fix: extra protection around tagName accessor

Author: tbiethman

src/getTag.js

@@ -6,6 +6,17 @@
  */
 export function getTag( el, filter )
 {
+  if (typeof el.tagName !== 'string') {
+    // If the tagName attribute has been overridden, we should 
+    // return null and not use tagName for selector generation.
+    //
+    // This can happen when a <form> element contains an <input>
+    // with an id of `tagName`. In this case, the form element's
+    // tagName property is a reference to the input element, not
+    // a string.
+    return null;
+  }
+
   const tagName = el.tagName.toLowerCase().replace(/:/g, '\\:')
 
   if (filter && !filter('tag', 'tag', tagName)) {

test/unique-selector.js

@@ -112,7 +112,6 @@ describe( 'Unique Selector Tests', () =>
     expect( uniqueSelector ).to.equal( 'span' );
   } );
 
-
   it( 'Tag', () =>
   {
     $( 'body' ).append( '<div class="test5"><span></span></div><div class="test5"><span></span></div>' );
@@ -129,6 +128,35 @@ describe( 'Unique Selector Tests', () =>
     expect( uniqueSelector ).to.equal( 'a' );
   } );
 
+  it( 'Tag - filtered due to property override', () =>
+  {
+    $( 'body' ).append(`
+      <div class="test2">
+        <form action="" method="get">
+          <div class="form-example">
+            <label for="name">Enter your name: </label>
+            <input type="text" name="name" id="tagName" required />
+          </div>
+        </form>
+      </div>
+    `);
+
+    const formNode = $( 'form' ).get( 0 );
+
+    // JSDOM doesn't actually exhibit this behavior;
+    // forcing the test to behave as a browser does.
+    Object.defineProperty(formNode, 'tagName', {
+      get: () => {
+        return $( 'input#tagName' ).get( 0 );
+      }
+    })
+
+    expect(typeof formNode.tagName).to.not.equal('string')
+
+    const uniqueSelector = unique( formNode );
+    expect( uniqueSelector ).to.equal( '.test2 > :nth-child(1)' );
+  } );
+
   it( 'Attributes', () =>
   {
     $( 'body' ).append( '<div class="test5" test="5"></div>' );