Simple ajax form using javascript no jQuery

The following is a far more elegant solution of the other answer, more fit for modern browsers.

My reasoning is that if you need support for older browser you already most likely use a library like jQuery, and thus making this question pointless.

/**
 * Takes a form node and sends it over AJAX.
 * @param {HTMLFormElement} form - Form node to send
 * @param {function} callback - Function to handle onload. 
 *                              this variable will be bound correctly.
 */

function ajaxPost (form, callback) {
    var url = form.action,
        xhr = new XMLHttpRequest();

    //This is a bit tricky, [].fn.call(form.elements, ...) allows us to call .fn
    //on the form's elements, even though it's not an array. Effectively
    //Filtering all of the fields on the form
    var params = [].filter.call(form.elements, function(el) {
        //Allow only elements that don't have the 'checked' property
        //Or those who have it, and it's checked for them.
        return typeof(el.checked) === 'undefined' || el.checked;
        //Practically, filter out checkboxes/radios which aren't checekd.
    })
    .filter(function(el) { return !!el.name; }) //Nameless elements die.
    .filter(function(el) { return el.disabled; }) //Disabled elements die.
    .map(function(el) {
        //Map each field into a name=value string, make sure to properly escape!
        return encodeURIComponent(el.name) + '=' + encodeURIComponent(el.value);
    }).join('&'); //Then join all the strings by &

    xhr.open("POST", url);
    // Changed from application/x-form-urlencoded to application/x-form-urlencoded
    xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");

    //.bind ensures that this inside of the function is the XHR object.
    xhr.onload = callback.bind(xhr); 

    //All preperations are clear, send the request!
    xhr.send(params);
}

The above is supported in all major browsers, and IE9 and above.

Here's a nifty function I use to do exactly what you're trying to do:

HTML:

<form action="/cs/Satellite">...</form>
<input type="button" value="Vote now" onclick="javascript:AJAXPost(this)">

JS:

function AJAXPost(myself) {
    var elem   = myself.form.elements;
    var url    = myself.form.action;    
    var params = "";
    var value;

    for (var i = 0; i < elem.length; i++) {
        if (elem[i].tagName == "SELECT") {
            value = elem[i].options[elem[i].selectedIndex].value;
        } else {
            value = elem[i].value;                
        }
        params += elem[i].name + "=" + encodeURIComponent(value) + "&";
    }

    if (window.XMLHttpRequest) {
        // code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp=new XMLHttpRequest();
    } else { 
        // code for IE6, IE5
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
    }

    xmlhttp.open("POST",url,false);
    xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    xmlhttp.setRequestHeader("Content-length", params.length);
    xmlhttp.setRequestHeader("Connection", "close");
    xmlhttp.send(params);

    return xmlhttp.responseText;
}

Nowadays using FormData is the easiest method. You construct it with a reference to the Form element, and it serializes everything for you.

MDN has an example of this here -- roughly:

const form = document.querySelector("#debarcode-form");
form.addEventListener("submit", e => {
    e.preventDefault();
    const fd = new FormData(form);
    const xhr = new XMLHttpRequest();
    xhr.addEventListener("load", e => {
        console.log(e.target.responseText);
    });
    xhr.addEventListener("error", e => {
        console.log(e);
    });
    xhr.open("POST", form.action);
    xhr.send(fd);
});

and if you want it as an object (JSON):

const obj = {};
[...fd.entries()].forEach(entry => obj[entry[0]] = entry[1]);

Expanding on Madara's answer: I had to make some changes to make it work on Chrome 47.0.2526.80 (not tested on anything else). Hopefully this can save someone some time.

This snippet is a modification of that answer with the following changes:

• filter !el.disabled,
• check type of input before excluding !checked
• Request type to x-www-form-urlencoded

With the following result:

function ajaxSubmit(form, callback) {
    var xhr = new XMLHttpRequest();
    var params = [].filter.call(form.elements, function (el) {return !(el.type in ['checkbox', 'radio']) || el.checked;})
    .filter(function(el) { return !!el.name; }) //Nameless elements die.
    .filter(function(el) { return !el.disabled; }) //Disabled elements die.
    .map(function(el) {
        return encodeURIComponent(el.name) + '=' + encodeURIComponent(el.value);
    }).join('&'); //Then join all the strings by &
    xhr.open("POST", form.action);
    xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    xhr.onload = callback.bind(xhr);
    xhr.send(params);
};