* vue生命周期
    ``` javascript
    export default {
        name: 'Test',
        data () {
            return {
                test: 'testmsg'
            }
        },
        components: {
        },
        methods:{
            back() {
            this.$router.push('/');
            },
        },
        beforeCreate() {
            //  此时获取不到data中的变量值
            console.log(this.test)
        },
        beforeMount() {
            console.log("beforeMount函数执行了----此时获取不到真实DOM");
            console.log(document.getElementById("myDiv"));
        },
        created() {
            console.log(this.test)
        },
        mounted() {
            // mounted 挂载结束后可以获取到真实的DOM
            console.log(document.getElementById("myDiv"));
        }
    }
    ```