index.js

export default (config, options) => {
	// find any first matching rule that contains postcss-loader
	filterPostCSSLoader(config.module.rules).forEach(rule => {
		filterPostCSSLoader(rule.oneOf).forEach(oneOf => {
			filterPostCSSLoader(oneOf.use || oneOf.loader).forEach(use => {
				// use the latest version of postcss-loader
				use.loader = require.resolve('postcss-loader');

				// conditionally replace options with a custom configuration
				use.options = options
					? Object.assign({ ident: 'postcss' }, options)
				: Object.assign(use.options, options)
			})
		})
	});

	// return the mutated configuration
	return config;
};

// return a filtered array that includes postcss-loader
const filterPostCSSLoader = array => array.filter(object => JSON.stringify(object).includes('postcss-loader'));