面试：Vue.js嵌套组件生命周期执行顺序是什么？

有组件A，组件B，组件C，组件C是组件B的子组件，组件B又是组件A的子组件，那么直观的层级结构如下：

ComponentA
--ComponentB
----ComponentC

下面的代码表示上述结构，全局注册了三个组件Comp、childrenA 和 childrenB。在 childrenA 中引用 childrenB，在 Comp 中 引用 childrenA。

Vue.component('comp', {
    template: '<div><children-a></children-a></div>',
    beforeCreate() {
        console.log('Comp beforeCreate')
    },
    created() {
        console.log('Comp created')
    },
    beforeMount() {
        console.log('Comp beforeMount')
    },
    mounted() {
        console.log('Comp mounted')
    },
})
Vue.component('childrenA', {
    template: '<div><children-b></children-b></div>',
    beforeCreate() {
        console.log('--childrenA beforeCreate')
    },
    created() {
        console.log('--childrenA created')
    },
    beforeMount() {
        console.log('--childrenA beforeMount')
    },
    mounted() {
        console.log('--childrenA mounted')
    },
})
Vue.component('childrenB', {
    template: '<div>{{text}}</div>',
    data() {
        return {
            text: 'childrenB'
        }
    },
    beforeCreate() {
        console.log('----childrenB beforeCreate')
    },
    created() {
        console.log('----childrenB created')
    },
    beforeMount() {
        console.log('----childrenB beforeMount')
    },
    mounted() {
        console.log('----childrenB mounted')
    },
})
new Vue({
    el: '#app'
})

输出结果如下：

Comp beforeCreate
Comp created
Comp beforeMount
--childrenA beforeCreate
--childrenA created
--childrenA beforeMount
----childrenB beforeCreate
----childrenB created
----childrenB beforeMount
----childrenB mounted
--childrenA mounted
Comp mounted

可以看到，先执行父级的beforeCreate、created 和 beforeMount，然后再去执行子组件的beforeCreate、created 和 beforeMount，如果子组件下面没有子组件了，就执行 mounted，然后再返回父级执行 mounted。