Just because name and onchange function are the same in the select is doesn't work. So, it's normal that the first select doesn't work ? (https://jsfiddle.net/ay1fvruz/)
function selectTypeParty(value) {
alert(value);
}
function test(value) {
alert(value);
}
don't work <br>
<form id="mainForm">
<select name="selectTypeParty" onchange="selectTypeParty(this.value)">
<option value="melee">Melee</option>
<option value="classic">Classic</option>
</select>
</form><br>
work <br>
<form id="mainForm2">
<select name="selectTypeParty" onchange="test(this.value)">
<option value="melee">Melee</option>
<option value="classic">Classic</option>
</select>
</form><br>
work <br>
<form id="mainForm3">
<select name="test" onchange="selectTypeParty(this.value)">
<option value="melee">Melee</option>
<option value="classic">Classic</option>
</select>
</form>
Thank you.