@vitejs/plugin-legacy breaks const hoisting in loops

[IlyaSemenov]

Describe the bug

I am compiling a legacy build for Chrome 49 with:

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

export default defineConfig({
  plugins: [
    legacy({
      targets: ['chrome 49'],
    }),
  ],
});

In the app code, I use variable hoisting inside a loop:

const messages = {
  good: 'You should see me',
  bad: 'You should not see me - hoisting broken!',
};
const obj = {};
for (const key in messages) {
  obj[key] = () => messages[key]; // <---- each closure uses a hoisted key
}
element.innerHTML = `${obj.good()}`;

What I expect is that the code works identically in both modern and legacy build.

Instead, the legacy build doesn't hoist key in the above loop. The resulting code is clearly wrong:

obj = {};
for (key in messages) {
  obj[key] = () => messages[key]; // <---- key is not hoisted
}
element.innerHTML = `${obj.good()}`;

Reproduction

https://stackblitz.com/edit/vitejs-vite-ndrwsg?file=vite.config.js,counter.js&terminal=dev

Steps to reproduce

Build and run production build:

pnpm build
pnpm preview

1. Load the modern build. The page will say: "You should see me"

2. Load the legacy build. To do so on a modern browser, edit dist/index.html and remove two lines:

• <script type="module" crossorigin src="/assets/index-9d88f3c8.js"></script>
• if (window.__vite_is_modern_browser) return;

The page will say: "You should not see me - hoisting broken!"

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 16.14.2 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 7.17.0 - /usr/local/bin/npm
  npmPackages:
    @vitejs/plugin-legacy: ^4.0.3 => 4.0.3 
    vite: ^4.3.2 => 4.3.3

Used Package Manager

pnpm

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 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.

[stackblitz]

Start a new pull request in StackBlitz Codeflow.

• vitejs-vite-ndrwsg (Open reproduction)

[sapphi-red]

This can be reproduced only with babel + preset-env.
https://stackblitz.com/edit/node-6scvhf?file=index.js

Closing as this seems to be tracked at babel/babel#15594.