Question

Is there a simple way to locally store a JSON value with a TTL in a Firefox SDK add-on?

As far as I can tell, Firefox forces you to use their 'simple-storage' library. I can’t use a third-party library such as jStorage.

Answer 1

No, simple-storage and DOM Storage are completely different things meaning that you cannot use a library like jStorage which is meant for work with DOM Storage.

Then again, storing JSON and implementing TTL is simple enough to implement yourself. For JSON you use JSON.parse and JSON.stringify. For TTL you simply store the TTL values somewhere and look them up when necessary. Something like this:

var AdvancedStorage = {
  _storage: require("simple-storage").storage,

  // Keep TTL data in a special storage field
  _ttlData: null,
  _readTTLData() {
    if (this._storage._ttl)
      this._ttlData = JSON.parse(this._storage._ttl);
    else
      this._ttlData = {};
  },
  _saveTTLData() {
    this._storage._ttl = JSON.stringify(this._ttlData);
  },

  // Special data manipulation functions
  set: function(key, value, options) {
    this._storage[key] = JSON.stringify(value);
    this.setTTL(key, options && "TTL" in options ? options.TTL : 0);
  },

  get: function(key, default) {
    if (!this._storage.hasOwnProperty(key))
      return default;

    // Check whether setting has expired
    if (!this._ttlData)
      this._readTTLData();
    if (this._ttlData.hasOwnProperty(key) && this._ttlData[key] <= Date.now())
      return default;

    return JSON.parse(this._storage[key]);
  },

  deleteKey: function(key) {
    delete this._storage[key];
  },

  // Setting the TTL value
  setTTL(key, ttl) {
    if (!this._ttlData)
      this._readTTLData();
    if (ttl > 0)
      this._ttlData[key] = Date.now() + ttl;
    else
      delete this._ttlData[key];
    this._saveTTLData();
  }
};

I didn’t test this code but this should be pretty much all the code you need to implement this kind of feature.