Web-to-Lead with JavaScript Spam Countermeasure

<html>
<head>
  <!--  ----------------------------------------------------------------------  -->
  <!--  NOTE: Please add the following <META> element to your page <HEAD>.      -->
  <!--  If necessary, please modify the charset parameter to specify the        -->
  <!--  character set of your HTML page.                                        -->
  <!--  ----------------------------------------------------------------------  -->

  <META HTTP-EQUIV="Content-type" CONTENT="text/html; charset=UTF-8">
  <script>
    /*
     * Function created specifically to set the Organization ID
     * pseudo-dynamically to prevent simple spam
     */
    function setOid() {
      document.getElementsByName("oid")[0].value = "YOUR_ORGANIZATION_ID";
    }
  </script>
</head>
<body>
  <!--  ----------------------------------------------------------------------  -->
  <!--  NOTE: Please add the following <FORM> element to your page.             -->
  <!--  NOTE: The form was modified to include an onsubmit attribute            -->
  <!--  ----------------------------------------------------------------------  -->

  <form action="https://www.salesforce.com/servlet/servlet.WebToLead?encoding=UTF-8" method="POST" onsubmit="setOid()">

  <!--  ----------------------------------------------------------------------  -->
  <!-- NOTE: This field value was removed for demo purposes, because we are     -->
  <!-- replacing it with a blank value that is instead set when the user        -->
  <!-- submits the form                                                         -->
  <!-- <input type=hidden name="oid" value="YOUR_ORGANIZATION_ID">                   -->
  <!--  ----------------------------------------------------------------------  -->
  <input type=hidden name="oid">
  <input type=hidden name="retURL" value="http://salesforce.com">

  <!--  ----------------------------------------------------------------------  -->
  <!--  NOTE: These fields are optional debugging elements. Please uncomment    -->
  <!--  these lines if you wish to test in debug mode.                          -->
  <!--  <input type="hidden" name="debug" value=1>                              -->
  <!--  <input type="hidden" name="debugEmail" value="martyc@slalom.com">       -->
  <!--  ----------------------------------------------------------------------  -->

  <label for="first_name">First Name</label><input  id="first_name" maxlength="40" name="first_name" size="20" type="text" /><br>

  <label for="last_name">Last Name</label><input  id="last_name" maxlength="80" name="last_name" size="20" type="text" /><br>

  <label for="email">Email</label><input  id="email" maxlength="80" name="email" size="20" type="text" /><br>

  <label for="company">Company</label><input  id="company" maxlength="40" name="company" size="20" type="text" /><br>

  <input type="submit" name="submit">

  </form>
</body>
</html>