a localStorage wrapper with time-to-live and JSON stringify/parsing

store.js

/**
 * store.js
 * a localStorage wrapper with time-to-live and JSON stringify/parsing
 * 
 * @author Matt Shwery
 * @license MIT (http://www.opensource.org/licenses/mit-license)
 */
store = (function () {

  // polyfill for localstorage into stale storage so this data store doesnt ever fail
  if (!('localStorage' in window)) {
    //if (!Modernizr.localstorage) {
    window.localStorage = {
      _data: {},
      setItem: function (id, val) { return this._data[id] = String(val); },
      getItem: function (id) { return this._data.hasOwnProperty(id) ? this._data[id] : undefined; },
      removeItem: function (id) { return delete this._data[id]; },
      clear: function () { return this._data = {}; }
    };
  }

  var PREFIX = 's';

  // prefixes the key with the store.js prefix
  function prefix(key) {
    return PREFIX + ':' + key;
  }
  
  // returns the current time in milliseconds
  function now() {
    return +new Date();
  }

  // returns the value associated with the given key, if it exists
  // returns null if the key doesn't exist
  function getItem(key) {
    var item = localStorage.getItem(key);

    if (item) {
      try {
        item = JSON.parse(item);
      } catch (err) {
        console.warn(err);
      }
    }

    return item;
  }

  // saves a new key/value pair if the key doesn't exist, otherwise
  // updates the value
  function setItem(key, value) {
    var valueString = value;
    
    try {
      valueString = JSON.stringify(valueString);
    } catch (err) {
      console.warn(err);
    }

    // try to set the item in localStorage        
    // if the attempt failed then this block executes
    if (!trySetItem(key, valueString)) {
      // try to remove expired stuff
      removeExpiredItems();
      
      // try again and return the item regardless of failure
      trySetItem(key, valueString);
    }
    
    // either way, return the original value, not the stringified one
    return value;
  }
  
  function trySetItem(key, value) {
    try {
      localStorage.setItem(key, value);
      return true;
    } catch (e) {
      return false;
    }
  }
  
  // removes expired items from cache
  function removeExpiredItems() {
    // get localStorage keys as array
    // filter out only ones that include our localStorage PREFIX
    var items = Object.keys(localStorage).filter(belongsToCache);

    items.forEach(getItemOrExpire);
  }
      
  function getItemOrExpire(key) {
    var item = getItem(key);
    
    // if there's a expired ttl, remove the item
    if (item && item.ttl && ttlIsExpired(item.ttl)) {
      return removeItem(key);    
    }

    return item;
  }
  
  function ttlIsExpired(ttl) {
    return ttl < now();
  }
  
  function belongsToCache (key) {
    return key.indexOf(PREFIX) !== -1;    
  }

  // accepts optional ttl (in minutes)
  function set(key, value, ttl) {
    // add prefix to key 
    key = prefix(key);

    // convert ttl from minutes to milleseconds
    if (ttl != null) ttl = now() + (ttl * 60000);

    // grab any previous items by this key
    var oldItem = getItem(key);

    // if the old item's ttl isn't expired and there's no ttl passed in use the old ttl
    // this allows you to continue to update an object with new values without updating its expiry
    // TODO: evaluate need for this
    if (!ttl && oldItem && oldItem.ttl && oldItem.ttl >= now()) {
      ttl = oldItem.ttl;
    }

    var item = {
      value: value,
      ttl: ttl
    };

    return setItem(key, item);
  }

  function get(key) {
    // add fs and user id prefix to key 
    key = prefix(key);

    var item = getItemOrExpire(key);

    // return the retrieved item's value if it exists,
    // or if it was removed return the removal's return (undefined)
    return (item && item.value) || item;
  }

  // remove an item from local storage
  function removeItem(key) {
    return localStorage.removeItem(key);
  }

  function remove(key) {
    return removeItem(prefix(key));
  }

  // delete all key/value pairs currently stored
  function clear() {
    // because we only want to delete stuff from localStorage that this function added,
    // loop through them one by one and remove them if they have this function's PREFIX
    for (var key in localStorage) {
      // remove only cached stuff that contains our prefix
      if (belongsToCache(key)) removeItem(key);
    }
  }

  return {
    get: get,
    set: set,
    remove: remove,
    clear: clear
  };
});