fix: catch fake collections throwing (#100)

Fixes #99

Author: ljharb

index.js

@@ -289,13 +289,18 @@ function regexpEqual(leftHandOperand, rightHandOperand) {
  */
 
 function entriesEqual(leftHandOperand, rightHandOperand, options) {
-  // IE11 doesn't support Set#entries or Set#@@iterator, so we need manually populate using Set#forEach
-  if (leftHandOperand.size !== rightHandOperand.size) {
+  try {
+    // IE11 doesn't support Set#entries or Set#@@iterator, so we need manually populate using Set#forEach
+    if (leftHandOperand.size !== rightHandOperand.size) {
+      return false;
+    }
+    if (leftHandOperand.size === 0) {
+      return true;
+    }
+  } catch (sizeError) {
+    // things that aren't actual Maps or Sets will throw here
     return false;
   }
-  if (leftHandOperand.size === 0) {
-    return true;
-  }
   var leftHandItems = [];
   var rightHandItems = [];
   leftHandOperand.forEach(function gatherEntries(key, value) {

test/new-ecmascript-types.js

@@ -95,6 +95,13 @@ describe('ES2015 Specific', function () {
       assert(eql(mapA, mapB), 'eql(Map { a => 1, b => 2, c => 3 }, Map { a => 1, b => 2, c => 3 })');
     });
 
+    (setExists ? it : it.skip)('returns false for fake Maps', function () {
+      var maplikeSet = new Set();
+      Object.defineProperty(maplikeSet, 'constructor', { enumerable: false, value: Map });
+
+      assert(eql(maplikeSet, new Map()) === false, 'eql(Set pretending to be a Map, Map { })');
+    });
+
   });
 
   describeIf(symbolAndMapExist && typeof Map.prototype[Symbol.iterator] === 'function')('map iterator', function () {
@@ -238,6 +245,13 @@ describe('ES2015 Specific', function () {
       assert(eql(setA, setB) === true, 'eql(Set { -> }, Set { <- }) === true');
     });
 
+    (mapExists ? it : it.skip)('returns false for fake Sets', function () {
+      var setlikeMap = new Map();
+      Object.defineProperty(setlikeMap, 'constructor', { enumerable: false, value: Set });
+
+      assert(eql(setlikeMap, new Set()) === false, 'eql(Map pretending to be a Set, Set { })');
+    });
+
   });
 
   describeIf(symbolAndSetExist && typeof Set.prototype[Symbol.iterator] === 'function')('set iterator', function () {