How to tell vite to output a specified file into a specified folder with rollupOptions?

I want to tell vite to only output the demo.js in src file into a build folder with a name bundle.js, here is my code, I run it, but vite only output the main.js and images into the dist folder, it doesn't output the demo.js I specified, what should I do?

I tried out.Dir and root, it just changes the output dir to a specified folder. But I need also change the input file to a specified file. I tried the root option, which vite did change the input file, but it need that file to have link to index HTML. I think vite still want to bundle all the files. But my goal is to bundle a single js file which has imports to a specified output file with the rollupOptions inside Vite config. I tried to just use the rollup's own config to do the job, which is successful. But I just wondering if we can just use the vite to do it, since it uses rollup.

import { defineConfig } from 'vite';
import vue from '@vitejs/plugin-vue';
import resolve from '@rollup/plugin-node-resolve';
import babel from '@rollup/plugin-babel';

// https://vitejs.dev/config/
export default defineConfig({
  plugins: [vue()],
  rollupOptions: {
    input: 'src/demo.js',
    output: {
      file: 'build/bundles.js',
      format: 'cjs'
    },
    // plugins: [
    //   resolve(),
    //   babel({ babelHelpers: 'bundled' })
    // ]
  }
})

You can build a single JS file (and no HTML) using Vite's library mode. Your example vite.config.js can be written as follows:

import { defineConfig } from 'vite';
import vue from '@vitejs/plugin-vue';
import resolve from '@rollup/plugin-node-resolve';
import babel from '@rollup/plugin-babel';

// https://vitejs.dev/config/
export default defineConfig({
  plugins: [vue()],
  build: {
    lib: {
      resolve(__dirname, 'src/demo.js'),
      name: 'demo',
      fileName: 'bundles',
      format: 'cjs',
    },
    outDir: 'build',
    rollupOptions: {
      // make sure to externalize deps that shouldn't be bundled
      // into your library
      external: ['vue'],
    },
  },
})

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