getFBPageId.js

(function () {
    var fbPage = function () {
        var that = {},
            _paths = location.pathname.split('/');
        var hasUserName = function () {
            return _paths.length == 2;
        };
 
        var getId = function () {
            if (hasUserName()) {
                var req = new XMLHttpRequest();
                req.onload = function () {
                    prompt("このページの ID を表示します。", 
						JSON.parse(this.responseText).id);
                };
                req.open("GET", "https://graph.facebook.com" + location.pathname);
                req.send();
            } else {
                prompt("このページの ID を表示します。",
					_paths[3]);
            }
        };
        that.getId = getId;
		
        return that;
    };
    var fb = fbPage().getId();
})();