Q:

Concat and minify all less files with Webpack without importing them

I have a folder of around 20 separate less files that I need to concatenate into a single file via Webpack and store this in my /dist folder. My current Webpack config file is as follows: 
const path = require('path');
const webpack = require('webpack');
const ExtractTextPlugin = require('extract-text-webpack-plugin');
const CheckerPlugin = require('awesome-typescript-loader').CheckerPlugin;
const bundleOutputDir = './wwwroot/dist';

module.exports = (env) => {
    const isDevBuild = !(env && env.prod);
    return [{
        stats: { modules: false },
        entry: { 'main': './ClientApp/boot.ts' },
        resolve: { extensions: ['.js', '.ts'] },
        output: {
            path: path.join(__dirname, bundleOutputDir),
            filename: '[name].js',
            publicPath: '/dist/'
        },
        module: {
            rules: [
                { test: /\.ts$/, include: /ClientApp/, use: 'awesome-typescript-loader?silent=true' },
                { test: /\.html$/, use: 'raw-loader' },
                { test: /\.css$/, use: isDevBuild ? ['style-loader', 'css-loader'] : ExtractTextPlugin.extract({ use: 'css-loader' }) },
                { test: /\.less/, use: ExtractTextPlugin.extract('style-loader', 'css-loader!less-loader') },
                { test: /\.(png|jpg|jpeg|gif|svg)$/, use: 'url-loader?limit=25000' }
            ]
        },
        plugins: [
            new CheckerPlugin(),
            new webpack.DllReferencePlugin({
                context: __dirname,
                manifest: require('./wwwroot/dist/vendor-manifest.json')
            })
        ].concat(isDevBuild ? [
            // Plugins that apply in development builds only
            new webpack.SourceMapDevToolPlugin({
                filename: '[file].map', // Remove this line if you prefer inline source maps
                moduleFilenameTemplate: path.relative(bundleOutputDir, '[resourcePath]') // Point sourcemap entries to the original file locations on disk
            })
        ] : [
                // Plugins that apply in production builds only
                new webpack.optimize.UglifyJsPlugin(),
                new ExtractTextPlugin('site.less'),
                new ExtractTextPlugin('site.css')
            ])
    }];
};

If I try and import each single .less file into the boot.ts entry file, I get a less error stating that the less variables that I've declared are not being recognised, which is how I came to the conclusion that I need to concat these files beforehand. I come from a gulp background, so any help to get me up and running with this would be greatly appreciated. 
If there is an alternative way to get all less compiled to css and working correctly, without the need for concat, then I'm open to suggestions.

A:

Webpack is a module bundler and uses the module syntax for JavaScript (ES6, CommonJS, AMD..), CSS (@import, url) and even HTML (through src attribute) to build the app's dependency graph and then serialize it in several bundles.
In your case, when you import the *.less files the errors are because you miss CSS modules. In other words, on the places where you have used variables defined in other file, that file was not @import-ed.
With Webpack it's recommended to modularize everything, therefore I would recommend to add the missing CSS modules. I had the same issue when I was migrating a project from Grunt to Webpack. Other temporary solution is to create an index.less file where you will @import all the less files (note: the order is important) and then import that file in app's entry file (ex. boot.ts).

