Typescript: Avoid stripping class properties when a decorator is set

[pmdartus]

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

Current behavior:
The babel-plugin-transform-typescript removes class properties without value regardless if decorators are assigned to it or not.

// Original
class C {
  @foo
  d;
  @foo
  e = 3;
}

// Transformed
class C {
  // <-- Missing `d` field
  @foo
  e = 3;
}

Expected behavior:
The babel-plugin-transform-typescript should only remove a class property if it has no value and no decorators.

// Original
class C {
  @foo
  d;
  @foo
  e = 3;
}

// Transformed
class C {
  @foo
  d;
  @foo
  e = 3;
}

REPL example

[pmdartus]

This code is a duplicate of the one in the Class visitor.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/8469/

[xtuc]

I don't have enough context here, sorry. Why do we want to emit decorators syntax? Shoudln't we strip out unsupported syntax?

[nicolo-ribaudo]

The typescript plugin should leave decorators there: they will be handled by the decorators plugin.

[diervo]

@nicolo-ribaudo @xtuc are you good with this changes?
The idea is for typescript to not touch any feature that will need further transform (in particular decorators and class fields - which they need to update anyway in their transpiler)

If so, shall bribe @hzoo or @loganfsmyth to merge this? :)

[xtuc]

I'm 👍 only if we can make sure the user is aware of this behavior:

• we should warn if decorator are not transformed (because no plugin loaded)

[nicolo-ribaudo]

I don't think that we should, the same way we don't warn if class properties or other proposals supported by typescript are not transpiled.
Users will see very easily that their code is not transformed, since it would cerash at the line containing the decorators as soon as it is loaded in a browser.

[xtuc]

That's right @nicolo-ribaudo, I was wondering if the "why it didn't transform it" would be confusing for users or not.

[nicolo-ribaudo]

If the user just adds the typescript plugin, Babylon throws and suggests the user to add the transform-decorators plugin.
The only problem would be if the users only enables the syntax plugin, but it is known that syntax plugins leave the input as-is.

[hzoo]

This is fine, it's expected behavior 👍.

[xtuc]

Ah yes Nicolo, perfect!