From dbb688164ba2c16e8514bf3bed9979d42f14ba55 Mon Sep 17 00:00:00 2001
From: Devin Ivy <devinivy@gmail.com>
Date: Sun, 19 Jul 2026 18:43:04 -0400
Subject: [PATCH] fix: harden define() branding and tests

---
 src/define.ts       |  9 +++++++++
 test/define.test.ts | 46 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+)

diff --git a/src/define.ts b/src/define.ts
index 6458491..9a189a6 100644
--- a/src/define.ts
+++ b/src/define.ts
@@ -34,6 +34,14 @@ export function define<T extends object>(importMeta: ImportMeta, value: T): T {
     throw new TypeError('define() requires an object or function value');
   }
 
+  if (Object.hasOwn(value, DEFINE_BRAND)) {
+    throw new TypeError('define() was already called on this value');
+  }
+
+  if (!Object.isExtensible(value)) {
+    throw new TypeError('define() cannot brand a frozen or sealed value');
+  }
+
   const index = counters.get(url) ?? 0;
   counters.set(url, index + 1);
   const id = `${url}#${index}`;
@@ -46,5 +54,6 @@ export function define<T extends object>(importMeta: ImportMeta, value: T): T {
 /** Returns the define id of a branded value, or undefined. */
 export function getDefineId(value: unknown): string | undefined {
   if (value === null || (typeof value !== 'object' && typeof value !== 'function')) return undefined;
+  if (!Object.hasOwn(value, DEFINE_BRAND)) return undefined;
   return (value as Record<symbol, string | undefined>)[DEFINE_BRAND];
 }
diff --git a/test/define.test.ts b/test/define.test.ts
index bc11f87..925b72a 100644
--- a/test/define.test.ts
+++ b/test/define.test.ts
@@ -1,12 +1,16 @@
 import { describe, it } from 'node:test';
 import assert from 'node:assert/strict';
 import { define } from 'moroutine';
+import { getDefineId } from '../src/define.ts';
 import { limits } from './fixtures/define.ts';
 
 describe('define()', () => {
   it('returns the same object reference', () => {
     assert.equal(limits.maxBatch, 64);
     assert.equal(limits.retry, 3);
+
+    const obj = { a: 1 };
+    assert.equal(define(import.meta, obj), obj);
   });
 
   it('throws on primitive values', () => {
@@ -14,4 +18,46 @@ describe('define()', () => {
     assert.throws(() => define(import.meta, 'nope' as any), TypeError);
     assert.throws(() => define(import.meta, null as any), TypeError);
   });
+
+  it('throws when called twice on the same value', () => {
+    const obj = define(import.meta, { x: 1 });
+    assert.throws(() => define(import.meta, obj), TypeError);
+  });
+
+  it('throws when branding a frozen or sealed value', () => {
+    const frozen = Object.freeze({ x: 1 });
+    assert.throws(() => define(import.meta, frozen), TypeError);
+
+    const sealed = Object.seal({ x: 1 });
+    assert.throws(() => define(import.meta, sealed), TypeError);
+  });
+});
+
+describe('getDefineId()', () => {
+  it('returns a `${url}#N`-shaped id for a defined value', () => {
+    const id = getDefineId(limits);
+    assert.match(id!, /^.+#\d+$/);
+  });
+
+  it('gives successive define() calls in one module distinct ids', () => {
+    const a = define(import.meta, { n: 1 });
+    const b = define(import.meta, { n: 2 });
+    assert.notEqual(getDefineId(a), getDefineId(b));
+  });
+
+  it('returns undefined for unbranded objects', () => {
+    assert.equal(getDefineId({}), undefined);
+  });
+
+  it('returns undefined for primitives', () => {
+    assert.equal(getDefineId(42), undefined);
+    assert.equal(getDefineId('nope'), undefined);
+    assert.equal(getDefineId(null), undefined);
+    assert.equal(getDefineId(undefined), undefined);
+  });
+
+  it('returns undefined for a value that only inherits the brand via its prototype chain', () => {
+    const derived = Object.create(limits);
+    assert.equal(getDefineId(derived), undefined);
+  });
 });
-- 
2.51.2