gatsby-node.js

const defaultBanner = '// This file is automatically generated. Do not modify this file manually -- YOUR CHANGES WILL BE ERASED!';

/** @type {import('gatsby').GatsbyNode} */
module.exports = {
	onCreateWebpackConfig: ({ getConfig, actions }, options) => {
		/** @type {import('webpack').Configuration} */
		const config = getConfig();

		if (!config.module?.rules) {
			return;
		}

		for (const rule of config.module.rules) {
			if (typeof rule === 'string') {
				return;
			}

			if ('oneOf' in rule && rule.oneOf !== undefined) {
				for (const oneOf of rule.oneOf) {
					injectBeforeCSSLoader(oneOf, options);
				}
			}
			else {
				injectBeforeCSSLoader(rule, options);
			}
		}

		actions.replaceWebpackConfig(config);
	},

	pluginOptionsSchema: ({ Joi }) => Joi.object({
		namedExport: Joi
			.boolean()
			.description('When the option is switched on classes exported as variables.')
			.default(true),

		banner: Joi
			.string()
			.description('Adds a "banner" prefix to each generated file.')
			.default(defaultBanner),

		customTypings: Joi
			.function()
			.description('A function that accepts classes (an array of string) and returns the content of declaration file.')
			.arity(1),

		dropEmptyFile: Joi
			.boolean()
			.description('If there are no classes, the typings file will not be generated, and the existing will be deleted.')
	})
}

/**
 * Place the `dts-css-modules-loader` before the `css-loader` if it exist.
 *
 * @param {import('webpack').RuleSetRule} rule
 * @param {import('gatsby').PluginOptions | undefined} options
 * @returns {void}
 */
function injectBeforeCSSLoader (rule, options) {
	if (!Array.isArray(rule.use)) {
		return;
	}

	const cssLoaderPath = getCSSLoaderPath();

	for (let index = 0; index < rule.use.length; index++) {
		const loader = rule.use[index];

		if (
			(typeof loader === 'string' && loader === cssLoaderPath) ||
			(typeof loader === 'object' && loader.loader === cssLoaderPath && isCSSModulesLoader(loader))
		) {
			rule.use.splice(index, 0, {
				loader: require.resolve('dts-css-modules-loader'),
				options: {
					namedExport: true,
					banner: defaultBanner,
					...options
				}
			});

			return;
		}
	}
}

/**
 * Find the path to "css-loader" from gatsby's point of view.
 *
 * @returns {string}
 */
function getCSSLoaderPath () {
	const resolveDefaultPaths = require.resolve.paths('css-loader') || [];

	return require.resolve('css-loader', { paths: [require.resolve('gatsby'), ...resolveDefaultPaths] })
}

/**
 * Basic detection of whether the CSS Modules option is activated in an object-based `css-loader` configuration.
 *
 * @param {Exclude<import('webpack').RuleSetUseItem, string | Function>} loader
 * @returns
 */
function isCSSModulesLoader (loader) {
	if (typeof loader.options === 'object') {
		return (loader.options.modules !== false);
	}

	return true;
}