How to add additional fields to form before submit?

Is there a way to use javascript and JQuery to add some additional fields to be sent from a HTTP form using POST?

I mean:

<form action="somewhere" method="POST" id="form">
  <input type="submit" name="submit" value="Send" />
</form>

<script type="text/javascript">
  $("#form").submit( function(eventObj) {
    // I want to add a field "field" with value "value" here
    // to the POST data

    return true;
  });
</script>

Yes.You can try with some hidden params.

  $("#form").submit( function(eventObj) {
      $("<input />").attr("type", "hidden")
          .attr("name", "something")
          .attr("value", "something")
          .appendTo("#form");
      return true;
  });
 

Try this:

$('#form').submit(function(eventObj) {
    $(this).append('<input type="hidden" name="field_name" value="value" /> ');
    return true;
});

$('#form').append('<input type="text" value="'+yourValue+'" />');

You can add a hidden input with whatever value you need to send:

$('#form').submit(function(eventObj) {
    $(this).append('<input type="hidden" name="someName" value="someValue">');
    return true;
});

This works:

var form = $(this).closest('form');

form = form.serializeArray();

form = form.concat([    {name: "customer_id", value: window.username},    {name: "post_action", value: "Update Information"}]);

$.post('/change-user-details', form, function(d) {
    if (d.error) {
        alert("There was a problem updating your user details")
    } 
});

May be useful for some:

(a function that allow you to add the data to the form using an object, with override for existing inputs, if there is) [pure js]

(form is a dom el, and not a jquery object [jqryobj.get(0) if you need])

function addDataToForm(form, data) {
    if(typeof form === 'string') {
        if(form[0] === '#') form = form.slice(1);
        form = document.getElementById(form);
    }

    var keys = Object.keys(data);
    var name;
    var value;
    var input;

    for (var i = 0; i < keys.length; i++) {
        name = keys[i];
        // removing the inputs with the name if already exists [overide]
        // console.log(form);
        Array.prototype.forEach.call(form.elements, function (inpt) {
             if(inpt.name === name) {
                 inpt.parentNode.removeChild(inpt);
             }
        });
        
        value = data[name];
        input = document.createElement('input');
        input.setAttribute('name', name);
        input.setAttribute('value', value);
        input.setAttribute('type', 'hidden');

        form.appendChild(input);
    }

    return form;
}

Use :

addDataToForm(form, {
    'uri': window.location.href,
     'kpi_val': 150,
     //...
});

you can use it like that too

var form = addDataToForm('myFormId', {
    'uri': window.location.href,
     'kpi_val': 150,
     //...
});

you can add # if you like too ("#myformid").

Expanding upon Khawer's answer, you could refrain from using jQuery - for consistency, eg.
<input type="hidden" name="field_name" value="value" />