rollup-plugin.js

const routesFilesPlugin = {
	name: 'route-files',
	buildStart: async (options) => {

		console.log('Generating routeModules.js');

		const routesDir = path.resolve(__dirname, 'src/components/routes');
		const routeFiles = fs.readdirSync(routesDir);

		const routesString = routeFiles.map((filename) => {
			const name = filename.replace('.svelte', '');
			return `
				{
					name: '${name}',
					async load () {
						if (loadedModules['${name}']) return loadedModules['${name}'];
						const imported = await import('routes/${filename}');
						const module = imported.default;
						loadedModules[name] = module;
						return module;
					}
				}
			`;
		});

		const script = `
			// FILE GENERATED DYNAMICALLY BY ROLLUP!!!

			const loadedModules = {};

			const routeLoaders = [${routesString}];

			export function getRouteLoader (name) {
				const loader = routeLoaders.find((route) => route.name === name);
				if (!loader) throw 'Route does not exist! ' + name;
				return loader;
			}
		`;

		fs.writeFileSync('src/router/routeModules.js', script, 'utf-8');
	}
};