[Vuejs]-How do I pass websocket instance to vue.js components and then call send() on it?

Try to create the socket instance in the global window scope. It will be created before Vue app is mounted and can be accessed across your app.

App.vue

<script>
 window.socket = new WebSocket('ws://localhost:3000/websocket');
 export default {
    name: 'app'
 };
</script>

HelloWorld.vue

methods: {
  send: () => {
    window.socket.addEventListener('open', () => {
      window.socket.send('Hello Server!');
    });
  },
},