Simple in-memory JavaScript object cache with a maxAge per key (in seconds) and maxEntries limit (per cache instance).

memory_cache.js

var MemoryCache = function (options) {
  options = $.extend(true, {
    maxEntries: null
  }, options || {});

  var self    = this;
  var cache   = {};
  var entries = 0;

  var ageOf = function (entry) {
    return Math.round((new Date() - entry.setAt) / 1000);
  };

  var deleteEntryByKey = function (key) {
    delete cache[key];
    if (entries > 0) entries--;
  };

  var deleteOldestEntry = function () {
    var keyOfOldestItem, dateOfOldestItem;
    for (var key in cache) {
      if (!dateOfOldestItem || dateOfOldestItem > cache[key].setAt) {
        dateOfOldestItem = cache[key].setAt;
        keyOfOldestItem  = key;
      }
    }

    if (keyOfOldestItem) {
      deleteEntryByKey(keyOfOldestItem);
    }
  };

  self.set = function (key, value, maxAge) {
    if (cache[key]) {
      cache[key].value  = value;
      cache[key].setAt  = new Date();
      cache[key].maxAge = maxAge || cache[key].maxAge;
    } else {
      if (options.maxEntries && entries >= options.maxEntries) {
        deleteOldestEntry();
      }

      cache[key] = {
        value:  value,
        setAt:  new Date(),
        maxAge: maxAge
      };

      entries++;
    }
  };

  self.get = function (key) {
    var entry = cache[key];

    if (entry) {
      if (entry.maxAge && entry.maxAge <= ageOf(entry)) {
        deleteEntryByKey(key);
      }

      return entry.value;
    }
  };
};