vite.config.js

import { defineConfig } from 'vite'
import { svelte } from '@sveltejs/vite-plugin-svelte'
import path from 'path'
import fs from 'fs'
// https://vitejs.dev/config/

const FILE_NAME = "main.user.js";
const headerText = fs.readFileSync("./src/header.js").toString()
export default ({mode}) => {
  return defineConfig({
    plugins: [
      svelte(),
      header(headerText, mode==="dev")
    ],
    build: {
      outDir: "dist",
      cssCodeSplit: true,
      minify: false,
      lib: {
        entry: "src/main.js",
        formats: ["iife"],
        fileName: () => {
          return FILE_NAME;
        },
        name: FILE_NAME
      },
      watch: mode === "dev"
    }
  })
}

function header(text, dev=true) {
  return {
    name: "vite-plugin-header",
    generateBundle(OutputOptions, ChunkInfo) {
      let filename =  String(OutputOptions.name).replace("dist/", "");
      let newCode = text + "\n" + ChunkInfo[filename].code;
      ChunkInfo[filename].code = newCode;
      if (dev) {
        let index = String(text).lastIndexOf("\n");
        let newText = text.slice(0, index) + `\n// @require file:///${path.join(__dirname, "/dist/" + FILE_NAME)}` + text.slice(index);
        if (!fs.existsSync("dist")) {
          fs.mkdirSync("dist");
        }
        fs.writeFileSync(path.join(__dirname, "/dist/" + FILE_NAME.replace(".js", ".dev.js")), newText);
      }
    }
  }
}