Caching Ajax requests with $.Deferred

gistfile1.js

var counters = {
	promises: {},
	fetch: function(service, url) {
		if (!counters.promises[service]) counters.promises[service] = {};
		var servicePromises = counters.promises[service];

		if (servicePromises[url]) {
			return servicePromises[url];
		}
		else {
			var options = services[service],
				deferred = $.Deferred(),
				jsonUrl = options.counterUrl && options.counterUrl.replace('{PAGE_URL}', url);

			if (typeof options.counter === 'function') {
				options.counter(jsonUrl, deferred);
			}
			else if (options.counterUrl) {
				$.getJSON(jsonUrl)
					.done(function(data) {
						try {
							var number = data;
							if (typeof options.convertNumber === 'function') {
								number = options.convertNumber(data);
							}
							deferred.resolve(number);
						}
						catch (e) {
							throw e;
							deferred.reject(e);
						}
					});
			}

			servicePromises[url] = deferred;
			return deferred.promise();
		}
	}
};