JSFiddle - React, Tailwind, and code Playground

HTML

<span id="prefix">Szukam</span>

CSS

.on {
  background: green;
}

.off {
  background: red;
}

JavaScript

var prefix = "on";
switch (prefix) {
    case 'on':
        $("#prefix").addClass("on");
        break;
    case 'off':
        $("#prefix").addClass("off");
        break;
    default:
      console.log('Nic z tego');
}