There were a few issues:
A common problem people run into with the button tag is by default, it has a type of ‘submit’ which will submit the form. There are a few ways to disable this, my preferred method is to set the type as button.
Another issue is you don’t have any content in the select box, which was causing an error trying to get the value of a select box with no options that can be selected.
I updated your radios, to use querySelectorAll and look for :checked that way you don’t need to create an if statement.
I also removed the paragraphArray from addParagraph() since it is a global variable.
<html>
<head>
<title>Promemoria esercizi</title>
</head>
<body>
<ul id="paragraphList">
</ul>
<form id="paragraphForm">
<br></br>
<textarea id="insertParagraph" rows="5" cols="100"></textarea>
<label>Inserisci il paragrafo:
<input type="radio" id="insertType" name="InsertType" value="last">In fondo
<input type="radio" id="insertType" name="InsertType" value="before">Dietro il paragrafo
<select id="beforeParagraph"></select><br></br>
</label>
<button type="button" id="add" onclick="addParagraph()">Inserisci</button><br></br>
</form>
<script>
var paragraphArray = [];
document.getElementById("paragraphList").innerHTML = paragraphArray;
function addParagraph(){
var text = document.getElementById("insertParagraph").value;
var radio = document.querySelectorAll("#insertType:checked");
var selectedInsertType = "";
var ul = document.getElementById("paragraphList");
var sel = document.querySelector("#beforeParagraph");
var selectedBeforeParagraph = (sel.selectedIndex > -1) ? sel.options[sel.selectedIndex].value : "";
for(i = 0; i < radio.length; i++){
selectedInsertType = radio[i].value;
}
if(selectedInsertType = "last"){
paragraphArray.push(text);
}else if(selectedInsertType = "before"){
paragraphArray.splice((selectedBeforeParagraph-1), 0, text);
}
var newChoice = document.createElement("option");
newChoice.value = paragraphArray.length.toString();
newChoice.text = paragraphArray.length.toString();
for(i = 0; i < paragraphArray.length; i++){
var li = document.createElement("li");
li.innerHTML = paragraphArray[i];
}
document.getElementById("paragraphList").innerHTML = paragraphArray;
}
</script>
</body>
</html>