support vue.config.cjs

[simon300000]

if "type": "module" was set in package.json, the vue service will emit error.

 ERROR  Error loading vue.config.js:
 ERROR  Error [ERR_REQUIRE_ESM]: Must use import to load ES Module: .../vue.config.js
require() of ES modules is not supported.
.../vue.config.js from .../node_modules/@vue/cli-service/lib/Service.js is an ES module file as it is a .js file whose nearest parent package.json contains "type": "module" which defines all .js files in that package scope as ES modules.
Instead rename vue.config.js to end in .cjs, change the requiring code to use import(), or remove "type": "module" from .../package.json.

this pr enable use of vue.config.cjs, as a simple solution in those environment.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Underlying tools
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

Other information:
close #5046

It would be nice if import() is used instead of require(), which will support both commonjs and module, but this makes the whole process asynchronous and many code need to be changed.

[simon300000]

I have no idea why AppVeyor is failed 😢

[haoqunjiang]

I've refactored some of the loading and testing logic in #5305

You can sync with the dev branch and then fix the Windows testing following that PR.

[simon300000]

I merged dev, the tests also passes, thank you!

[simon300000]

I worked out a different solution which uses import(vue.config.js),

But I don’t know which one is more suitable.

dev...simon300000:import(vue.config.js)

Should I open a different PR/issue to discuss about it?

[haoqunjiang]

Thank you very much for the help!

I'd very like to support the import('vue.config.js') form too. But I'm worried about the extra implementation complexity that the deasync package brings in. It is a native module that requires the node-gyp toolchain to be present in the user environment, thus may cause compatibility issues under some circumstances.

Out of the same concern, we've made the fibers dependency optional for the sass setup: #3628

So, as of now, I think we'd go with the implementation in this PR to support vue.config.js.
For true ESM support, please open a separate issue to discuss.

IMO we should support async loading of the config before supporting ESM imports, so that we can avoid using deasync.