Use cjs for commonjs exports

[huozhi]

Fixes #1822

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[shuding]

My only concern is if threre's some bundler that doesn't support extensions other than .js...

[huozhi]

My only concern is if threre's some bundler that doesn't support extensions other than .js...

At least we need to move forward with the new standard. Now esm.js is the fallback for .mjs, and .js is the fallback for .cjs, containing all of them in the publish assets to fit the bundler whos don't support it is wired, which makes swr keep staying behind the new standard.

There's some bundler like metro (for RN) seems doesn't support cjs nor the exports field so far, and the issue is not updated. It's uncertain if they're going to support it or not (probably not).
However expo bundler supports them pretty well so it works if you play RN with expo.

One idea is to learn from RN community:

• if there's any possibility to switch bundler?
• how much the rough percentage is metro taking so far?

Then could propbably make better decisions to support it or drop it.

[shuding]

At least we need to move forward with the new standard.

I agree this is important, however we have to maintain backward compatibility as well, if possible. I'd personally choose "not broken" over "featureful".

[huozhi]

Then maybe we could test with 3 extensions together js, esm.js, cjs, mj together for now. And decide a time to drop the esm.js and js together with announcement

[huozhi]

Since it's the yarn 3 issue we can close the inproper fix