Javascript Validation for all field with Required attribute

Question

I've searched high and low for the answer to this but can't find it anywhere.

I have a form which has the HTML 'required' attributes and it does a fine job of highlighting the fields that need to filled in before submission...or would do, but the system which my form is bolted onto (of which I have no control over) submits the form anyway after a few seconds. It relies on Javascript for it's submission. Therefore I'd like to write a Javascript script to check all fields for a required attribute. Currently I have a script that specifies the fields I want to be mandatory, but if it could look up the attribute instead, that would be brilliant.

Answer 1

In case that input[type=submit] is used, you don't need any JavaScript

<form id="theForm" method="post" acion="">
  <input type="firstname" value="" required />
  <input type="lastname" value="" required />
  <input type="submit" name="submit" value="Submit" />  
</form>

Working jsBin

But if input[type=button] is used for submitting the form, use the snippet below

<form id="theForm" method="post" acion="">
  <input type="firstname" value="" required />
  <input type="lastname" value="" required />
  <input type="button" name="button" value="Submit" />  
</form>

window.onload = function () {
  var form = document.getElementById('theForm');
  form.button.onclick = function (){
    for(var i=0; i < form.elements.length; i++){
      if(form.elements[i].value === '' && form.elements[i].hasAttribute('required')){
        alert('There are some required fields!');
        return false;
      }
    }
    form.submit();
  }; 
};

Wotking jsBin