sapper-template/rollup/client.config.js

import resolve from 'rollup-plugin-node-resolve';
import replace from 'rollup-plugin-replace';
import commonjs from 'rollup-plugin-commonjs';
import svelte from 'rollup-plugin-svelte';
import babel from 'rollup-plugin-babel';
import { terser } from 'rollup-plugin-terser';
import config from 'sapper/config/rollup.js';

const mode = process.env.NODE_ENV;
const dev = mode === 'development';
const legacy = !!process.env.SAPPER_LEGACY_BUILD;

export default {
	input: config.client.input(),
	output: config.client.output(),
	plugins: [
		resolve(),
		replace({
			'process.browser': true,
			'process.env.NODE_ENV': JSON.stringify(mode)
		}),
		commonjs(),
		svelte({
			dev,
			hydratable: true
		}),

		legacy && babel({
			extensions: ['.js', '.html'],
			include: [
				'app/**',
				'routes/**'
			],
			runtimeHelpers: true
		}),

		!dev && terser({
			module: true
		})
	],
	experimentalCodeSplitting: true
};