Multiple export defaults (T7242)

[babel-bot]

Issue originally made by James Whitney (whitneyit)

Bug information

• Babel version: Babel 6.7.4
• Node version: v5.4.0
• npm version: v3.5.3

Input code

export default class A {
}

export default class B {
}

Description

I'm not sure if this is intended or has just been overlooked, but the above code does not produce an error.

I was able to find a few references to support this, namely a reference on [[ https://developer.mozilla.org/en/docs/web/javascript/reference/statements/export#Description | MDN]] and a post by Dr. Axel Rauschmayer's on [[ http://www.2ality.com/2014/09/es6-modules-final.html#an_overview_of_the_es6_module_syntax | 2ality ]], but I couldn't track it down specifically in the ES spec.

In addition, this is reported as an error on the [[ http://www.typescriptlang.org/Playground#src=export%20default%20class%20A%20%7B%0A%7D%0A%0Aexport%20default%20class%20B%20%7B%0A%7D| Typescript Playground]].

[babel-bot]

Comment originally made by @loganfsmyth

Good call, I don't think we have any error handling for duplicate exports at the moment.

[babel-bot]

Comment originally made by @jmm

If this is a spec violation, someone please provide a spec citation.

[babel-bot]

Comment originally made by @kaicataldo

Cross-posting from the open PR:

Thanks for the comment! I'll be up front - my experience reading the spec is limited at best at the moment, but I think this section references the error we're reporting in this PR. The relevant part seems to be the note at the bottom:

The duplicate ExportedNames rule implies that multiple export default ExportDeclaration items within a ModuleBody is a Syntax Error. Additional error conditions relating to conflicting or duplicate declarations are checked during module linking prior to evaluation of a Module. If any such errors are detected the Module is not evaluated.

Does that look right to you?

[babel-bot]

Comment originally made by @jmm

Thanks @kaicataldo! That does seem like the relevant reference. (Hmm...I'm not sure off the top of my head what they're referring to with "Additional error conditions [...]".) Actually that was kind of my bad 🤦 for forgetting that multiple export default is just a special case of attempting to export the same name multiple times in general, as indicated by that spec reference. Probably this issue should be updated or absorbed into a new issue to reflect that it's not specific to export default.

[babel-bot]

Comment originally made by @kaicataldo

I'm happy to continue working on this, and would be willing to try my hand at the relevant Babylon change, if need be - what are your thoughts on @hzoo's suggestion [[ https://github.com//pull/3518#issuecomment-225709319 | here ]]?

[babel-bot]

Comment originally made by @danez

It seems this landed in master now and for the proper fix in the parser there is already an issue.
babel/babylon#69