Class providing caching for data objects (e.g. translations).

DataCache.js

/**
 * Class providing caching for data objects.
 *
 * It's assumed that the data comes from some remote service (otherwise there is no point in caching).
 *
 * @param {String} dataKey Globally(!) unique key to store data.
 * @param {Storage} storageService Storage service that should have two functions:
 *		storageService.set(key, value)	-- set data
 *		storageService.get(key)			-- get data
 * 	Will attempt fallback to `$.jStorage` if not secified.
 * @class DataCache
 */
function DataCache(dataKey, storageService) {

	// init storage service
	if (typeof (storageService) !== "object") {
		storageService = $.jStorage;
	}
	this.storage = storageService;
		
	/**
	 * Load limit in seconds.
	 *
	 * After that many seconds we will attempt to refresh data (to be available next time).
	 */
	this.loadLimit = 10 * 60;

	/**
	 * Forced translations reload limit in seconds.
	 *
	 * After that many seconds we will wait for new data even if we have them saved in storage.
	 *
	 * Set to `0` to ignore this limit
	 */
	this.forcedReloadLimit = 30 * 24 * 60 * 60;

	/**
	 * Storage key for data.
	 * @private
	 */
	this.dataStorageKey = 'DataCache.data.' + dataKey;
	/**
	 * Storage key for time.
	 * @private
	 */
	this.timeStorageKey = 'DataCache.time.' + dataKey;

	/**
	 * Data state information.
	 */
	this.dataState = {
		available: false,
		needRefresh: true,
		needReload: true
	};

	/**
	 * The data.
	 */
	this.data;
}

/**
 * Initial check to be done before loading anything.
 * 
 * @returns {Boolean} true if data is available (it's considered unavailable after `forcedReloadLimit` has passed).
 */
DataCache.prototype.check = function() {
	this.dataState = {
		available: false,
		needRefresh: true,
		needReload: true
	};
	if (this.storage.get(this.timeStorageKey) === null) {
		return false;
	}

	var lastLoadTime = this.storage.get(this.timeStorageKey);
	var timeDiffSeconds = Math.abs((new Date()).getTime() - lastLoadTime) / 1000;
	// can preapre saved data if not too old
	if (!this.forcedReloadLimit || timeDiffSeconds < this.forcedReloadLimit) {
		this.data = this.storage.get(this.dataStorageKey);
		this.dataState.available = true;
		this.dataState.needReload = false;
	}
	// should skip loading
	if (timeDiffSeconds < this.loadLimit) {
		this.dataState.needRefresh = false;
	}

	return this.dataState.available;
};

/**
 * Initial check to be done before loading cache.
 */
DataCache.prototype.save = function(data) {
	this.storage.set(this.dataStorageKey, data);
	this.storage.set(this.timeStorageKey, (new Date()).getTime());
};

usage.js

function onData(data) {
	// do whatever with data
}
var cache = new DataCache('MyApp.CachedThings');
if (cache.check()) {
	// immediately recieve data
	onData(cache.data);
	// no need to refresh (for next time) - skip the rest
	if (!cache.dataState.needRefresh) {
		return;
	// will load data, but will not wait for it
	} else {
		onData = function(){};
	}
}

SomeAjaxLib.request({
url: 'example.com/somethind',
success: function(response) {
	cache.save(response.responseText);
	onData(response.responseText);
});