fix is undefined when component has no props bug (vuejs#6263)

Author: hzzhaoxinhui

flow/component.js

@@ -29,7 +29,7 @@ declare interface Component {
   $slots: { [key: string]: Array<VNode> };
   $scopedSlots: { [key: string]: () => VNodeChildren };
   $vnode: VNode; // the placeholder node for the component in parent's render tree
-  $attrs: ?{ [key: string] : string };
+  $attrs: { [key: string] : string };
   $listeners: ?{ [key: string]: Function | Array<Function> };
   $isServer: boolean;
 

src/core/instance/render.js

@@ -49,16 +49,17 @@ export function initRender (vm: Component) {
   // $attrs & $listeners are exposed for easier HOC creation.
   // they need to be reactive so that HOCs using them are always updated
   const parentData = parentVnode && parentVnode.data
+
   /* istanbul ignore else */
   if (process.env.NODE_ENV !== 'production') {
-    defineReactive(vm, '$attrs', parentData && parentData.attrs, () => {
+    defineReactive(vm, '$attrs', parentData && parentData.attrs || {}, () => {
       !isUpdatingChildComponent && warn(`$attrs is readonly.`, vm)
     }, true)
     defineReactive(vm, '$listeners', vm.$options._parentListeners, () => {
       !isUpdatingChildComponent && warn(`$listeners is readonly.`, vm)
     }, true)
   } else {
-    defineReactive(vm, '$attrs', parentData && parentData.attrs, null, true)
+    defineReactive(vm, '$attrs', parentData && parentData.attrs || {}, null, true)
     defineReactive(vm, '$listeners', vm.$options._parentListeners, null, true)
   }
 }

test/unit/features/instance/properties.spec.js

@@ -146,6 +146,20 @@ describe('Instance properties', () => {
     }).then(done)
   })
 
+  // #6263
+  it('$attrs should not be undefined when no props passed in', () => {
+    const vm = new Vue({
+      template: `<foo/>`,
+      data: { foo: 'foo' },
+      components: {
+        foo: {
+          template: `<div>{{ this.foo }}</div>`
+        }
+      }
+    }).$mount()
+    expect(vm.$attrs).toBeDefined()
+  })
+
   it('warn mutating $attrs', () => {
     const vm = new Vue()
     vm.$attrs = {}