Add prefer-default-export rule (fixes #304)

[gavriguy]

Add prefer-default-export rule

When there is only a single export from a module prefer using default export over named export.

[jfmengels]

missing = here

[jfmengels]

Thanks a lot @gavriguy, looks good!

Could you add a line about it in the README, and add an entry in the changelog? You can thank yourself there too :)

[gavriguy]

fixed. thanks for the warm welcome

[benmosher]

Just making a note: would like to support

• count export { x, y, ... } as >=2 named exports
• handle export { z as default } as a valid default export
• add valid + invalid tests for these cases

I think the first is solved by counting ExportSpecifiers instead of ExportNamedDeclarations.

The latter just needs to inspect the exported.name on ExportSpecifiers and assert hasDefaultExport if specifier.exported.name === 'default'. or as code:

// replaces `ExportNamedDeclaration` visitor. 
'ExportSpecifier': function(node) {
  if (node.exported.name === 'default') {
    hasDefaultExport = true
  } else {
    namedExportCount++
    namedExportNode = node
  }
}

[benmosher]

@gavriguy let me know if you're up for making those changes in this PR. I can also do it post-merge if you'd rather not. 😎

[gavriguy]

great ideas ill work on it. thanks

[gavriguy]

I've made the changes.
It was a bit more complicated than your suggestion as ExportSpecifier isn't called on all named exports - its only called when the named export is inside curly braces.
To solve this - I had to create two counters: namedExportCount & specifierExportCount.

Anyways now everything works and all tests are passing.

[gavriguy]

one of the build failed because of timeout - I think its a travis issue, but I don't see how to rebuild it again (maybe because I don't have the right permissions at Travis) Can you please have a look?

[benmosher]

Changes look good, no worries on the build.

You could rebase against the tip, if you want, but I will probably tweak the change log a little before merging anyway, so you don't need to worry about it. Thanks for the updates! 😄

[gavriguy]

Thanks, I rebased against master - please let me know if you need anything more. thanks

[benmosher]

Merged via cf45512. Thanks!