vuetify data table expanded icon example

  clicked (value) {
    const index = this.expanded.indexOf(value)
    if (index === -1) {
      this.expanded.push(value)
    } else {
      this.expanded.splice(index, 1)
    }
  },

<v-data-table
  :headers="headers"
  :items="desserts"
  :expanded="expanded"
  item-key="name"
  show-expand
  class="elevation-1"
  @click:row="clicked">