example.vue

<template>
  <p>{{ counterInfo.currentValue }} / {{ counterInfo.nextValue }}</p>
  <button @click="pokus().a(50)">Pokus</button>
</template>

<script>
      methods: {
        pokus() {
            return {
                a: n => {
                    this.counter = n;
                },
            };
        },
    },
    computed: {
        counterInfo() {
            return {
                currentValue: this.counter,
                nextValue: this.counter + 1,
                // tohle nejde :D
                beforeValue: () => {
                    return this.counter - 1;
                },
            };
        },
    },
  
</script>