SSR import in development can't resolve default import

[cyco130]

Describe the bug

For some packages, when one does import whatever from "whatever", the default export ends up in whatever.default instead of whatever as expected. This only happens in 2.7.0 beta (2.6 is fine), in development mode of SSR. Everything works in the production build and in development in the browser.

Reproduction

This repo illustrates the issue with styled-components: https://github.com/cyco130/vite-issue-ssr-default-import

• Clone the repo
• Install dependencies with npm install
• Start dev server with npm run dev
• Open the link in the browser
• Observe the message "BEEP! imported.default has what we expect in imported!" on the terminal
• Also observe the lack thereof in the browser

The problem is caused by src/App.jsx where styled-component doesn't work in dev SSR mode when correctly imported but it does work when one accesses imported.default.

The issue appears to be caused by the proxyESM function in ssrModuleLoader.ts not following the xxx.default chain as many times as style-component's build system expects. The following change seems to solve the issue but might be too heavy-handed:

  let defaultExport = mod;
  while ("default" in defaultExport && defaultExport.__esModule) {
    defaultExport = defaultExport.default;
  }

System Info

System:
    OS: Linux 5.4 Linux Mint 20.2 (Uma)
    CPU: (8) x64 Intel(R) Core(TM) i7-3630QM CPU @ 2.40GHz
    Memory: 4.80 GB / 15.53 GB
    Container: Yes
    Shell: 5.0.17 - /bin/bash
  Binaries:
    Node: 14.18.1 - /usr/local/bin/node
    Yarn: 1.22.11 - ~/.npm-global/bin/yarn
    npm: 7.24.1 - ~/.npm-global/bin/npm
  Browsers:
    Chrome: 96.0.4664.45
    Firefox: 94.0
  npmPackages:
    @vitejs/plugin-react: ^1.1.0-beta.0 => 1.1.0-beta.0 
    vite: ^2.7.0-beta.6 => 2.7.0-beta.6

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/vue-next 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.

[brillout]

@cyco130 How about this?

  let defaultExport = mod;
  const visited = new WeakMap()
  while ("default" in defaultExport && defaultExport.__esModule) {
    if (visited.has(defaultExport)) {
      break
    }
    visited.set(defaultExport, true);
    defaultExport = defaultExport.default;
  }

[cyco130]

@brillout Much better. I did run into a pathological module that broke mine today.