Asp.net error: server tag not well formed at for (var I = 1; statement

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ASP.NET

function changeSystemDDLIndex() {
            // Select the Site in SitesDDL = CustomerSiteIdOfSystem
            // Save the System ID into a hidden variable
            var systemsDDL = document.getElementById('ddlSystems');
            var system = systemsDDL.options[systemsDDL.selectedIndex].value;
            var sitesDDL = document.getElementById('ddlSites');
            var hidCustomerSiteId = document.getElementById("hdnCustomerSiteId");
            var hidCustomerSystemId = document.getElementById("hdnCustomerSystemId");

            if (systemsDDL.options[systemsDDL.selectedIndex].value == '') {
                // if System = N/A, Customer_Site_Id = '' then SitesDDL.SelectedIndex = 0 (N/A)
                sitesDDL.options[0].selected = true;

                // Set new values of hidden variables
                hidCustomerSiteId.value = '';
                hidCustomerSystemId.value = '';
            }
            else {
                var isFound = false;
                var CustomerSiteIdOfSystem = systemsDDL.options[systemsDDL.selectedIndex].attributes['Customer_Site_Id'].value;

                // Find the Customer_Site_Id in SitesDDL options and set it
                for (var i = 1; i < sitesDDL.options.length; i++) {
                    if (sitesDDL.options[i].value == CustomerSiteIdOfSystem) {
                        sitesDDL.options[i].selected = true;

                        isFound = true;
                    }
                    else {
                        sitesDDL.options[i].selected = false;
                    }
                }

                if (isFound == false) {
                    sitesDDL.options[0].selected = true;
                    sitesDDL.options[0].disabled = false;
                }

                // Set new values of hidden variables
                hidCustomerSiteId.value = CustomerSiteIdOfSystem;
                hidCustomerSystemId.value = system;
            }
            return;
        }

What I have tried:

I tried differet simple for statements. I tried using different .Net targetFramework. I also tried to add AJAX extension, ScriptManager.