How to create alert when nothing is selected

This is my html code. It will print the book details along with checkbox option to select a particular book.

  <div id="SearchBook" class="tabcontent">
      <h3 align="center">Select the books</h3>
      <center>
      <form action= "searchbook" method="post" name="search" onsubmit="return searchForm()" >
      <table cellpadding='10' cellspacing='5' border='2'>
      <tr><td><b> Book ID </b></td><td><b> Book Name </b></td><td><b> Quantity </b></td><td><b> Select </b></td></tr>  
      <%
      try{
            Class.forName("org.postgresql.Driver");
            Connection con=DriverManager.getConnection("jdbc:postgresql://localhost:5432/postgres","postgres","*");
            Statement st= con.createStatement();
            ResultSet resultSet = st.executeQuery("select * from librarydata1");
            while(resultSet.next()){
      %>
      <tr><td><b><%= resultSet.getString(1) %></b></td><td><b><%= resultSet.getString(2) %></b></td><td><b><%= resultSet.getString(3) %></b></td><td><b> <input type="checkbox" name="checkselected" value="<%= resultSet.getString(1) %>" id = "checked" ></b></td></tr><br>

    <%
            }
    %>
    </table>
    <br><input type="submit" name="select" value="Select" onclick ="bookselect()" ></form></center>
    <%
      }catch(Exception e){
          out.println(e.getMessage());
      }
    %>
    </div>

I tried this javascript code but it is not working

function searchForm() {
  var x = document.forms["search"]["checkselected"].value;
  if (x.checked==true ) {
    alert("please select atleast one");
    return false;
  }

}

help me out.