isMap

Author: mtrezza

eslint.config.js

@@ -60,6 +60,10 @@ module.exports = [
           selector: "BinaryExpression[operator='instanceof'][right.name='Promise']",
           message: "Use Utils.isPromise() instead of instanceof Promise (cross-realm safe).",
         },
+        {
+          selector: "BinaryExpression[operator='instanceof'][right.name='Map']",
+          message: "Use Utils.isMap() instead of instanceof Map (cross-realm safe).",
+        },
       ]
     },
   },

spec/Utils.spec.js

@@ -333,6 +333,7 @@ describe('Utils', () => {
     });
     it('should return true for a cross-realm Map', () => {
       const crossRealmMap = vm.runInNewContext('new Map()');
+      // eslint-disable-next-line no-restricted-syntax -- intentional: proving instanceof fails cross-realm
       expect(crossRealmMap instanceof Map).toBe(false);
       expect(Utils.isMap(crossRealmMap)).toBe(true);
     });

spec/eslint.config.js

@@ -81,6 +81,10 @@ module.exports = [
           selector: "BinaryExpression[operator='instanceof'][right.name='Promise']",
           message: "Use Utils.isPromise() instead of instanceof Promise (cross-realm safe).",
         },
+        {
+          selector: "BinaryExpression[operator='instanceof'][right.name='Map']",
+          message: "Use Utils.isMap() instead of instanceof Map (cross-realm safe).",
+        },
       ],
     },
   },

src/Utils.js

@@ -498,7 +498,7 @@ class Utils {
   static getCircularReplacer() {
     const seen = new WeakSet();
     return (key, value) => {
-      if (value instanceof Map) {
+      if (Utils.isMap(value)) {
         return Object.fromEntries(value);
       }
       if (value instanceof Set) {