ES decorator metadata emit throws when a decorated class extends null #65363

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1. visualkhh commented on Nov 8, 2026

   

   visualkhh (visualkhh)

   on Nov 8, 2026

   Last edited by visualkhh

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   Reproduced on released 5.9.3, not just main — and confirmed the bug is exclusive to the standard-decorator path, which narrows the blast radius usefully.

   Repro (tsc 5.9.3, --target es2022 --module esnext):

   let _classSuper = null;
   var C = class extends _classSuper {
       // ...
       const _metadata = typeof Symbol === "function" && Symbol.metadata ? Object.create(_classSuper[Symbol.metadata] ?? null) : void 0;

   Running the emitted module with a Symbol.metadata polyfill throws TypeError: Cannot read properties of null (reading 'Symbol(metadata)'). Without the polyfill the whole expression short-circuits to void 0, so the crash is invisible until something installs Symbol.metadata — which every metadata-aware decorator library does, so it surfaces exactly where decorators are actually used.

   The legacy path is clean. Same class compiled with --experimentalDecorators --emitDecoratorMetadata emits the old __decorate([dec], L) form and never touches Symbol.metadata, so it loads fine. Anyone on experimentalDecorators is unaffected — that includes most DI frameworks, and the one I run on (@dooboostore/simple-boot), since none of them read Symbol.metadata; the failure is confined to standard decorators.

   On the expected value: the reporter's reading checks out at runtime — Object.getPrototypeOf(class extends null {}) is Function.prototype, and with the polyfill installed Function.prototype[Symbol.metadata] is undefined, so Object.create(undefined ?? null) gives the null-prototype metadata object they describe. That means the fix is to read the parent metadata from the constructor's prototype chain rather than from _classSuper directly — i.e. (_classSuper ?? Function.prototype)[Symbol.metadata], or equivalently capturing Object.getPrototypeOf(ctor).

   One test-shape suggestion since this is easy to regress: assert both halves in one fixture — the module loads (no throw) and Object.getPrototypeOf(C[Symbol.metadata]) === null. A load-only assertion passes if someone "fixes" it by skipping metadata entirely, which would silently drop inherited metadata for the normal (non-extends null) case.