[Vuejs]-Output dates between selected days via indexOf

Try this :

<template>
  <div class="datepicker">
    <div class="month-header">
      <i class="prev-month" @click="prevMonth"></i>
      {{ currentMonth }} {{ currentYear }}
      <i class="next-month" @click="nextMonth"></i>
    </div>
    <div class="weekdays">
      <div class="weekday" v-for="weekday in weekdays" :key="weekday">
        {{ weekday }}
      </div>
    </div>
    <div class="days">
      <div
        v-for="date in daysInMonth(
          currentYear,
          currentMonthInNumber,
          firstDay,
          lastDay
        )"
        :key="date"
        class="day"
        :class="{
          active: date === firstDay || date === lastDay,
          between: between.includes(date),
        }"
        @click="chooseDate(date)"
      >
        {{ date }}
      </div>
    </div>
    {{ between }}
  </div>
</template>

<script>
export default {
  data() {
    return {
      weekdays: ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"],
      months: [
        "January",
        "February",
        "March",
        "April",
        "May",
        "June",
        "July",
        "August",
        "September",
        "October",
        "November",
        "December",
      ],
      currentMonth: "",
      currentMonthInNumber: "",
      currentYear: "",
      firstDay: false,
      between: [],
      lastDay: false,
    };
  },
  mounted() {
    const date = new Date();
    this.currentMonth = this.months[date.getMonth()];
    this.currentMonthInNumber = date.getMonth();
    this.currentYear = date.getFullYear();
  },
  methods: {
    prevMonth() {
      this.currentMonthInNumber--;
      if (this.currentMonthInNumber < 0) {
        this.currentMonthInNumber = 11;
        this.currentYear--;
      }
      this.currentMonth = this.months[this.currentMonthInNumber];
    },
    nextMonth() {
      this.currentMonthInNumber++;
      if (this.currentMonthInNumber > 11) {
        this.currentMonthInNumber = 0;
        this.currentYear++;
      }
      this.currentMonth = this.months[this.currentMonthInNumber];
    },
    daysInMonth(year, month, firstDay, lastDay) {
      let date = new Date(year, month, 1);
      let days = [];
      while (date.getMonth() === month) {
        days.push(date.getDate());
        date.setDate(date.getDate() + 1);
      }
      return days;
    },
    chooseDate(date) {
      if (this.firstDay === false) {
        this.firstDay = date;
      } else if (this.lastDay === false) {
        this.lastDay = date;
        this.setBetween();
      } else {
        this.firstDay = date;
        this.lastDay = false;
        this.between = [];
      }
    },
    setBetween() {
      if (this.firstDay > this.lastDay) {
        [this.firstDay, this.lastDay] = [this.lastDay, this.firstDay];
      }
      let date = new Date(
        this.currentYear,
        this.currentMonthInNumber,
        this.firstDay
      );
      while (date.getDate() <= this.lastDay) {
        this.between.push(date.getDate());
        date.setDate(date.getDate() + 1);
      }
    },
  },
};
</script>

<style scoped>
.datepicker {
  background-color: #fff;
  border-radius: 5px;
  box-shadow: 0 2px 5px 0 rgba(0, 0, 0, 0.1);
  padding: 20px;
  text-align: center;
}

.month-header {
  display: flex;
  justify-content: space-between;
  margin-bottom: 20px;
}

.prev-month,
.next-month {
  cursor: pointer;
}

.weekdays {
  display: flex;
}

.weekday {
  flex: 1;
  font-weight: bold;
  padding: 10px 0;
}

.days {
  display: grid;
  grid-template-columns: repeat(7, 1fr);
  grid-gap: 10px;
  margin-top: 20px;
}

.day {
  background-color: #fff;
  border: 1px solid #ddd;
  border-radius: 5px;
  cursor: pointer;
  height: 40px;
  line-height: 40px;
  text-align: center;
  user-select: none;
}

.day.active {
  background-color: #3c8dbc;
  color: #fff;
}

.day.between {
  background-color: #ddd;
}
</style>