Ensure properties is only called once

Author: TimvdLippe

lib/mixins/properties-mixin.js

@@ -86,8 +86,12 @@ export const PropertiesMixin = dedupingMixin(superClass => {
    if (!constructor.hasOwnProperty(JSCompiler_renameProperty('__ownProperties', constructor))) {
      let props = null;
 
-     if (constructor.hasOwnProperty(JSCompiler_renameProperty('properties', constructor)) && constructor.properties) {
-       props = normalizeProperties(constructor.properties);
+     if (constructor.hasOwnProperty(JSCompiler_renameProperty('properties', constructor))) {
+       const properties = constructor.properties;
+
+       if (properties) {
+        props = normalizeProperties(properties);
+       }
      }
 
      constructor.__ownProperties = props;

test/unit/polymer.properties-mixin.html

@@ -44,6 +44,8 @@
 class MyElement extends PropertiesMixin(HTMLElement) {
 
   static get properties() {
+    this._calledProperties++;
+
     return {
       prop: String,
       noStomp: String,
@@ -111,6 +113,8 @@
 MyElement.prototype._calledReady = 0;
 MyElement.prototype._callAttributeChangedCallback = 0;
 
+MyElement.constructor.prototype._calledProperties = 0;
+
 customElements.define('my-element', MyElement);
 
 window.MyElement = MyElement;
@@ -236,6 +240,7 @@
     assert.equal(el._calledConnectedCallback, 1);
     assert.equal(el._calledReady, 1);
     assert.equal(el._callAttributeChangedCallback, 0);
+    assert.equal(el.constructor._calledProperties, 1);
   });
 
   test('listeners', function() {