toFixed

{{ num | toFixed }}

{{ num | toFixed(2) }}

<div id="app">
  <p><input v-model="num"></p>
  <p>{{ num | toFixed }}</p>
  <p>{{ num | toFixed(2) }}</p>
</div>
new Vue({
  el: '#app',
  data: {
    num: 123.456
  },
  filters: {
    toFixed(num, n) {
      if (!num) return
      num = +num
      if (n > 1) {
        return num.toFixed(n)
      } else {
        return num.toFixed()
      }
    }
  }
})