Loading items into array from SQL Server
Main issues

Access


DOM operations take time. You're selecting the element N^2 + 1 times. This can be reduced to 1: document.getElementById('itemSelect')
You're constantly getting arrayValues[i], which only needs to be done once.


Style


You should put spaces after commas: arg, arg2
You should put spaces before and after operations, for readability: var name = value
Spaces after semicolons: i &lt; length; i++


Rewrite

function loadItem(arrayValues, arrayNames) {
    var itemSelect = document.getElementById('itemSelect'),
        j = itemSelect.options.length;
    for(var i = 0, length = arrayValues.length; i &lt; length; i++) {
        let val = arrayValues[i];
        if(!existsElement(itemSelect, val)) {
            itemSelect.options[j] = new Option(val + " - " + arrayNames[i], val, null);
            j++;
        }
    }
}