<div id="app">
  <button @click="add">Add</button>
  <button @click="remove">Remove</button>
  <transition-group name="list" tag="p">
    <span v-for="item in items" :key="item" class="list-item">
      {{ item }}
    </span>
  </transition-group>
</div>

html, body {
  display: grid;
  justify-content: center;
  align-content: center;
  height: 100%;
  
  font-family: sans-serif;
  font-size: 1rem;
}

button {
  margin-right: 10px;
}

.list-item {
  display: inline-block;
  margin-right: 10px;
}

.list-enter-active,
.list-leave-active {
  transition: all 1s ease;
}

.list-enter-from,
.list-leave-to {
  opacity: 0;
  transform: translateY(30px);
}

const app = Vue.createApp({
  data: function () {
    return {
      items: [1, 2, 3, 4, 5, 6, 7, 8, 9],
      nextNum: 10
    }
  },
  methods: {
    randomIndex: function () {
      return Math.floor(Math.random() * this.items.length)
    },
    add: function () {
      this.items.splice(this.randomIndex(), 0, this.nextNum++)
    },
    remove: function () {
      this.items.splice(this.randomIndex(), 1)
    }
  }
})

const vm = app.mount('#app')