Multiple text-areas being added after an update event
Change your JS to this:
let editor;
function update() {
var select = $('#lang').val();
editor = editor ?? CodeMirror.fromTextArea($('#text'), {
theme: "blackboard",
});
editor.setOption('mode', select);
}
update();
This makes it so if editor already exists, it is not created again, but instead re-used.