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

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Seit 27.10.2026.

  • #64982 von @1-DARK — offen

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Issue-Typ
Bug
Klarheit
Klar beschrieben
Aktivitätsstatus
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Tech-Stack
typescript
Bereich
compilers

Rechercherichtung

Start by reading typeeraser.go around the decorated declare and abstract property handling, then follow visitPropertyDeclaration in legacydecorators.go to see why the preserved node emits a field. Compare with the visitPropertyDeclaration handling in ts.ts described in the issue. Done means the abstract property is erased while its legacy decorator call remains, matching the expected output; an open pull request is already linked.

Vom Indexierungsmodell aus dem Issue-Text verfasst.

Beschreibung

Bug
🔎 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.

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