Reserve empty import statements for there could have side effects

[hyf0]

Describe the feature

Currently

import { unused } from 'unused'
import { unused2 } from './unused'

will be compressed to

with unused option.

playground link.

IMO, there should be an option called is_imported_module_has_side_effects enabled by default.

With is_imported_module_has_side_effects and unused enabled,

import { unused } from 'unused'
import { unused2 } from './unused'

will be compressed to

import 'unused'
import './unused'

Babel plugin or link to the feature description

No response

Additional context

No response

--edited
Wrong playground link. Fixed now.

[kdy1]

I think it can be the default behavior even without an option. It's simply not implemented because it's not useful for next.js bundles

[kdy1]

Sorry, I misread it. I'm not sure if an export of ESM can have a side effect on access

[hyf0]

Sorry, I misread it. I'm not sure if an export of ESM can have a side effect on access

I guess the key is that top-level statements of imported modules could have side effects, not the exports.

// index.js
import { foo }  from './foo'

// foo.js
console.log('foo.js')
export const foo = 'foo'

[kdy1]

This behavior is shared among lots of tools.
If you want to preserve imports, you can use import 'foo', without {}

[kdy1]

Well terser simply preserves it. Hmm...

[hyf0]

This behavior is shared among lots of tools. If you want to preserve imports, you can use import 'foo', without {}

The above example I gave was wrong, which cause some misunderstanding. I fixed just now.

[kdy1]

I think we should align this behavior with terser. I hope terser playground had a feature to share code :/

[Austaras]

There's already a PR but seems nobody is reviewing it.

[hyf0]

@kdy1 I like to give a try on this. If we align this behavior with terser, I guess we just simply preserve specifiers?

[kdy1]

Yeap, and you would need to patch the DCE pass ( https://github.com/swc-project/swc/blob/e74ef8d9ed5b1a70dc57a97a7877ccbe099dd23a/crates/swc_ecma_transforms_optimization/src/simplify/dce/mod.rs )

Thank you in advance!