packages/react-server-cli/src/compileClient.js
function getWebpackConfig(userWebpackConfigOpt, wpAffectingOpts) {

	let extend = (data) => { return data }
	if (userWebpackConfigOpt) {
		const userWebpackConfigPath = path.resolve(process.cwd(), userWebpackConfigOpt);
		const userWebpackConfigFunc = require(userWebpackConfigPath);
		extend = userWebpackConfigFunc.default;
	}

	const baseConfig = require(path.join(__dirname, "webpack/webpack4.config.fn.js")).default(wpAffectingOpts);
	return extend(baseConfig);
}
function getWebpackConfig(userWebpackConfigOpt, wpAffectingOpts) {

	let extend = (data) => { return data }
	if (userWebpackConfigOpt) {
		const userWebpackConfigPath = path.resolve(process.cwd(), userWebpackConfigOpt);
		const userWebpackConfigFunc = require(userWebpackConfigPath);
		extend = userWebpackConfigFunc.default;
	}

	const baseConfig = require(path.join(__dirname, "webpack/webpack4.config.fn.js")).default(wpAffectingOpts);
	return extend(baseConfig);
}
get the webpack configuration object loads data from default configuration at webpack/webpack4.config.fn.js, and extends it by calling user-supplied function, if one was provided