doc: WeakSet and WeakMap comparison details

ishon19 · aduh95 · commit ae360c30dc10 · 2025-02-06T01:10:37.000+01:00
PR-URL: #56648 Reviewed-By: Luigi Pinca <luigipinca@gmail.com> Reviewed-By: James M Snell <jasnell@gmail.com>
diff --git a/doc/api/assert.md b/doc/api/assert.md
@@ -799,8 +799,10 @@ are recursively evaluated also by the following rules.
 * [`Map`][] keys and [`Set`][] items are compared unordered.
 * Recursion stops when both sides differ or both sides encounter a circular
   reference.
-* [`WeakMap`][] and [`WeakSet`][] comparison does not rely on their values. See
-  below for further details.
+* [`WeakMap`][] and [`WeakSet`][] instances are **not** compared structurally.
+  They are only equal if they reference the same object. Any comparison between
+  different `WeakMap` or `WeakSet` instances will result in inequality,
+  even if they contain the same entries.
 * [`RegExp`][] lastIndex, flags, and source are always compared, even if these
   are not enumerable properties.
 
@@ -877,23 +879,40 @@ assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol2]: 1 });
 // }
 
 const weakMap1 = new WeakMap();
-const weakMap2 = new WeakMap([[{}, {}]]);
-const weakMap3 = new WeakMap();
-weakMap3.unequal = true;
+const weakMap2 = new WeakMap();
+const obj = {};
 
+weakMap1.set(obj, 'value');
+weakMap2.set(obj, 'value');
+
+// Comparing different instances fails, even with same contents
 assert.deepStrictEqual(weakMap1, weakMap2);
-// OK, because it is impossible to compare the entries
+// AssertionError: Values have same structure but are not reference-equal:
+//
+// WeakMap {
+//   <items unknown>
+// }
+
+// Comparing the same instance to itself succeeds
+assert.deepStrictEqual(weakMap1, weakMap1);
+// OK
 
-// Fails because weakMap3 has a property that weakMap1 does not contain:
-assert.deepStrictEqual(weakMap1, weakMap3);
+const weakSet1 = new WeakSet();
+const weakSet2 = new WeakSet();
+weakSet1.add(obj);
+weakSet2.add(obj);
+
+// Comparing different instances fails, even with same contents
+assert.deepStrictEqual(weakSet1, weakSet2);
 // AssertionError: Expected inputs to be strictly deep-equal:
 // + actual - expected
 //
-//   WeakMap {
-// +   [items unknown]
-// -   [items unknown],
-// -   unequal: true
-//   }
+// + WeakSet { <items unknown> }
+// - WeakSet { <items unknown> }
+
+// Comparing the same instance to itself succeeds
+assert.deepStrictEqual(weakSet1, weakSet1);
+// OK
 ```
 
 ```cjs
@@ -969,23 +988,40 @@ assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol2]: 1 });
 // }
 
 const weakMap1 = new WeakMap();
-const weakMap2 = new WeakMap([[{}, {}]]);
-const weakMap3 = new WeakMap();
-weakMap3.unequal = true;
+const weakMap2 = new WeakMap();
+const obj = {};
 
+weakMap1.set(obj, 'value');
+weakMap2.set(obj, 'value');
+
+// Comparing different instances fails, even with same contents
 assert.deepStrictEqual(weakMap1, weakMap2);
-// OK, because it is impossible to compare the entries
+// AssertionError: Values have same structure but are not reference-equal:
+//
+// WeakMap {
+//   <items unknown>
+// }
+
+// Comparing the same instance to itself succeeds
+assert.deepStrictEqual(weakMap1, weakMap1);
+// OK
 
-// Fails because weakMap3 has a property that weakMap1 does not contain:
-assert.deepStrictEqual(weakMap1, weakMap3);
+const weakSet1 = new WeakSet();
+const weakSet2 = new WeakSet();
+weakSet1.add(obj);
+weakSet2.add(obj);
+
+// Comparing different instances fails, even with same contents
+assert.deepStrictEqual(weakSet1, weakSet2);
 // AssertionError: Expected inputs to be strictly deep-equal:
 // + actual - expected
 //
-//   WeakMap {
-// +   [items unknown]
-// -   [items unknown],
-// -   unequal: true
-//   }
+// + WeakSet { <items unknown> }
+// - WeakSet { <items unknown> }
+
+// Comparing the same instance to itself succeeds
+assert.deepStrictEqual(weakSet1, weakSet1);
+// OK
 ```
 
 If the values are not equal, an [`AssertionError`][] is thrown with a `message`
