Provide better error message for invalid default export declaration

[dczombera]

Q	A
Fixed Issues?	Fixes #6708
Patch: Bug Fix?	n
Major: Breaking Change?	n
Minor: New Feature?	n
Tests Added + Pass?	Yes
Documentation PR
Any Dependency Changes?
License	MIT

I was not sure if an error message like the one suggested in the issue is consistent with the other error messages that babylon provides (is consistency even an issue when it comes to error messages?).
Apart from that, it's my first contribution to the project. I welcome any type of feedback. Thanks! :)

REPL from build

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/7579/

[nicolo-ribaudo]

Thank you for this PR!

[nicolo-ribaudo]

It can also take function and class declarations.

[dczombera]

So would you suggest to just add both to the error message?

[existentialism]

Maybe just:

Only expressions, functions or class declarations are allowed as the default export.

(Let's add tests for those too).

[dczombera]

Yeah, I like that compact message.
So, I will change the message then and will also add tests for class declarations. Tests for functions declarations already exist as I've just seen.

[dczombera]

Alright, I changed the error message as discussed and added tests for class declarations/expressions. ☺️

[hzoo]

Nice work, a great first PR!

[existentialism]

@dczombera thanks!

[jcurtis]

This one always catches me off-guard (export default const). Thanks!

This might be the wrong forum but can anyone explain why you can't do that but you can do this?:

export const foo = 'bar';

[nicolo-ribaudo]

An easy way to think about it is that named exports must have a name, while default exports not.

The real reason is that named exports export bindings, or variables (infact, if you modify an already exported variable the modules which import it will see the changes), while default exports export values.

[Jessidhia]

But there's an asterisk... there's always an asterisk 😇

If your default export is a function or class declaration (no parens wrapping them), then those will be mutable bindings, and reassigning the function or class name will update the default export.

It's also possible to make anything you want the default export by using export { foo as default }.