IN operator now returns false if string indexer is used to search for property and returns null

src/AngleSharp.Js.Tests/OperatorsTests.cs

@@ -0,0 +1,47 @@
+namespace AngleSharp.Js.Tests
+{
+    using NUnit.Framework;
+    using System.Threading.Tasks;
+
+    [TestFixture]
+    public class OperatorsTests
+    {
+        [Test]
+        public async Task InOperatorExistingAttribute()
+        {
+
+            var result = await "'action' in document.createElement('form')".EvalScriptAsync();
+            Assert.AreEqual("True", result);
+        }
+
+        [Test]
+        public async Task InOperatorNonExistingAttribute()
+        {
+
+            var result = await "'action' in document.createElement('div')".EvalScriptAsync();
+            Assert.AreEqual("False", result);
+        }
+
+        [Test]
+        public async Task InOperatorInputWithinForm()
+        {
+            var result = await "'input1' in new DOMParser().parseFromString(`<form><input name='input1'/></form>`, 'text/html').body.firstChild".EvalScriptAsync();
+            Assert.AreEqual("True", result);
+        }
+
+        [Test]
+        public async Task InOperatorInputWithinDiv()
+        {
+            var result = await "'input1' in new DOMParser().parseFromString(`<div><input name='input1'/></div>`, 'text/html').body.firstChild".EvalScriptAsync();
+            Assert.AreEqual("False", result);
+        }
+
+        [Test]
+        public async Task InOperator_Issue104()
+        {
+            var result = await "'somethingThatDoesntExist' in document.createElement('form')".EvalScriptAsync();
+            Assert.AreEqual("False", result);
+        }
+
+    }
+}

src/AngleSharp.Js/Proxies/DomPrototypeInstance.cs

@@ -70,7 +70,15 @@ public Boolean TryGetFromIndex(Object value, String index, out PropertyDescripto
             if (_stringIndexer != null && !HasProperty(index))
             {
                 var args = new Object[] { index };
-                var prop = _stringIndexer.GetMethod.Invoke(value, args).ToJsValue(_instance);
+                var valueAtIndex = _stringIndexer.GetMethod.Invoke(value, args);
+
+                if (valueAtIndex == null)
+                {
+                    result = PropertyDescriptor.Undefined;
+                    return false;
+                }
+
+                var prop = valueAtIndex.ToJsValue(_instance);
                 result = new PropertyDescriptor(prop, false, false, false);
                 return true;
             }