vue component - How to use Vue.js in an external js file

Why is it that I can't get a basic hello world to print using Vue when I put it in an external js file, yet when I put it in the html file surrounded by a script tag, it works fine

When I copy and paste the contents of the that script tag inside a file called js_vue.js it doesn't work at all

I'm new to learning Vue so I don't tell get what's going on here. Surely there's a way to put all my js code in a separate file like I would with raw html and jquery

    
    

    
    





    {{ message }}

As a Snippet:

var app = new Vue({
  el: '#app',
  data: {
    message: 'Hello Vue!'
  }
})

  {{ message }}