[Breaking] non-Date objects with different [[Prototypes]] are not equal

Date objects are equal when:
 - the timestamp is the same
 - if strict, the [[Prototype]] is the same
 - own properties match, same as normal objects

.eslintrc

@@ -16,6 +16,7 @@
       "files": ["example/**", "test/**"],
       "rules": {
         "array-bracket-newline": 0,
+        "max-lines": 0,
         "max-params": 0,
         "max-statements": 0,
         "no-console": 0,

index.js

@@ -7,6 +7,7 @@ var isArray = require('isarray');
 var isDate = require('is-date-object');
 
 var getTime = Date.prototype.getTime;
+var gPO = Object.getPrototypeOf;
 
 function deepEqual(actual, expected, options) {
   var opts = options || {};
@@ -51,7 +52,7 @@ function isBuffer(x) {
 }
 
 function objEquiv(a, b, opts) {
-  /* eslint max-statements: [2, 50] */
+  /* eslint max-statements: [2, 60] */
   var i, key;
   if (typeof a !== typeof b) { return false; }
   if (isUndefinedOrNull(a) || isUndefinedOrNull(b)) { return false; }
@@ -72,9 +73,13 @@ function objEquiv(a, b, opts) {
     return a.source === b.source && flags(a) === flags(b);
   }
 
-  if (isDate(a) && isDate(b)) {
-    return getTime.call(a) === getTime.call(b);
-  }
+  var aIsDate = isDate(a);
+  var bIsDate = isDate(b);
+  if (aIsDate !== bIsDate) { return false; }
+  if (aIsDate || bIsDate) { // && would work too, because both are true or both false here
+    if (getTime.call(a) !== getTime.call(b)) { return false; }
+    if (opts.strict && gPO && gPO(a) !== gPO(b)) { return false; }
+  } else if (gPO && gPO(a) !== gPO(b)) { return false; } // non-Dates always compare [[Prototype]]s
 
   var aIsBuffer = isBuffer(a);
   var bIsBuffer = isBuffer(b);

test/cmp.js

@@ -90,7 +90,13 @@ test('arguments class', function (t) {
 test('dates', function (t) {
   var d0 = new Date(1387585278000);
   var d1 = new Date('Fri Dec 20 2013 16:21:18 GMT-0800 (PST)');
-  t.deepEqualTest(d0, d1, 'equivalent Dates', true, true);
+
+  t.deepEqualTest(d0, d1, 'two Dates with the same timestamp', true, true);
+
+  d1.a = true;
+
+  t.deepEqualTest(d0, d1, 'two Dates with the same timestamp but different own properties', false, false);
+
   t.end();
 });
 
@@ -342,3 +348,29 @@ test('functions', function (t) {
 
   t.end();
 });
+
+test('[[Prototypes]]', { skip: !Object.getPrototypeOf }, function (t) {
+  function C() {}
+  var instance = new C();
+  delete instance.constructor;
+
+  t.deepEqualTest({}, instance, 'two identical objects with different [[Prototypes]]', false, false);
+
+  t.test('Dates with different prototypes', { skip: !Object.setPrototypeOf }, function (st) {
+    var d1 = new Date(0);
+    var d2 = new Date(0);
+
+    t.deepEqualTest(d1, d2, 'two dates with the same timestamp', true, true);
+
+    var newProto = {};
+    Object.setPrototypeOf(newProto, Date.prototype);
+    Object.setPrototypeOf(d2, newProto);
+    st.ok(d2 instanceof Date, 'd2 is still a Date instance after tweaking [[Prototype]]');
+
+    t.deepEqualTest(d1, d2, 'two dates with the same timestamp and different [[Prototype]]', true, false);
+
+    st.end();
+  });
+
+  t.end();
+});