[BugFix] Compile export default from

[nveenjain]

Q	A
Fixed Issues?	Fixes #7293
Patch: Bug Fix?	Yes
Major: Breaking Change?	No
Minor: New Feature?	No
Tests Added + Pass?	NO
Documentation PR
Any Dependency Changes?	No
License	MIT

Now we check in export default statements that if from is either an assignment or from is exported then valid AST are generated, which doesn't require any sort of plugin.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/6780/

[nicolo-ribaudo]

1. Can you add some tests?
2. To avoid using lookahead (which needs to clone the whole Babylon state), we can do something like this:

const declaration = this.parseMaybeAssign();
if (declaration.type === "Identifier" && declaration.value === "from" && this.state.type === tt.string) {
  // export default from "module";
} else {
  node.declaration = declaration;
  return ExportDefaultDeclaration;
}

[nicolo-ribaudo]

What if I have another token? e.g. export default from + 2;

[nveenjain]

Yeah Sorry, didn't consider that.. Will update the PR

[nicolo-ribaudo]

I just read the spec and it looks like this is disallowed? 🤔
cc @leaves4j

https://tc39.github.io/proposal-export-default-from/#prod-ExportDeclaration

export default [lookahead ∉ { function, class, from }] AssignmentExpression[In];


> NOTE 1
The addition of from to the lookahead after export default allows for the disambiguation
of export default from "mod", which is the proposed syntactic form for exporting
"mod"'s default export as this module's default export, from export default from, which
is the potentially confusing export of a local named variable from.

[nveenjain]

but since it is still in proposal, should i work on this PR or not? like first we should check whether export-default-from plugin is used or not.
Shouldn't export default from work without export-default-from plugin and don't work with export-default-from plugin enabled?

[nicolo-ribaudo]

For now I think we should wait: even if it is currently allowed, we should prevent users from depending on export default from;.

Usually the spec doesn't includes breaking changes, but since people who are using modules are also using Babel (because native esm support is really new), that change wouldn't break anyone's existing code and thus I think tc39 folks could decide do make it.

cc @leebyron I'd like to know your opinion, since you are the author of the proposal 🙂

[leebyron]

I believe @jdalton has taken over for these. I’m still in support of them, but have not been attending TC39 meetings and don’t know their status

[JLHwung]

@nveenjain Although this PR is closed by #11676, I want to thank you for your inspiring work here!