in module.rules. Until loaders are updated one can use the LoaderOptionsPlugin to pass these options to the loader: plugins: [ new webpack.LoaderOptionsPlugin({ // test: /.xxx$/, // may apply this only for some modules options: { mode: ... } }) ] at webpack (C:\Users\asdf\WebstormProjects\node_modules\webpack\lib\webpack.js:19:9) at Object. ( at Module._compile (internal/modules/cjs/loader.js:689:30) at Object.Module._extensions..js (internal/modules/cjs/loader.js:700:10) at Module.load (internal/modules/cjs/loader.js:599:32) at tryModuleLoad (internal/modules/cjs/loader.js:538:12) at Function.Module._load (internal/modules/cjs/loader.js:530:3) at Function.Module.runMain (internal/modules/cjs/loader.js:742:12) at startup (internal/bootstrap/node.js:282:19) at bootstrapNodeJSCore (internal/bootstrap/node.js:743:3)

    /* eslint-disable */
const path = require('path');
const webpack = require('webpack');
const webpackMerge = require('webpack-merge');
const HtmlWebpackPlugin = require('html-webpack-plugin');

const commonConfig = require('./webpack.config.common');

module.exports = webpackMerge(
  commonConfig,
  {
    devtool: 'cheap-module-eval-source-map',
    entry: {
      main: ['babel-polyfill', 'webpack-hot-middleware/client', './app/index.js'],
    },
    output: {
      path: __dirname,
      publicPath: '/',
      filename: '[hash].bundle.js',
    },
    module: {
      rules: [
        {
          test: /\.mspcss/,
          use: [
            'style-loader',
            'css-loader?modules=true&importLoaders=1&localIdentName=[local]___[hash:base64:5]',
            'resolve-url-loader',
            'sass-loader?sourceMap'
          ]
        },
        {
          test: /\.scss$/,
          use: ['style-loader', 'css-loader', 'sass-loader?sourceMap']
        },
        {
          test: /\.css$/,
          use: ['style-loader', 'css-loader']
        },
      ],
    },
    plugins: [
      new webpack.DefinePlugin({
        'process.env': {
          NODE_ENV: JSON.stringify('development'),
          BABEL_ENV: JSON.stringify('development'),
        },
        __DEV__: true,
      }),
      new webpack.HotModuleReplacementPlugin(),
      new webpack.optimize.OccurrenceOrderPlugin(),
      new webpack.NoErrorsPlugin(),
      new HtmlWebpackPlugin({
        title: 'some- Development',
        template: path.resolve(__dirname, 'index.ejs'),
        filename: path.resolve(__dirname, 'index.html'),
        favicon: 'favicon.ico',
        inject: 'body'
      }),

    ]
  }
)
/* eslint-enable */

What version of webpack are you using? You are probably on version 2 or 3 and the latest version of webpack-dev-server (3.2.1) targets webpack 4. I had the same issue and fixed it by installing webpack-dev-server version 2.11.5

npm uninstall webpack-dev-server
npm i -D webpack-dev-server@2.11.5