test(runtime-dom): treat 'class' as non-attr vuejs#3173

Author: heyunfei.i

packages/runtime-dom/__tests__/patchAttrs.spec.ts

@@ -34,4 +34,13 @@ describe('runtime-dom: attrs patching', () => {
     patchProp(el, 'onwards', 'a', null)
     expect(el.getAttribute('onwards')).toBe(null)
   })
+
+  // #3173
+  test('treat class as non-attr', () => {
+    const el = document.createElement('div')
+    patchProp(el, 'class', null, null)
+    expect(el.getAttribute('class')).toBe('')
+    patchProp(el, 'class', undefined, undefined)
+    expect(el.getAttribute('class')).toBe('')
+  })
 })

packages/runtime-dom/__tests__/patchClass.spec.ts

@@ -9,6 +9,8 @@ describe('runtime-dom: class patching', () => {
     expect(el.className).toBe('foo')
     patchProp(el, 'class', null, null)
     expect(el.className).toBe('')
+    patchProp(el, 'class', undefined, undefined)
+    expect(el.className).toBe('')
   })
 
   test('transition class', () => {