From 21285dd7a30afc2c9caf950023e4a19339b5f24e Mon Sep 17 00:00:00 2001
From: Alec Gibson <12036746+alecgibson@users.noreply.github.com>
Date: Thu, 24 Sep 2026 09:55:27 +0000
Subject: [PATCH] =?UTF-8?q?=F0=9F=90=9B=20Only=20use=20the=20fallback=20pr?=
 =?UTF-8?q?ovider=20if=20`Reflect`=20has=20the=20whole=20metadata=20API?=
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Fixes #165

When `Reflect.defineMetadata` exists but the registry doesn't,
`CreateMetadataRegistry` wraps the existing `Reflect` in a fallback
provider. The provider captures five methods, and `isProviderFor` calls
`getOwnMetadataKeys` on every lookup. So if some other library put only
part of the API on `Reflect`, every metadata lookup throws
`TypeError: getOwnMetadataKeys is not a function`.

To reproduce:

1. Install `reflect-metadata@0.2.2` and `@abraham/reflection`, which
   defines `defineMetadata` but not `getOwnMetadataKeys`.
2. Import `@abraham/reflection` before `reflect-metadata`.
3. Call `Reflect.getOwnMetadata("a", {})`. It throws the error above.

With this change, the fallback is only used when all five methods it
captures are functions. Otherwise `reflect-metadata` installs its own
implementation, as it does when no metadata API is present. The
trade-off is that metadata written through an incomplete implementation
before `reflect-metadata` loads can't be read afterwards, which is
better than failing on every lookup.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>
---
 Reflect.ts           |  6 +++++-
 ReflectLite.ts       |  6 +++++-
 ReflectNoConflict.ts |  6 +++++-
 test/registry.ts     | 16 ++++++++++++++++
 4 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/Reflect.ts b/Reflect.ts
index b3468b8..dea1eb1 100644
--- a/Reflect.ts
+++ b/Reflect.ts
@@ -1497,7 +1497,11 @@ namespace Reflect {
             if (!IsUndefined(registrySymbol) &&
                 typeof root.Reflect !== "undefined" &&
                 !(registrySymbol in root.Reflect) &&
-                typeof root.Reflect.defineMetadata === "function") {
+                typeof root.Reflect.defineMetadata === "function" &&
+                typeof root.Reflect.hasOwnMetadata === "function" &&
+                typeof root.Reflect.getOwnMetadata === "function" &&
+                typeof root.Reflect.getOwnMetadataKeys === "function" &&
+                typeof root.Reflect.deleteMetadata === "function") {
                 // interoperate with older version of `reflect-metadata` that did not support a registry.
                 fallback = CreateFallbackProvider(root.Reflect);
             }
diff --git a/ReflectLite.ts b/ReflectLite.ts
index 858da75..5e523e2 100644
--- a/ReflectLite.ts
+++ b/ReflectLite.ts
@@ -1457,7 +1457,11 @@ namespace Reflect {
             if (!IsUndefined(registrySymbol) &&
                 typeof root.Reflect !== "undefined" &&
                 !(registrySymbol in root.Reflect) &&
-                typeof root.Reflect.defineMetadata === "function") {
+                typeof root.Reflect.defineMetadata === "function" &&
+                typeof root.Reflect.hasOwnMetadata === "function" &&
+                typeof root.Reflect.getOwnMetadata === "function" &&
+                typeof root.Reflect.getOwnMetadataKeys === "function" &&
+                typeof root.Reflect.deleteMetadata === "function") {
                 // interoperate with older version of `reflect-metadata` that did not support a registry.
                 fallback = CreateFallbackProvider(root.Reflect);
             }
diff --git a/ReflectNoConflict.ts b/ReflectNoConflict.ts
index f7707fd..0d50e76 100644
--- a/ReflectNoConflict.ts
+++ b/ReflectNoConflict.ts
@@ -1334,7 +1334,11 @@ function CreateMetadataRegistry(): MetadataRegistry {
     if (!IsUndefined(registrySymbol) &&
         typeof Reflect !== "undefined" &&
         !(registrySymbol in Reflect) &&
-        typeof Reflect.defineMetadata === "function") {
+        typeof Reflect.defineMetadata === "function" &&
+        typeof Reflect.hasOwnMetadata === "function" &&
+        typeof Reflect.getOwnMetadata === "function" &&
+        typeof Reflect.getOwnMetadataKeys === "function" &&
+        typeof Reflect.deleteMetadata === "function") {
         // interoperate with older version of `reflect-metadata` that did not support a registry.
         fallback = CreateFallbackProvider(Reflect);
     }
diff --git a/test/registry.ts b/test/registry.ts
index 43ca659..8b9645e 100644
--- a/test/registry.ts
+++ b/test/registry.ts
@@ -115,5 +115,21 @@ for (const { name, header, context } of suites) {
             assert.isDefined(provider2);
             assert.strictEqual(provider1, provider2);
         });
+        it("ignores incomplete implementation", () => {
+            const { missing, value } = script({ ...context })`
+                Reflect.defineMetadata = function() {};
+
+                {
+                    ${header}
+
+                    const obj = {};
+                    exports.missing = Reflect.getOwnMetadata("a", obj);
+                    Reflect.defineMetadata("a", 1, obj);
+                    exports.value = Reflect.getOwnMetadata("a", obj);
+                }
+            `;
+            assert.isUndefined(missing);
+            assert.strictEqual(value, 1);
+        });
     });
 }
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