How to convert Jsoup connect object code into XMLHttpRequest object javascript?

Question

I am new to learning AJAX. My question is: How would you connect to a website using Javascript instead of Java? In Java, you could use the JSoup library and the following code.

Response res = Jsoup.connect(EDLINE_URL + "/InterstitialLogin.page").timeout(0).userAgent("Chrome/12.0.742.122").execute();

    Map<String, String> preLogCookies = res.cookies();
    System.out.println("Pre Login Cookies: " + preLogCookies);

    res = Jsoup.connect(EDLINE_URL + "/post/InterstitialLogin.page")
            .data("TCNK", "authenticationEntryComponent", 
                    "submitEvent", "1",
                    "guestLoginEvent", "",
                    "enterClicked", "false",
                    "bscf", "",
                    "bscv", "",
                    "targetEntid", "",
                    "ajaxSupported", "yes",
                    "screenName", username,
                    "kclq", password)
                    .cookies(preLogCookies)
                    .userAgent("Chrome/12.0.742.122")
                    .method(Method.POST)
                    .timeout(0)
                    .execute();

    loginCookies = res.cookies();
    loginCookies.putAll(preLogCookies);
    System.out.println("Login Cookies: " + loginCookies);

    System.out.println("Connected to: " + res.url());
    if (res.url().toString().contains("Notification.page")) {
        System.out.println("Username and Password Incorrect");
        System.exit(1);
    }

    return res;
}

Obviously, you can't use Jsoup in javascript. So, how could you achieve the same connection? What I am trying to do is provided a "username" and a "password" enter that into a website and then access the data provided in the user-specific page. I think that you would use an XMLHttpRequest object but I am not sure how to 'feed' the information into the object/what to do with the object. For reference to how I think it could be done:

var clientId = "MyApp";
var clientSecret = "MySecret";

var authorizationBasic = $.base64.btoa(clientId + ':' + clientSecret);

var request = new XMLHttpRequest();
request.open('POST', oAuth.AuthorizationServer, true);
request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8');
request.setRequestHeader('Authorization', 'Basic ' + authorizationBasic);
request.setRequestHeader('Accept', 'application/json');
request.send("username=John&password=Smith&grant_type=password");

request.onreadystatechange = function () {
    if (request.readyState == 4) {
    alert(request.responseText);
    }
};

This code is 'irrelevant' and sample from a website...but could prove valuable in understanding what I am trying to do...I don't understand what is happening or how I could modify it though. Any help would be wonderful!