sapper-template/webpack/client.config.js

const webpack = require('webpack');
const config = require('sapper/config/webpack.js');

const mode = process.env.NODE_ENV;
const dev = mode === 'development';

module.exports = {
	entry: config.client.entry(),
	output: config.client.output(),
	resolve: {
		extensions: ['.js', '.json', '.html'],
		mainFields: ['svelte', 'module', 'browser', 'main']
	},
	module: {
		rules: [
			{
				test: /\.html$/,
				use: {
					loader: 'svelte-loader',
					options: {
						dev,
						hydratable: true,
						hotReload: true
					}
				}
			}
		]
	},
	mode,
	plugins: [
		dev && new webpack.HotModuleReplacementPlugin(),
		new webpack.DefinePlugin({
			'process.browser': true,
			'process.env.NODE_ENV': JSON.stringify(mode)
		}),
	].filter(Boolean),
	devtool: dev && 'inline-source-map'
};