Add data when form is submitted

Sometimes we want to add extra data to our form and submit them to the server.

We can do this like :

var form = document.querySelector('form');
    form.onsubmit = function(){
    var input = document.createElement('input');
    input.setAttribute('type','hidden');
        input.setAttribute('name', elementName);
        input.setAttribute('value',elementValue);
        form.appendChild(input);
    }