Simple object to make Ajax requests

ajax.js

var ajax = (function() {
    'use strict';

    var xmlHttpRequest;

    var states = {
        UNSET: 0,
        OPENED: 1,
        HEADERS_RECEIVED: 2,
        LOADING: 3,
        DONE: 4
    };

    var defaultHeaders = [
        {
            'Content-Type': 'application/x-www-form-urlencoded'
        }
    ];

    function send(url, options) {
        xmlHttpRequest = new XMLHttpRequest();
        options.method = options.method || '';
        options.data = options.data || '';

        xmlHttpRequest.addEventListener('readystatechange', function() {
            if (this.readyState == states.DONE) {
                if (options.callback && typeof options.callback == 'function') {
                    options.callback(xmlHttpRequest);
                }
            }
        });

        xmlHttpRequest.open(options.method, url);
        configure(options.configuration || {});
        xmlHttpRequest.send(options.data || null);
    }

    function addHeaders(headersList) {
        var headerItem,
            index,
            headerValue,
            headerName;
        for (index in headersList) {
            headerItem = headersList[index];
            for (headerName in headerItem) {
                headerValue = headerItem[headerName];
                xmlHttpRequest.setRequestHeader(headerName, headerValue);
            }
        }
    }

    function configure(configuration) {
        var headersList = configuration.headers || defaultHeaders;
        addHeaders(headersList);
        xmlHttpRequest.timeout = configuration.timeout || 0;
        xmlHttpRequest.responseType = configuration.responseType || "";
    }

    return {
        send: send
    };
})();