function xhr(options, cb) {
    if(!options) {
        throw '[ajax] missing required options';
    }

    var url = options.url ? options.url : window.location.href;
    var method = options.method ? options.method.toUpperCase() : 'GET';
    var data = options.data ? JSON.stringify(options.data) : null;
    var contentType = (typeof options.contentType !== 'undefined') ? options.contentType : null;
    var headers = (options.headers && typeof options.headers === 'object') ? options.headers : {};
    var username = options.username ? options.username : null;
    var password = options.password ? options.password : null;
    var timeout = options.timeout ? options.timeout : 0;
    var withCredentials = (typeof options.withCredentials !== 'undefined') ? options.withCredentials : false;
    var async = (typeof options.async !== 'undefined') ? options.async : true;
    var onError = (options.onError && typeof options.onError === 'function') ? options.onError : null;
    var onSuccess = (options.onSuccess && typeof options.onSuccess === 'function') ? options.onSuccess : null;
    var onTimeout = (options.onTimeout && typeof options.onTimeout === 'function') ? options.onTimeout : null;

    var _xhr = new XMLHttpRequest();
    _xhr.open(method, url, async, username, password);

    // can only set a timeout on asynchronous requests
    if(async) {
        _xhr.timeout = timeout;
    }

    if(withCredentials) {
        _xhr.withCredentials = withCredentials;
    }

    if(contentType) {
        _xhr.setRequestHeader('Content-Type', contentType);
    } else if(contentType !== false && method === 'POST') {
        _xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8');
    }

    for (var key in headers) {
        _xhr.setRequestHeader(key, headers[key]);
    }

    _xhr.onreadystatechange = function() {
        if (this.readyState === 4) {
            if (this.status >= 200 && this.status < 400) {
                if(onSuccess) {
                    onSuccess(this.responseText, this.status, this);
                }
            } else {
                if(onError) {
                    onError(this, this.status);
                }
            }
        }
    };

    if(onTimeout) {
        _xhr.ontimeout = onTimeout;
    }

    _xhr.send(data);
    _xhr = null;
}

xhr({
    url: "http://foo.bar/customers"
}

xhr({
    url: "http://foo.bar/customers",
    method: "POST",
    data: 'foo=bar'
}

xhr({
    url: "http://foo.bar/customers",
    method: "POST",
    contentType: "application/json; charset=UTF-8"
    data: { foo: "bar" },
    onSuccess: function(response, statusCode, xhr) {
    }
    onError: function(xhr, statusCode) {
    },
    onTimeout: function(event) {
    }
}