Output dates between selected days via indexOf

I make a datepicker on vue3, When choosing two dates, I need the days between the selected dates to be hover. I use the “indexOf” method, but the desired result is not obtained

<div
            v-for="date in daysInMonth(currentYear, currentMonthInNumber, firstDay, lastDay)"
            :key="date"
            ref="date"
            class="day"
            :class="{ active: date === firstDay || date === lastDay, between: between.indexOf(date)}"
            @click="choosingDates(date)" > {{ date }} </div>
<script>
      firstDay: false,
      between: [],
      lastDay: false,
      firstDaySelected: false,
    };
  },
  methods: {
    choosingDates(date) {
      if (this.firstDay === false) {
        this.firstDay = date;
      } else if (this.lastDay === false) {
        this.lastDay = date;;
      }
    },

With this code, “between” hover all days of the month (css styles are written)
:class=”{ between: between.indexOf(date)} ”
what i do wrong