fix: CTabs - unable posibility of undefined active tab

Author: woothu

src/components/tabs/CTab.vue

@@ -31,16 +31,13 @@ const props = Object.assign(propsFactory(), {
 
 export default {
   name: 'CTab',
-  inject: ['distributed', 'tabs'],
+  inject: ['distributed', 'tabs', 'initialTab'],
   components: {
     CLink
   },
   beforeMount () {
     this.index = Array.from(this.$parent.$children).indexOf(this)
-    if (
-      this.active && this.distributed.header 
-      && typeof this.tabs.activeTab !== 'number'
-    ) {
+    if (this.active && this.distributed.header && this.initialTab === undefined) {
       this.distributed.changeTabTo(this.index)
     }
   },

src/components/tabs/CTabs.vue

@@ -37,7 +37,7 @@ export default {
     Object.defineProperty(tabs, 'fade', {
       get: () => this.fade
     })
-    return { tabs }
+    return { tabs, initialTab: this.activeTab }
   },
   props: {
     fill: Boolean,
@@ -61,12 +61,12 @@ export default {
   },
   data () {
     return {
-      activeTabIndex: this.activeTab
+      activeTabIndex: this.activeTab || 0
     }
   },
   watch: {
     activeTab (val) {
-      this.activeTabIndex = val
+      this.activeTabIndex = val || 0
     }
   },
   computed: {

src/components/tabs/tests/CTabs.spec.js

@@ -87,6 +87,10 @@ describe(ComponentName, () => {
       activeTab: 1
     })
     expect(defaultWrapper.vm.activeTabIndex).toBe(1)
+    defaultWrapper.setProps({
+      activeTab: null
+    })
+    expect(defaultWrapper.vm.activeTabIndex).toBe(0)
   })
   it('properly changes vertical classes', () => {
     defaultWrapper.setProps({ vertical: true })