rollup.config.js

import svelte from "rollup-plugin-svelte";
import commonjs from "@rollup/plugin-commonjs";
import resolve from "@rollup/plugin-node-resolve";
import livereload from "rollup-plugin-livereload";
import { terser } from "rollup-plugin-terser";
import sveltePreprocess from 'svelte-preprocess'
import { less } from "svelte-preprocess";
import css from "rollup-plugin-css-only";

const production = !process.env.ROLLUP_WATCH;

function serve() {
    let server;

    function toExit() {
        if (server) server.kill(0);
    }

    return {
        writeBundle() {
            if (server) return;
            server = require("child_process").spawn(
                "npm", ["run", "start", "--", "--dev"], {
                    stdio: ["ignore", "inherit", "inherit"],
                    shell: true,
                }
            );

            process.on("SIGTERM", toExit);
            process.on("exit", toExit);
        },
    };
}

export default [{
        input: "src/scripts/main.js",
        output: {
            sourcemap: true,
            format: "iife",
            name: "app",
            file: "public/build/bundle.js",
        },

        plugins: [
            svelte({
                compilerOptions: {
                    // enable run-time checks when not in production
                    dev: production,
                },
                preprocess: sveltePreprocess({})
            }),
            // we'll extract any component CSS out into
            // a separate file - better for performance
            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(),

            // In dev mode, call `npm run start` once
            // the bundle has been generated
            !production && serve(),

            // Watch the `public` directory and refresh the
            // browser on changes when not in production
            !production && livereload("public"),

            // If we're building for production (npm run build
            // instead of npm run dev), minify
            production && terser(),
        ],
        watch: {
            clearScreen: false,
        },
    },
    {
        input: "src/scripts/injection.js",
        output: {
            sourcemap: false,
            format: "iife",
            file: "public/build/injection.js",
        },
        plugins: [resolve(), commonjs()],
        watch: {
            clearScreen: false,
        },
    },
    {
        input: "src/scripts/update.js",
        output: {
            sourcemap: false,
            format: "iife",
            file: "public/build/update.js",
        },
        plugins: [resolve(), commonjs()],
        watch: {
            clearScreen: false,
        },
    },
    {
        input: "src/scripts/background.js",
        output: {
            sourcemap: false,
            format: "iife",
            file: "public/build/background.js",
        },
        plugins: [resolve(), commonjs()],
        watch: {
            clearScreen: false,
        },
    },
];