javascript disable submit button until form is fully validated

1function fieldValidation() {
2
3var name = document.forms['RegForm']['Name'].value;
4var address = document.forms['RegForm']['Address'].value;
5var email = document.forms['RegForm']['EMail'].value;
6var password = document.forms['RegForm']['Password'].value;
7var telephone = document.forms['RegForm']['Telephone'].value;
8var job = document.forms['RegForm']['Job'].value;
9var comment = document.forms['RegForm']['Comment'].value;
10var fullName = /^[a-zA-Z]+ [a-zA-Z]+$/;
11var phnFormat = /((\(\d{3}\) ?)|(\d{3}-))?\d{3}-\d{4}/;
12
13if (name === '') {
14  alert('Please enter your name.'); 
15  return false;
16}
17
18if (!fullName.test(name)) {
19  alert('Please make sure we have your full name.');
20  return false;
21}
22
23if (address === '') { 
24  alert('Please enter your address.'); 
25  return false; 
26} 
27
28if (email === '') { 
29  alert('Please enter your e-mail address.'); 
30  return false; 
31} 
32
33if (password === '') { 
34  alert('Please enter a password.'); 
35  return false; 
36}   
37
38if (telephone === '') { 
39  alert('Please enter your telephone number.'); 
40  return false; 
41}
42
43if (!phnFormat.test(telephone)) {
44  alert('Please enter your phone number in the following format: (123) 555-1212)');
45  return false;
46}
47
48if (job.value === '') { 
49  alert('Please select a job choice.'); 
50  return false; 
51} 
52
53if (comment.value === '') { 
54  alert('Please enter a comment.'); 
55  return false; 
56}
57  return true;
58}