$emitメソッドで子コンポーネントから親コンポーネントに情報を渡す

Vue.js 
<!DOCTYPE html>
<html lang="ja">
<head>
  <meta charset="UTF-8">
  <title>Vue.js</title>
</head>
<body>
<div id="app">
  <p>現在値：{{ current }}</p>

  <my-counter step="1" v-on:plus="onplus"></my-counter>
  <my-counter step="2" v-on:plus="onplus"></my-counter>
  <my-counter step="-1" v-on:plus="onplus"></my-counter>
</div>
<script src="https://cdn.jsdelivr.net/npm/vue@2.6.10/dist/vue.js"></script>
<script src="https://cdn.jsdelivr.net/npm/lodash@4.17.21/lodash.min.js"></script>
<script src="js/emit_basic.js"></script>
</body>
</html>

Vue.component('my-counter', {
  props: [ 'step'],
  template: `<button type="button" v-on:click="onclick">
  {{ step }}</button>`,
  methods: {
    onclick() {
      this.$emit('plus', Number(this.step));
    }
  }
});

new Vue({
  el: '#app',
  data: {
    current: 0
  },
  methods: {
    onplus: function(e) {
      this.current += e;
    }
  }
})