Webpack4にバージョンアップし、ファイルの圧縮がうまくいかない

以前の3からWebpack4にバージョンアップし、
それまでファイルの圧縮に使用してきた「uglifyjs-webpack-plugin」に変わり、
「terser-webpack-plugin」を導入してうまくファイル更新の後コンパイルできるのですが、
今までできていたファイルの中身（ソースコードを一行に）というのができなくなっております。
以下が設定したwebpack.config.jsファイルです。

webpack.config.js

/* ----------------------------------------------------------------------------------
外部ファイル読み込み
---------------------------------------------------------------------------------- */
const path = require('path');
const TerserPlugin = require('../../node_modules/terser-webpack-plugin');
//const UglifyJsPlugin = require('../../node_modules/uglifyjs-webpack-plugin');


/* ----------------------------------------------------------------------------------
変数定義
---------------------------------------------------------------------------------- */
var webpack = require('../../node_modules/webpack');
var jsRootDir = './src/js/';


/* ----------------------------------------------------------------------------------
コンパイルの仕方を定義
---------------------------------------------------------------------------------- */
module.exports = {
    mode: 'development',
    // インポートファイル（ここに新規ファイルを追加していく）
    entry: {
        main: jsRootDir + 'main.js', // 共通
        component: jsRootDir + 'jsx/component.jsx' // JSXページ
    },
    // ビルド実行して実際に出力されるファイル
    output: {
        path: path.join(__dirname, 'dist'),
        publicPath: './public/js/',
        filename: "[name].js"
    },
    module: {
        rules: [
            {
                test: /.(js|jsx)$/,
                loader: 'jsx-loader',
                exclude: /node_modules/
            }
        ]
    },
    resolve: {
        extensions: ['.js', '.jsx']
    },
    externals: {
        jQuery: 'jQuery'
    },
    node: {
        setImmediate: false
    },

    optimization: {
        minimizer: [
            new TerserPlugin({
                parallel: true,
                cache: true,
                terserOptions: {
                    ie8: false,
                    // sourceMap: true,
                    compress: {
                        dead_code: true,
                        warnings: false,
                        properties: true,
                        drop_debugger: true,
                        conditionals: true,
                        booleans: true,
                        loops: true,
                        unused: true,
                        toplevel: true,
                        if_return: true,
                        inline: true,
                        join_vars: true
                    },
                    output: {
                        comments: false,
                        beautify: false,
                        indent_level: 2
                    }
                }
            })
        ]
    },

    plugins: [
        new webpack.optimize.OccurrenceOrderPlugin(),
        new webpack.optimize.AggressiveMergingPlugin(),
        new webpack.ProvidePlugin({
            $: 'jquery',
            jQuery: 'jquery'
        }),
    /*
        new webpack.optimize.UglifyJsPlugin({
            compress: {
                screw_ie8: false,
                dead_code: true,
                warnings: false
            },
            comments: false,
            mangle: false,
            sourceMap: true
        }),
    */
        new webpack.HotModuleReplacementPlugin()
    ],

    devtool: 'source-map'
};

package.json

{
  "name": "template",
  "version": "0.0.0",
  "description": "template",
  "main": "gulpfile.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "start": "webpack-dev-server --mode development --open"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "babel-cli": "^6.26.0",
    "babel-core": "^6.26.3",
    "babel-preset-env": "^1.7.0",
    "babel-preset-es2015": "^6.24.1",
    "babel-preset-react": "^6.24.1",
    "browser-sync": "^2.18.2",
    "css-loader": "^1.0.0",
    "css-mqpacker": "^6.0.2",
    "gulp": "^3.9.1",
    "gulp-autoprefixer": "^3.1.1",
    "gulp-cached": "^1.1.1",
    "gulp-changed": "^1.3.2",
    "gulp-clean-css": "^3.7.0",
    "gulp-combine-media-queries": "^0.2.0",
    "gulp-compass": "^2.1.0",
    "gulp-concat": "^2.6.1",
    "gulp-connect-php": "0.0.8",
    "gulp-csscomb": "^3.0.8",
    "gulp-cssmin": "^0.1.7",
    "gulp-csso": "^3.0.1",
    "gulp-ejs": "^3.1.1",
    "gulp-gzip": "^1.4.1",
    "gulp-htmlhint": "^0.3.1",
    "gulp-imagemin": "^3.1.1",
    "gulp-notify": "^2.2.0",
    "gulp-plumber": "^1.1.0",
    "gulp-postcss": "^7.0.1",
    "gulp-rename": "^1.2.2",
    "gulp-sass": "^3.0.0",
    "gulp-sftp": "^0.1.5",
    "gulp-uglify": "^2.0.0",
    "gulp-util": "^3.0.8",
    "gulp-watch": "^4.3.11",
    "gulp-webpack": "^1.5.0",
    "imagemin-pngquant": "^5.0.0",
    "jquery": "^3.3.1",
    "jsx-loader": "^0.13.2",
    "postcss-cssnext": "^3.1.0",
    "react": "^16.5.2",
    "react-dom": "^16.5.2",
    "rimraf": "^2.6.1",
    "run-sequence": "^1.2.2",
    "style-loader": "^0.23.0",
    "vinyl-ftp": "^0.5.0",
    "webpack": "^4.20.2",
    "webpack-cli": "^3.1.2",
    "webpack-stream": "^4.0.0"
  },
  "dependencies": {
    "@fancyapps/fancybox": "^3.5.1",
    "ajv": "^6.5.4",
    "animejs": "^2.2.0",
    "del": "^2.2.2",
    "imagemin-pngquant": "^5.0.0",
    "slick-carousel": "^1.8.1",
    "terser": "^3.9.3",
    "terser-webpack-plugin": "^1.1.0"
  }
}

ご教授のほどよろしくお願いいたします。