<template>
  <div id="counter">
    Counter: {{ counter }}
    
  </div>
</template>

<script>
export default {
  name: 'HelloWorld',
  data() {
      return {
        counter: 0
      }
      },
  Mounted() {
    setInterval(() => {
      this.counter++
    },1000)
  }
  }
</script>

应改为：

<template>
  <div id="counter">
    Counter: {{ counter }}
    
  </div>
</template>

<script>
export default {
  name: 'HelloWorld',
  data() {
      return {
        counter: 0
      }
      },
  mounted() {
    setInterval(() => {
      this.counter++
    },1000)
  }
  }
</script>