title: vue-property-decorator: How to modify the parameters of @Emit
body: <p>When I saw <code>@Emit</code>, I saw the example on GitHub.</p>
  
  <pre><code>import { Vue, Component, Emit } from 'vue-property-decorator'
     @Component
     export default class YourComponent extends Vue {
        count = 0
        @Emit()
        addToCount(n: number) {
           this.count += n
        }
     }
  </code></pre>
  
  <p>is equivalent to</p>
  
  <pre><code>export default {
    data() {
      return {
        count: 0
      }
    },
    methods: {
      addToCount(n) {
        this.count += n
        this.$emit('add-to-count', n)
      }
    }
  }
  </code></pre>
  
  <p><strong>Problem:</strong></p>
  
  <p>I want to know how to modify this parameter <code>n</code>.
  I use return statement but it doesn't work.</p>
  
  <pre><code>@Emit()
    startChange(num:any){
  }
  </code></pre>
  
  <p>The type of the <code>num</code> parameter is <code>Date</code>.
  I want to transform it into <code>timestamp</code> and pass it on to the parent component.
  How do you do it?</p>