fix(tabindex): don't error when tabindex property is overridden (#1910)

* fix(tabindex): dont error when tabindex attribute is overridden

* solve for NaN

* add radix

* test rolling build

lib/checks/keyboard/tabindex.js

@@ -1 +1,6 @@
-return node.tabIndex <= 0;
+const tabIndex = parseInt(node.getAttribute('tabindex'), 10);
+
+// an invalid tabindex will either return 0 or -1 (based on the element) so
+// will never be above 0
+// @see https://www.w3.org/TR/html51/editing.html#the-tabindex-attribute
+return isNaN(tabIndex) ? true : tabIndex <= 0;

test/checks/keyboard/tabindex.js

@@ -22,4 +22,21 @@ describe('tabindex', function() {
 
 		assert.isTrue(checks.tabindex.evaluate(node));
 	});
+
+	it('should look at the attribute and not the property', function() {
+		var node = document.createElement('div');
+		node.setAttribute('tabindex', '1');
+		node.tabindex = null;
+		fixture.appendChild(node);
+
+		assert.isFalse(checks.tabindex.evaluate(node));
+	});
+
+	it('should pass if tabindex is NaN', function() {
+		var node = document.createElement('div');
+		node.setAttribute('tabindex', 'foobar');
+		fixture.appendChild(node);
+
+		assert.isTrue(checks.tabindex.evaluate(node));
+	});
 });