// A generic function for checking to see if an input element has
  // had information entered into it
  function checkRequired( elem ) {
      if ( elem.type == "checkbox" || elem.type == "radio" )
          return getInputsByName( elem.name ).numChecked;
      else
          return elem.value || elem.value == elem.defaultValue;
  }
  
  // Find all input elements that have a specified name (good for finding
  // and dealing with checkboxes or radio buttons)
  function getInputsByName( name ) {
      // The array of input elements that will be matched
      var results = [];
      // Keep track of how many of them were checked
      results.numChecked = 0;
  
      // Find all the input elements in the document
      var input = document.getElementsByTagName("input");
      for ( var i = 0; i < input.length; i++ ) {
          // Find all the fields that have the specified name
          if ( input[i].name == name ) {
              // Save the result, to be returned later
              results.push( input[i] );
  
              // Remember how many of the fields were checked
              if ( input[i].checked )
                  results.numChecked++;
          }
      }
  
      // Return the set of matched fields
      return results;
  }
  
  // Get an input element to check
  var elem = document.getElementById("age");
  
  // Make sure that the required age field has been checked
  if ( ! checkRequired( elem ) ) {
      alert( "Required field is empty – you must be over 13 to use this site." );
  }
  
  // Get an input element to check
  var elem = document.getElementById("name");
  
  // Make sure that some text has been entered into the name field
  if ( ! checkRequired( elem ) ) {
      alert( "Required field is empty – please provide your  name." );
  }