RFC: Remove vars follow up

[blling]

Most of the var has been removed after the PR #11780(thanks @raphamorim again) merged, so we could lint against developers use var again, but this is a bit tricky because some files have to use var. we should have a way to override lint configuration for a subset of the files, so we should discuss the solutions here.

[gaearon]

For Prettier, we have separate configurations for files that should be ES5 and ES6. We need to extract these paths to a shared config, and then split the ESLint task into two runs, one of them would assume the code is ES5 and another, ES6.

[davscro]

hi @gaearon
could we write custom eslint preprocessor and for "var" files (~40) override 'no-var'
rule?

[gaearon]

Here's how we separate configs for Prettier. Since we now call ESLint through its Node API too, I imagine we would use a similar strategy there. Divide it into two calls, and override rules for one of the calls.

[ctxhou]

Hi @gaearon, I want to work on this.
Can you give me some instructions? Like what kind of file we will permit to use var?

[gaearon]

These are files that need to use ES5 (and thus var).

These are files that should use ES6 only.

Does that help?

[ctxhou]

Ok thanks, it helps!

[blling]

@ctxhou i think this should be done as fast as you can, otherwise vars will be add again.

[ctxhou]

There is one thing I want to discuss.
Currently, these is prettier es6 files pattern:

[
   // Internal forwarding modules
   'packages/*/*.js',
   // Source files
   'packages/*/src/**/*.js',
   'packages/shared/**/*.js',
 ]

But for eslint, the es6 lint pattern should be all js files without es5 files?
So it means that the shared es6 pattern will not be the same with prettier and eslint config.
Is it correct?

[gaearon]

I don’t understand why it would be different. Those are ES6 files. The ES5 files are listed separately below.

[ctxhou]

Sorry, I misunderstand the pattern.
Here I create a pr to fix this issue: #11794

[gaearon]

Fixed in #11794.