Validate standard/custom Salesforce web-to-lead forms

// JavaScript Document
function validation(form) {


if(form.first_name.value == '') {

alert('Please enter your first name');
form.first_name.focus();
return false;
}

if(form.last_name.value == '') {

alert('Please enter your last name');
form.last_name.focus();
return false;
}

if(form.email.value == '') {

alert('Please enter your email address');
form.email.focus();
return false;
}

if(form.company.value == '') {

alert('Please enter your company');
form.company.focus();
return false;
}

var chooselocation = document.getElementById('00ND0000004qjQL');
if(chooselocation.value == '') {
  alert('Please select a location');
  chooselocation.focus();
  return false;
}

var comment = document.getElementById('00ND0000004oVyD');
if(comment.value == '') {
  alert('Please enter a message');
  comment.focus();
  return false;
}

return true;
}