JSFiddle - React, Tailwind, and code Playground

HTML

<select id="telefoonkeuze" onchange="Checkfunction()">
        <option value="gsm" id="gsm">gsm</option>
        <option value="telefoon">telefoon</option>
</select>

JavaScript

var gsm = document.getElementById("gsm").value;

        Checkfunction = function() {
            var x = document.getElementById("telefoonkeuze").value;
            if (x == gsm) {
                alert("nice it works!");
            }
        }