$tempClass = "navbar-dark bg-dark";
return view("folder.view", compact("...your other arrays", "tempClass"));

@php
$tempClass = (!empty($tempClass) ? $tempClass : 'navbar-transparent')
@endphp

<nav id="nav" class="navbar fixed-top navbar-toggleable-md navbar-expand-lg navbar-transparent">

<nav id="nav" class="navbar fixed-top navbar-toggleable-md navbar-expand-lg {{ $tempClass }}">