plugin-legacy: modernTargets not respected when renderLegacyChunks is false

[jgosmann]

Describe the bug

I'm using @vitejs/plugin-legacy with this configuration:

export default defineConfig({
  plugins: [
    legacy({
      modernTargets: ['Firefox >= 68', 'Chrome >= 72', 'Edge >= 91'],
      modernPolyfills: true,
      renderLegacyChunks: false,
    }),
  ],
});

The modernTargets option is only respected if I also set renderLegacyChunks to true.

Reproduction

https://stackblitz.com/edit/vitejs-vite-nagbo9?file=vite.config.js

Steps to reproduce

A (fairly) minimal reproduction is provided under the Stackblitz link. Run DEBUG=vite:legacy npm run build and you will see that the modernTargets option is not logged. In the generated bundle (dist/assets/index-[hash].js), you will be able to find a null coalescing operation (??) that isn't compatible with the browsers given in modernTargets.

If you change renderLegacyChunks to true, you get the expected log output:

plugin-legacy 'modernTargets' option overrode the builtin targets of modern chunks. Some versions of browsers between legacy and modern may not be supported.
[@vitejs/plugin-legacy] targets: last 2 versions and not dead, > 0.3%, Firefox ESR
[@vitejs/plugin-legacy] modernTargets: [ 'Firefox >= 68', 'Chrome >= 72', 'Edge >= 91' ]

Also, in the generated bundle the null coalescing operator does not longer exist and has been replaced with code compatible with older browsers.

System Info

# Stackblitz environment


  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: 18.18.0 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.2.3 - /usr/local/bin/npm
    pnpm: 8.14.0 - /usr/local/bin/pnpm
  npmPackages:
    @vitejs/plugin-legacy: ^5.3.0 => 5.3.0 
    vite: ^5.0.12 => 5.0.12 

Local environment

  System:
    OS: macOS 14.3
    CPU: (12) arm64 Apple M2 Max
    Memory: 6.17 GB / 64.00 GB
    Shell: 5.9 - /opt/local/bin/zsh
  Binaries:
    Node: 20.11.0 - /opt/local/bin/node
    npm: 10.4.0 - /opt/local/bin/npm
  Browsers:
    Chrome: 121.0.6167.139
    Edge: 121.0.2277.98
    Safari: 17.3
  npmPackages:
    @vitejs/plugin-legacy: ^5.3.0 => 5.3.0
    @vitejs/plugin-react: ^4.2.1 => 4.2.1
    vite: ^5.0.12 => 5.0.12

### Used Package Manager

npm

### Logs

_No response_

### Validations

- [X] Follow our [Code of Conduct](https://github.com/vitejs/vite/blob/main/CODE_OF_CONDUCT.md)
- [X] Read the [Contributing Guidelines](https://github.com/vitejs/vite/blob/main/CONTRIBUTING.md).
- [X] Read the [docs](https://vitejs.dev/guide).
- [X] Check that there isn't [already an issue](https://github.com/vitejs/vite/issues) that reports the same bug to avoid creating a duplicate.
- [X] 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](https://github.com/vuejs/core) instead.
- [X] Check that this is a concrete bug. For Q&A open a [GitHub Discussion](https://github.com/vitejs/vite/discussions) or join our [Discord Chat Server](https://chat.vitejs.dev/).
- [X] The provided reproduction is a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) of the bug.

[stackblitz]

Start a new pull request in StackBlitz Codeflow.

• vitejs-vite-nagbo9 (Open reproduction)

[jgosmann]

After pondering this comment in the review I realized that the bug described here isn't quite accurate. The syntax transforms are applied correctly if config.build.target is properly configured in the Vite configuration. (Though, the documentation on this could probably be improved.) However, there is still a bug as modernTargets is not correctly initialized before detecting the necessary polyfills.