how to select gridview textbox value using javascript

<asp:GridView ID="dgSecurityDisplay" runat="server">
                              <Columns>
 <asp:TemplateField HeaderText="Mature/Mortgage Date">
                 <ItemTemplate>
   <asp:Label ID="lblDocDate" runat="server" Text='<%#Eval("docmatdate")%>'></asp:Label>
                                      </ItemTemplate>
                                  </asp:TemplateField>
                                  <asp:TemplateField HeaderText="Alloted">
                                      <ItemTemplate>
                                          <asp:TextBox ID="txtAlloted" runat="server" Text='<%# Eval("assigned") %>'></asp:TextBox>
                                      </ItemTemplate>
                                  </asp:TemplateField>
                                  <asp:TemplateField HeaderText="Assigned">
                                      <ItemTemplate>
                                          <asp:CheckBox ID="chkAssignedvalue" runat="server" />
                                      </ItemTemplate>
                                  </asp:TemplateField>
                                  <asp:TemplateField HeaderText="Available">
                                      <ItemTemplate>
                                          <asp:Label ID="lblAvailable" runat="server" Text=""></asp:Label>
                                      </ItemTemplate>
                                      <EditItemTemplate>
                                          <asp:TextBox ID="txtAvailable" runat="server" Text=""></asp:TextBox>
                                      </EditItemTemplate>
                                  </asp:TemplateField>
                              </Columns>
                          </asp:GridView>

<script type="text/javascript">
function fillAvailableSecurity() 
{
    var grid1 = document.getElementById("<%= dgSecurityDisplay.ClientID %>");
    for (i = 1; i < grid1.rows.length ; i++) 
    {
        var cellValue = grid1.rows[i].cells[2].value;
                  alert(cellValue);
               
    }
}
</script>