--experimentalDecorators: decorated abstract property is emitted as a class field (regression in 7.0) #64980

🔎 Search Terms

experimentalDecorators, abstract property, decorator, class field, useDefineForClassFields, typeeraser, legacy decorators

🕗 Version & Regression Information

• Regression from tsc 6.0.3, which erases the field.
• Reproduces in 7.0.2, 7.1.0-beta and 7.1.0-dev.20261026.1.

💻 Code

function dec(target: object, key: string) {
  Object.defineProperty(target, key, { get: () => 'from prototype', configurable: true });
}

abstract class Base {
  @dec abstract data: string;
}

class Sub extends Base {
  declare data: string;
}

const s = new Sub();
console.log(Object.hasOwn(s, 'data'), s.data);

{ "compilerOptions": { "target": "es2022", "experimentalDecorators": true } }

🙁 Actual behavior

Base gets a data; field, so every instance has an own data: undefined that shadows whatever the decorator put on the prototype. Prints true undefined.

class Base {
    data;
}
__decorate([
    dec
], Base.prototype, "data", void 0);

🙂 Expected behavior

Same as tsc 6.0.3: the field is erased and only the __decorate call remains. Prints false from prototype.

class Base {
}
__decorate([
    dec
], Base.prototype, "data", void 0);

Additional information about the issue

The field appears only when all three hold: experimentalDecorators, a decorator on the abstract property, and define semantics (useDefineForClassFields, the default at target ES2022 and later). emitDecoratorMetadata makes no difference. An undecorated abstract property and a decorated declare property are both still erased.

The likely cause is in typeeraser.go. For a decorated declare/abstract property under experimentalDecorators, it keeps the node so that LegacyDecoratorsTransformer can emit the __decorate call. But it rebuilds the node with VisitModifiers, which elides AbstractKeyword. visitPropertyDeclaration in legacydecorators.go then finds neither NodeFlagsAmbient nor ModifierFlagsAbstract and emits a field. declare escapes this because the node keeps NodeFlagsAmbient. Strada's visitPropertyDeclaration in ts.ts handles the same case by appending a declare modifier to the preserved node (concatenate(modifiers, factory.createModifiersFromModifierFlags(ModifierFlags.Ambient))).

We hit this in TypeORM entities: an abstract base declares a decorated abstract column, each subclass redeclares it with declare, and every entity instance gains an own data: undefined it doesn't have under tsc 6.