SOURCE

 var app= new Vue({
        el:"#app",
        data:{
            counter:0
            
       
        },

        watch:{
            // counter:function(counter1), 不管有没有超过30都会重置
            output:function(counter1){
                var vm = this;
                setTimeout(function(){
                    vm.counter=0;
                },5000);
            }
        },

        computed:{
            output:function(){
                return this.counter>30?"大于30":"小于30";
            }
        }

 })

<script src="https://cdn.jsdelivr.net/npm/vue@2/dist/vue.js"></script>
<div id="app">
    <button @click="counter++">加1</button>
    <button @click="counter--">减1</button> 
    <br>
    {{counter}}
    <br>
    {{output}}
</div>