rollup.config.js

import copy     from  'rollup-plugin-copy'
import svelte   from  'rollup-plugin-svelte'
import resolve  from  'rollup-plugin-node-resolve'

const build = process.env.BUILD ? process.env.BUILD : false
const dev   = !!process.env.ROLLUP_WATCH;
const tasks = []

if (!build || build === 'package') {
    tasks.push({
        input: 'src/index.js',
        output: [
            {
                file: 'index.mjs',
                format: 'esm'
            },
            {
                file: 'index.js',
                format: 'cjs'
            }
        ],
        plugins: [
            svelte({
                compilerOptions: {
                    // enable run-time checks when not in production
                    dev: dev
                }
            }),
            resolve(),
            copy({
                targets: [
                    {
                        dest: './',
                        src: ['src/*', '!src/index.js']
                    }
                ]
            })
        ]
    })
}

export default tasks