Describe the bug

After running vite build, Worker is built in legacy mode with the {type: "module"} option, which is undesirable in this case and results in the application breaking in older browser versions.

vite.config.ts

import vue from '@vitejs/plugin-vue';
import legacy from '@vitejs/plugin-legacy';
import { resolve } from 'path';
import { defineConfig } from 'vite';

export default defineConfig({
  plugins: [vue(), legacy()],
  build: {
    reportCompressedSize: false,
  },
  worker: {
    format: 'iife',  // 'es' produces the same output
  },
  resolve: {
    alias: {
      '@': resolve(__dirname, './src'),
    },
  },
});

index-legacy.057ce815.js

return new Worker("/assets/tv.worker.b73a6f8e.js", {type: "module"})

Reproduction

https://stackblitz.com/edit/node-kfimtt

System Info

System:
    OS: macOS 12.2
    CPU: (8) x64 Intel(R) Core(TM) i5-1038NG7 CPU @ 2.00GHz
    Memory: 1.82 GB / 16.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 17.4.0 - /usr/local/bin/node
    Yarn: 1.22.17 - /usr/local/bin/yarn
    npm: 8.4.0 - /usr/local/bin/npm
  Browsers:
    Brave Browser: 98.1.35.100
    Chrome: 60.0.3112.113
    Safari: 15.3
  npmPackages:
    @vitejs/plugin-legacy: ^1.6.4 => 1.6.4 
    @vitejs/plugin-vue: ^2.1.0 => 2.1.0 
    vite: ^2.8.0-beta.5 => 2.8.0-beta.5

Used Package Manager

npm

Logs

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Make sure this is a Vite issue and not a framework-specific issue. For example, if it's a Vue SFC related bug, it should likely be reported to https://github.com/vuejs/core instead.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.