Vue

HTML

// pages/index.vue
<template>
  <h1>Error: navigator is not defined</h1>
</template>

<script>
    import { u } from '../assets/scripts/utils' // Error: navigator is not defined

    export default {
        created () {
            if (process.client) {
                console.log(u)
            }
        }
    }
</script>

Vue

// assets/scripts/utils.js
export const u = navigator.userAgent