Class Property without initializer parsed when classProperties plugin is not enabled

[babel-bot]

Issue originally reported by @DrewML in babel/babylon#242

Input Code

class Bar {
   abc; 
}

Babylon/Babel Configuration

Calling babylon.parse() without a plugins []

Expected Behavior

Unexpected token error

Current Behavior

Class Property is parsed

Your Environment

software	version
Babylon	6.14.1
node	All
npm	All
Operating System	All

---

The only check for the classProperties plugin is currently in pp.parseClassProperty in statement.js:

pp.parseClassProperty = function (node) {
  if (this.match(tt.eq)) {
    if (!this.hasPlugin("classProperties")) this.unexpected();
    this.next();
    node.value = this.parseMaybeAssign();
  } else {
    node.value = null;
  }
  this.semicolon();
  return this.finishNode(node, "ClassProperty");
};

If an initializer is not provided, it drops passed the branch that has the plugin check. I think we can just fix this by moving the plugin check to the top of parseClassProperty, since we've already done a check prior (in pp.parseClassBody) to verify we're dealing with a class prop.

[babel-bot]

Comment originally made by @loganfsmyth

This does indeed seem like a bug. Just a general note, any fix for this needs to take https://github.com/babel/babylon/blob/48873f886aa91c4d250ccce0c5e220c82f538f11/src/plugins/flow.js#L1050 into account. If I had to guess, that's the motivation for the current behavior.

You can do

class Foo {
    prop: string;
}

with only flow turned on, without classProperties.

[babel-bot]

Comment originally made by @DrewML

Good call on the flow stuff, hadn't considered that. Will keep in mind. I plan on addressing this, just trying to get through work on the Private Class Fields proposal first.

[babel-bot]

Comment originally made by @bakkot

I believe this was fixed by #300.

[hzoo]

Fixed by babel/babylon#300