projectFolder
  ...
  src
    components
      Component.vue
    utils
      numberWithComma.js

<template>
  <span>{{ valueChanged }}</span>
</template>

<script>
import numberWithComma from '../utils/numberWithComma.js';
// или настройте алиасы, тогда путь к утилитам может выглядеть так: '@/numberWithComma.js'

export default {
  data: ({
    value: 0.123456789  
  }),
  computed: {
    valueChanged() {
      return numberWithComma(this.value);
    }
  }
};
</script>

const numberWithComma = (x) => x.toString().replace(/\B(?<!\.\d*)(?=(\d{3})+(?!\d))/g, ", ");

export default numberWithComma;