why @babel/plugin-proposal-class-properties and can't set loose true

[su6838354]

office document：make(Auto)Observable only supports properties that are already defined. Make sure your compiler configuration is correct, or as work-around, that a value is assigned to all properties before using make(Auto)Observable. Without correct configuration, fields that are declared but not initialized (like in class X { y; }) will not be picked up correctly.

try use @babel/plugin-proposal-class-properties and set loose true

class Bork {
  static a = "foo";
  static b;

  x = "bar";
  y;
}

transform to

var Bork = function Bork() {
  babelHelpers.classCallCheck(this, Bork);
  this.x = "bar";
  this.y = void 0;
};

Bork.a = "foo";
Bork.b = void 0;

the class property y is still initialized with undefined？
of course ts will has promble，but the @babel/plugin-proposal-class-properties and set loose true has no promble

[urugator]

Because our implementation assumes false (standard ES behavior) and we don't have to worry about true.