Json fetcher in JavaScript

JsonFetcher.js

function JsonFetcher(url) {
    this.url = url;
    this.request = new XMLHttpRequest();
    this.request.addEventListener("readystatechange",
                    goog.bind(this._onReadyStateChange, this));
}

JsonFetcher.prototype = {
    url: "",
    request: null,
    callback: null,

    _onReadyStateChange: function JsonFetcher$_onReadyStateChange() {
        if(this.request.readyState == readyStates.DONE)
        {
            var jsonobject = goog.json.parse(this.request.responseText);
            this.callback(jsonobject);
        }
    },

    fetch: function JsonFetcher$fetch(callback) {
        this.callback = callback;
        this.request.open("GET", this.url);
        this.request.send();
    },
};