Even though the original solution is effective and impressive, achieving the same result can be done in a simpler way using Vue.js watchers:

<template>
    {{ timerCount }}
</template>

<script>

    export default {

        data() {
            return {
                timerCount: 30
            }
        },

        watch: {

            timerCount: {
                handler(value) {

                    if (value > 0) {
                        setTimeout(() => {
                            this.timerCount--;
                        }, 1000);
                    }

                },
                immediate: true // This ensures the watcher is triggered upon creation
            }

        }
    }

</script>

The advantage of using this approach is that resetting the timer immediately is as simple as changing the value of timerCount.

If you want to implement play/pause functionality for the timer, you can do so by following this method (although it may round to the nearest second):

<template>
    {{ timerCount }}
</template>

<script>

    export default {

        data() {
            return {
                timerEnabled: true,
                timerCount: 30
            }
        },

        watch: {

            timerEnabled(value) {
                if (value) {
                    setTimeout(() => {
                        this.timerCount--;
                    }, 1000);
                }
            },

            timerCount: {
                handler(value) {

                    if (value > 0 && this.timerEnabled) {
                        setTimeout(() => {
                            this.timerCount--;
                        }, 1000);
                    }

                },
                immediate: true // This ensures the watcher is triggered upon creation
            }

        }

        methods: {

            play() {
                this.timerEnabled = true;
            },

            pause() {
                this.timerEnabled = false;
            }

        }

    }

</script>

Presented here is a custom component designed for a countdown timer:

<template>
  <div>
    <slot :hour="hour" :min="min" :sec="sec"></slot>
  </div>
</template>

<script>
export default {
  props : {
    endDate : {  // specify the date object representing when the timer should end
      type : Date,
      default(){
        return new Date()
      }
    },
    negative : {  // optional, indicating whether the countdown should continue into negative values after reaching 0
      type : Boolean,
      default : false
    }
  },
  data(){
    return{
      now : new Date(),
      timer : null
    }
  },
  computed:{
    hour(){
      let h = Math.trunc((this.endDate - this.now) / 1000 / 3600);
      return h>9?h:'0'+h;
    },
    min(){
      let m = Math.trunc((this.endDate - this.now) / 1000 / 60) % 60;
      return m>9?m:'0'+m;
    },
    sec(){
      let s = Math.trunc((this.endDate - this.now)/1000) % 60
      return s>9?s:'0'+s;
    }
  },
  watch : {
    endDate : {
      immediate : true,
      handler(newVal){
        if(this.timer){
          clearInterval(this.timer)
        }
        this.timer = setInterval(()=>{
          this.now = new Date()
          if(this.negative)
            return
          if(this.now > newVal){
            this.now = newVal
            this.$emit('endTime')
            clearInterval(this.timer)
          }
        }, 1000)
      }
    }
  },
  beforeDestroy(){
    clearInterval(this.timer)
  }
}
</script>

Transform it into a reusable component.

<header>
    <div id="app">
        <timer></timer>
        <timer></timer>
        <timer></timer>
    </div>
    <script>
        Vue.component('timer', {
            template: '<button v-on:click="startCountdown()">{{ countdown }}</button>',
            data: function () {
                return {
                    countdown: 10,
                    startCountdown() {
                        if (this.countdown > 0) {
                            setTimeout(() => {
                                this.countdown -= 1
                                this.startCountdown();
                            }, 1000)
                        }
                    }
                }
            }
        })

        const app = new Vue({
            el: '#app'
        })
    </script>
</header>

If you happen to be using Luxon's DateTime object instead of the native JS Date object, here is a sample code snippet for a CountDownTimer component:

<template>
  <span v-if="timer">
    {{ timeCalculated }}
  </span>
</template>

<script>
import { DateTime } from 'luxon'

export default {
  name: 'CountDownTimer',

  props: {
    endDate: {
      type: String,
      required: true
    }
  },

  data () {
    return {
      now: DateTime.local(),
      timer: null
    }
  },

  computed: {
    timeCalculated () {
      const endDateDateTimeObj = DateTime.fromISO(this.endDate)
      const theDiff = endDateDateTimeObj.diff(this.now, ['hours', 'minutes', 'seconds'])

      return `${theDiff.hours}:${theDiff.minutes}:${Math.round(theDiff.seconds)}`
    }
  },

  watch: {
    endDate: {
      immediate: true,

      handler (endDateTimeStr) {
        const endDateTimeObj = DateTime.fromISO(endDateTimeStr)

        if (this.timer) {
          clearInterval(this.timer)
        }

        this.timer = setInterval(() => {
          this.now = DateTime.local()

          if (this.now > endDateTimeObj) {
            this.now = endDateTimeObj
            clearInterval(this.timer)
          }
        }, 1000)
      }
    }
  },

  beforeDestroy () {
    clearInterval(this.timer)
  }
}
</script>

In my situation, the endDate parameter has a String type due to JSON retrieval. You can easily convert it back to the original DateTime object if needed.

Utilize timeframes.

<template>
  <div>{{ clock }}</div>
</template>

<script>
export default {
  name: 'Clock',

  props: ['minutes'],

  data: () => ({
    interval: undefined,
    deadline: new Date(0, 0, 0),
    current_time: new Date(0, 0, 0, 0, this.minutes)
  }),

  computed: {
    clock: {
      get() {
        return this.current_time.getMinutes();
      },

      set(t) {
        this.current_time = new Date(0, 0, 0, 0, this.current_time.getMinutes() + t);
      }
    }
  },

  methods: {
    startTimer() {
      this.deadline >= this.current_time
        ? clearInterval(this.interval)
        : (this.clock = -1);
    }
  },

  created() {
    this.interval = setInterval(this.startTimer, 1000);
  }
};
</script>

If you're searching for the most efficient way to achieve this task, utilizing the setInterval function is key. This method is inherently recursive, eliminating unnecessary code clutter. Assign it to a variable called ''interval'' so that you can easily halt the recursion using "clearInterval".

Solution to the specified query:

data () {
   return {
      countDown: 10
   }
},

interval = setInterval(() => { 
    if(this.countDown == 0) clearInterval(interval)
    this.countDown--; 
}, 1000)

For those seeking a typical timer implementation:

// mounted
interval = setInterval(() => { this.seconds += 1;  }

//html or computed
Math.floor(this.seconds/ 60) % 60 // seconds
Math.floor(this.recordTimer / (60 * 60)) % 60 // hours