<div id="app" class="demo">
   <child @greet="sayHello"></child>
  </div>
   <script src="https://unpkg.com/vue@next"></script>
   <script>
   const app = Vue.createApp({});
   app.component('child', {
     data: function() {
       return {
         name: '张三'
       }
      },
       methods: {
         handleClick() {
           this.$emit('greet', this.name);
         }
       },
       template: '<button @click="handlerClick">开始欢迎</button>',
       sayHello(name) {
         alert("Hello," + name )
       }
   })
   
.mount('#app')
     </script>