Fix reflection errors when explicitly “false”.

CHANGELOG.md

@@ -15,6 +15,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   predicted from developers (#351).
 
 ### Fixed
+- Explicitly setting `reflect` to `false` now works. Previously, setting this
+  explicitly (versus omitting entirely) would cause a runtime error (#354).
 - Custom element constructors are no longer invoked during template analysis.
   Previously, the internal computation and caching of a `DocumentFragment` would
   cause custom element’s constructors to run. Now, an inert document is used at

test/test-reflected-properties.js

@@ -24,6 +24,10 @@ class TestElement extends XElement {
         initial: false,
         reflect: true,
       },
+      explicitlyNotReflected: {
+        type: Boolean,
+        reflect: false,
+      },
     };
   }
   static template(html) {
@@ -104,3 +108,14 @@ it('does not reflect next false value (Boolean)', async () => {
   await Promise.resolve();
   assert(el.hasAttribute('boolean-property-true'));
 });
+
+// Setting “reflect: false” explicitly should behave the same as omitting it.
+//  See https://github.com/Netflix/x-element/issues/353.
+it('does not error when reflect is explicitly set to false', async () => {
+  const el = document.createElement('test-element');
+  document.body.append(el);
+  el.explicitlyNotReflected = true;
+  await Promise.resolve();
+  assert(el.explicitlyNotReflected === true);
+  assert(el.hasAttribute('explicitly-not-reflected') === false);
+});

x-element.js

@@ -563,6 +563,8 @@ export default class XElement extends HTMLElement {
           : host.setAttribute(property.attribute, serialization);
         hostInfo.reflecting = false;
       };
+    } else {
+      delete property.reflect;
     }
   }