Listbox selectindexchange not showing proper index value

[kunalDASGUPTA]

I am using visual basic 2017 for a web application, I am using a listbox and I write a code for searching data and if I click on another index item (rather than last search)
listbox selectindexchange not showing last clicked index. my code are like that pleas tell me where I am wrong

<asp:TextBox ID="TextBox21" runat="server" Onkeyup ="FilterItems(this.value)"  AutoPostBack ="True"
 
function FilterItems(value) {
 
ddl = document.getElementById("<%=ListBox1.ClientID %>");
 
        j1 = 0;
        j = 1;
        ok = 0;
        if (ddl1 != null) 
        {
    for (i = 1; i < ddl1.options.length; i++) {
                ok = 0;
 
                if(value.localeCompare((ddl1.options[i].innerHTML.substr(0, value.length))) !== 1) {
                    if (value !== "") {
                        ok = 1;
                        j = i;
 
         ddl1.options[i].scrollIntoView(true);
        ddl1.options[i].selected = true;
        i = ddl1.options.length;
        return false;
 
 
                    }
                }
            }
         }
           }