#__PURE__ on member expressions should be handled

[dgreensp]

Describe the feature

If I have an export like export const foo = ..., this export will only be removed if the RHS can be determined to be side-effect-free, understandably. Member accesses in general are considered to potentially have side effects (with the exception of Math.PI; see implementation of is_pure_callee), which is very conservative. I have exports like export const HTMLElement = window.HTMLElement in a module, and it's inconvenient that SWC is worried about the side effects of an expression like window.HTMLElement.

There is a quite ugly workaround, involving temporarily reassigning the global Math. It would be great if there were some other way to mark an expression (or export) as side-effect-free, such as with a comment, a wrapper function with a magic name, or even some identifier besides Math that you can access properties on and have it considered to be pure, like if you name your local variable __pure you are ok.

Babel plugin or link to the feature description

No response

Additional context

No response

[Austaras]

terser/terser#513

[Austaras]

Reread the terser thread, seems like the only objection is maintenance cost.

[kdy1]

Honestly, I think the problem is that you have

export const HTMLElement = window.HTMLElement in a module.

I'm fine with the maintenance cost, but not sure about adding it to getter instead of a member expression

[Austaras]

The maintenance cost of swc is sky high anyway

const href = location.href seems like a fairly common pattern

[kdy1]

I think access to global location can be marked as pure.

[kdy1]

I think we can add the pure comment support for member expressions, and I think we should.

Currently, this can be done via

export const MyExport = /*#__PURE__*/ (function () { return window.MyExport; })()

but this does not look like a good idea.

[dgreensp]

I didn't know about /*#__PURE__*/. I will try that out.

[dgreensp]

For what I'm doing (Deno bundling), the comment here doesn't seem to have any effect:

export const MyExport = /*#__PURE__*/ (function () { return window.MyExport; })()

I was grepping the SWC code for __PURE__, and it looks like it is checked for in the minifier, not the optimizer? I'm not sure if that's right.

[kdy1]

Yeah, it's only handled by the minifier

[swc-bot]

This closed issue has been automatically locked because it had no new activity for a month. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.