grey out submit button until form filled out

Question

I have the following form:

Answer 1

If you are a beginner of javascript, a better option would be

Add HTML as follows

   First name: 
   

Now add validateForm() as

function validateForm() {
   // Validate all fields in the form
   return true; // if all form entries are valid. else return false
}

Note: This will not grey-out the submit button. But in this case submit will work only if all the entries are valid.