preview.proxy does not work when server.proxy is empty

[liuyang678]

Describe the bug

Configuring preview.proxy doesn't work, and running vite preview will give an error when server.proxy is not configured.

pr: #7415

Reproduction

https://stackblitz.com/edit/vitejs-vite-b5medm?file=vite.config.ts&terminal=dev

System Info

System:
    OS: Linux 5.10 Ubuntu 20.04.4 LTS (Focal Fossa)
    CPU: (8) x64 Intel(R) Core(TM) i7-9700 CPU @ 3.00GHz
    Memory: 17.19 GB / 24.88 GB
    Container: Yes
    Shell: 5.0.17 - /bin/bash
  Binaries:
    Node: 16.13.2 - ~/.nvm/versions/node/v16.13.2/bin/node
    Yarn: 1.22.17 - /mnt/d/Programs/nodejs/yarn
  npmPackages:
    vite: ^2.8.0 => 2.8.6

Used Package Manager

yarn

Logs

> vite preview

error when starting preview server:
TypeError: Cannot convert undefined or null to object
    at Function.keys (<anonymous>)
    at proxyMiddleware (/Users/liuyang/Desktop/work/yiyao_fe_shangjia_react/b2c_welcome/node_modules/vite/dist/node/chunks/dep-9c153816.js:51440:12)
    at preview (/Users/liuyang/Desktop/work/yiyao_fe_shangjia_react/b2c_welcome/node_modules/vite/dist/node/chunks/dep-9c153816.js:69529:17)
    at processTicksAndRejections (node:internal/process/task_queues:96:5)
    at async CAC.<anonymous> (/Users/liuyang/Desktop/work/yiyao_fe_shangjia_react/b2c_welcome/node_modules/vite/dist/node/cli.js:783:24)

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.

[gao-zhan]

I have the same problem, can anyone help me

[Jinjiang]

I met the same problem. Then I digged a little bit and found:

The current process: first detect whether config.preview.proxy exists:

vite/packages/vite/src/node/preview.ts

Line 81 in 212d454

if (config.preview.proxy) {

second read config.server.proxy to proceed:

vite/packages/vite/src/node/server/middlewares/proxy.ts

Line 35 in 212d454

const options = config.server.proxy!

which leads a bug when users preview with config.preview.proxy but without config.server.proxy.

To fix this, I'm afraid this part of code needs a little revamp since the middleware doesn't differentiate which mode it's under.

Hope this information is helpful.

Thanks.