use mix env variables

resources/assets/js/bootstrap.js

@@ -50,7 +50,7 @@ if (token) {
 
 // window.Echo = new Echo({
 //     broadcaster: 'pusher',
-//     key: 'your-pusher-key',
-//     cluster: 'mt1',
+//     key: process.env.MIX_PUSHER_APP_KEY,
+//     cluster: process.env.MIX_PUSHER_APP_CLUSTER,
 //     encrypted: true
 // });