rollup.config.js

import svelte from 'rollup-plugin-svelte';
import resolve from '@rollup/plugin-node-resolve';
import commonjs from '@rollup/plugin-commonjs';
import pkg from './package.json';
import css from 'rollup-plugin-css-only';

const production = !process.env.ROLLUP_WATCH;

const external = [
	...Object.keys(pkg.devDependencies || {}),
	...Object.keys(pkg.peerDependencies || {}),
	...Object.keys(pkg.dependencies || {})
];

export default {
    input: 'src/index.js',
		output: [
			{ file: pkg.main, format: 'cjs' },
			{ file: pkg.module, format: 'es' }
		],
    external,
    plugins: [
        svelte({
            compilerOptions: {
                dev: !production,
            },
        }),
        css({ output: 'bundle.css' }),

        // If you have external dependencies installed from
        // npm, you'll most likely need these plugins. In
        // some cases you'll need additional configuration -
        // consult the documentation for details:
        // https://github.com/rollup/plugins/tree/master/packages/commonjs
        resolve({
            browser: true,
            dedupe: ['svelte'],
        }),
        commonjs({
            include: 'node_modules/**',
        })
    ],
};