Use it as follows to get a completion
1. create global.d.ts at resources/js with the following

import Echo from 'laravel-echo';
import Pusher from 'pusher-js';

declare global {
    interface Window {
        Pusher: Pusher;
        // replace "pusher" with "ably", "reverb" or whatever you're using
        Echo: Echo<"pusher">; 
    }
}

2. access it via window.Echo and you'll get a completion list