It's not possible to modify the default configuration.

You can use the optimization.minimizer setting to instantiate your own UglifyJsPlugin, however. Using 4.0 we used this example to get source maps even when mode is set to 'production' for example (no longer necessary as of 4.1.1):

const UglifyJsPlugin = require('uglifyjs-webpack-plugin');

module.exports = {
  optimization: {
    minimizer: [
      // we specify a custom UglifyJsPlugin here to get source maps in production
      new UglifyJsPlugin({
        cache: true,
        parallel: true,
        uglifyOptions: {
          compress: false,
          ecma: 6,
          mangle: true
        },
        sourceMap: true
      })
    ]
  }
};

You can use this:

const UglifyJsPlugin = require('uglifyjs-webpack-plugin')

...

optimization: {
  minimizer: [
    new UglifyJsPlugin({
      uglifyOptions: { ... }
    })
  ]
}

And some more options:

module.exports = {
  // ...
  plugins: [
    // ... other plugins
    optimization: {
    minimizer: [
        //https://github.com/mishoo/UglifyJS2/tree/harmony
        new UglifyJsPlugin({
            uglifyOptions: {
                output: {
                    comments: false
                },
                minify: {},
                compress: {
                    booleans: true,
                    //...
                }
            }
        }),
    ]
  },
  ]
  // ...
}

more discussion: https://github.com/webpack-contrib/uglifyjs-webpack-plugin/issues/234#issuecomment-369134047

similar question in stackoverflow: https://stackoverflow.com/a/49059746/6200607

In Webpack docs: https://webpack.js.org/plugins/uglifyjs-webpack-plugin/

** Edit: replaced UglifyJSPlugin with UglifyJsPlugin **

webpack.config.js:

const webpack = require("webpack");

module.exports = {
  entry: {
    "bundle": "./entry.js",
    "bundle.min": "./entry.js",
  },
  devtool: "source-map",
  output: {
    path: "./dist",
    filename: "[name].js"
  },
  plugins: [
    new webpack.optimize.UglifyJsPlugin({
      include: /\.min\.js$/,
      minimize: true
    })
  ]
};

Since Webpack 4, webpack.optimize.UglifyJsPlugin has been deprecated and its use results in error:

webpack.optimize.UglifyJsPlugin has been removed, please use config.optimization.minimize instead

As the manual explains, the plugin can be replaced with minimize option. Custom configuration can be provided to the plugin by specifying UglifyJsPlugin instance:

const webpack = require("webpack");
const UglifyJsPlugin = require('uglifyjs-webpack-plugin');

module.exports = {
  // ...
  optimization: {
    minimize: true,
    minimizer: [new UglifyJsPlugin({
      include: /\.min\.js$/
    })]
  }
};

This does the job for a simple setup. A more effective solution is to use Gulp together with Webpack and do the same thing in one pass.

https://medium.com/webpack/webpack-4-mode-and-optimization-5423a6bc597a

If you look at the url it will explain all optimization options.

By default in dev mode webpack 4 won't minimize js, this is to speed up development. As soon as you switch mode to production or use the -p while running webpack it will automatically minimize your JS there is no need for uglifyjs setup anymore in webpack 4.

I use webpack4 on production and have to use UglifyJsPlugin as well.

First of all I would ensure that you have proper version of webpack and UglifyJsPlugin in your package.json I currently have "webpack": "4.20.2", and "uglifyjs-webpack-plugin": "2.0.1", To ensure that they are properly installed I would advise to double check that proper versions are installed by running: rm -rf node_modules && npm install OR rm -rf node_modules && yarn install whatever works for you.

Next I would check the config. My webpack.production.js which works is the following

 // ...

 const UglifyJsPlugin = require('uglifyjs-webpack-plugin');

 // ...

     mode: 'production',
     optimization: {
        minimizer: [
            new UglifyJsPlugin({
                cache: true,
                parallel: true,
                sourceMap: false, // set to true if you want JS source maps
            }),
        ],
    },
    module: {
        //...
   },

Ok, after a bit of reading/searching it turned out Webpack > 4 doesn't require the UglifyJsPlugin so simply removing references from both webpack.config and webpack.config.vendor files fixed the issue.

You're still putting it in config.plugins, have you tried putting it in config.optimization.minimizer?

const UglifyJSPlugin = require('uglifyjs-webpack-plugin')

...

optimization: {
  minimizer: [
    new UglifyJSPlugin({
      uglifyOptions: {
        compress: {
          drop_console: true,
        }
      }
    })
  ]
}

As you may notice that the plugin uglifyjs-webpack-plugin is being deprecated and in the terser-webpack-plugin comes in as replacement. So UglifyJsPlugin plugin is likely unavailable in webpack.optimize. So here is a possible way to fix your issue:

• Just remove the following line in your config file:

new webpack.optimize.UglifyJsPlugin({ sourceMap: true })
// Remove this ^

• and add the plugin to optimizer:

const TerserPlugin = require("terser-webpack-plugin");

module.exports = {
  // ...
  optimization: {
    minimize: true,
    minimizer: [new TerserPlugin()],
  },
  // ...
};

Regarding NamedModulesPlugin is not a constructor, it's also be deprecated you can find out here if you're using webpack 5. Basically you can remove that plugin and replace it with optimization option:

module.exports = {
  //...
  // NamedModulesPlugin → optimization.moduleIds: 'named'
  optimization: {
    moduleIds: 'named',
  },
};