AJAX Database: The JavaScript File
Book information will be listed here...
When a user selects a book in the dropdown list above, a function called showBook as below is executed. The showBook function does the following:
  1. Check if a book is selected.

  2. Create an XMLHttpRequest object.

  3. Create the function to be executed when the server response is ready.

  4. Send the request off to a file on the server.

  5. A parameter (q) with the content of the dropdown list is added to the URL. 
function showBook( str ) {
  if ( str == "" ) {
    document.getElementById("txtHint").innerHTML = "";
    return;
  }
  if ( window.XMLHttpRequest ) {
    // code for IE7+, Firefox, Chrome, Opera, Safari
    xmlhttp = new XMLHttpRequest( );
  }
  else {
    // code for IE6, IE5
    xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
  }
  xmlhttp.onreadystatechange = function( ) {
    if ( ( xmlhttp.readyState == 4 ) && 
         ( xmlhttp.status     == 200 ) ) {
      document.getElementById("txtHint").innerHTML = 
        xmlhttp.responseText;
    }
  }
  xmlhttp.open( "GET", "GetBook.php?q="+str, true );
  xmlhttp.send( );
}