Add a CustomElements.instanceOf(obj, actor) method to support custom …

…elements implemented on systems without Object.__proto__ (e.g. IE10).

src/CustomElements.js

@@ -407,6 +407,32 @@ if (useNative) {
   scope.upgrade = upgradeElement;
 }
 
+// Create a custom 'instanceof'. This is necessary when CustomElements
+// are implemented via a mixin strategy, as for example on IE10.
+var isInstance;
+if (!Object.__proto__ && !useNative) {
+  isInstance = function(obj, ctor) {
+    var p = obj;
+    while (p) {
+      // NOTE: this is not technically correct since we're not checking if
+      // an object is an instance of a constructor; however, this should
+      // be good enough for the mixin strategy.
+      if (p === ctor.prototype) {
+        return true;
+      }
+      p = p.__proto__;
+    }
+    return false;
+  }
+} else {
+  isInstance = function(obj, base) {
+    return obj instanceof base;
+  }
+}
+
+// exports
+scope.instanceof = isInstance;
+
 // bc
 document.register = document.registerElement;
 

test/js/customElements.js

@@ -413,6 +413,36 @@
       CustomElements.takeRecords();
       assert.deepEqual(['a', 'b', 'c', 'd', 'e'], log);
   });
+
+  test('instanceof', function() {
+    var p = Object.create(HTMLElement.prototype);
+    var PCtor = document.registerElement('x-instance', {prototype: p});
+    var x = document.createElement('x-instance');
+    assert.isTrue(CustomElements.instanceof(x, PCtor), 'instanceof failed for x-instance');
+
+    var p2 = Object.create(PCtor.prototype);
+    var P2Ctor = document.registerElement('x-instance2', {prototype: p2});
+    var x2 = document.createElement('x-instance2');
+    assert.isTrue(CustomElements.instanceof(x2, P2Ctor), 'instanceof failed for x-instance2');
+    assert.isTrue(CustomElements.instanceof(x2, PCtor), 'instanceof failed for x-instance2');
+  });
+
+
+  test('instanceof typeExtension', function() {
+    var p = Object.create(HTMLButtonElement.prototype);
+    var PCtor = document.registerElement('x-button-instance', {prototype: p, extends: 'button'});
+    var x = document.createElement('button', 'x-button-instance');
+    assert.isTrue(CustomElements.instanceof(x, PCtor), 'instanceof failed for x-button-instance');
+    assert.isTrue(CustomElements.instanceof(x, HTMLButtonElement), 'instanceof failed for x-button-instance');
+
+    var p2 = Object.create(PCtor.prototype);
+    var P2Ctor = document.registerElement('x-button-instance2', {prototype: p2, extends: 'button'});
+    var x2 = document.createElement('button','x-button-instance2');
+    assert.isTrue(CustomElements.instanceof(x2, P2Ctor), 'instanceof failed for x-button-instance2');
+    assert.isTrue(CustomElements.instanceof(x2, PCtor), 'instanceof failed for x-button-instance2');
+    assert.isTrue(CustomElements.instanceof(x2, HTMLButtonElement), 'instanceof failed for x-button-instance2');
+  });
+
 });
 
 htmlSuite('customElements (html)', function() {