C# - Create Editable Dropdown List With Dictionary Option Using JavaScript?

Eg:-- When I type "A" all the elements starting with A should be displayed... If "B" then elements with B....etc)

Ex:

<asp:DropDownList ID="ddlLocation" style="width:140px" runat="server">
<asp:ListItem Value="1" >India</asp:ListItem>
<asp:ListItem Value="2" >India - Hyderabad</asp:ListItem>
<asp:ListItem Value="3">South Africa</asp:ListItem>
<asp:ListItem Value="4">Australia</asp:ListItem>
</asp:DropDownList>

javascript :

function DisplayText()
{
var textboxId = '<% = txtText.ClientID %>';
var dropdownListId = '<% = ddlLocation.ClientID %>';
document.getElementById(textboxId).value = document.getElementById(dropdownListId).value;
document.getElementById(textboxId).focus();
}

code behind :

ddlLocation.Attributes.Add("onChange", "DisplayText();");