lscache-extra

This is a simple library that emulates memcache functions using HTML5 localStorage, so that you can cache data on the client and associate an expiration time with each piece of data. If the localStorage limit (~5MB) is exceeded, it tries to create space by removing the items that are closest to expiring anyway. If localStorage is not available at all in the browser, the library degrades by simply not caching and all cache requests return null.

Additional Functionality

This library is a fork of @pamelafox's lscache library, with a little extra functionality exposed. Namely, the skipRemove and allowExpired parameters for get() and the isExpired() and setExpiryUnitMs() functions.

Methods

The library exposes 8 methods: set(), get(), isExpired(), setExpiryUnitMs(), remove(), flush(), flushExpired(), and setBucket().

---

lscache.set

Stores the value in localStorage. Expires after specified number of minutes.

Arguments

1. key (string)
2. value (Object|string)
3. time (number: optional)

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lscache.get

Retrieves specified value from localStorage, if not expired.

Arguments

1. key (string)
2. skipRemove (boolean) Don't remove the item if it is found to be expired [Default: false]
3. allowExpired (boolean) Allow returning of expired values [Default: false]

Returns

string | Object : The stored value. If no value is available, null is returned.

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lscache.isExpired

Returned whether or not the given key is expired

Arguments

1. key (string)

Returns

boolean : Whether or not the cached value is expired

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lscache.setExpiryUnitMs

Change the units used for cache expiration. Default is minutes (60 * 1000).

Note: this flushes the lscache as well if the units differ from what was previously used, to ensure that no prior data, using a different unit, remains in an invalid cache state

Arguments

1. ms (number)

---

lscache.remove

Removes a value from localStorage.

Arguments

1. key (string)

---

lscache.flush

Removes all lscache items from localStorage without affecting other data.

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lscache.flushExpired

Removes all expired lscache items from localStorage without affecting other data.

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lscache.setBucket

Appends CACHE_PREFIX so lscache will partition data in to different buckets

Arguments

1. bucket (string)

---

lscache.setExpiryUnitMs

Change the units used for cache expiration. Default is minutes (60 * 1000).

Note: this flushes the lscache as well if the units differ from what was previously used, to ensure that no prior data, using a different unit, remains in an invalid cache state

Arguments

1. ms (number)

Usage

The interface should be familiar to those of you who have used memcache, and should be easy to understand for those of you who haven't.

For example, you can store a string for 2 minutes using lscache.set():

lscache.set('greeting', 'Hello World!', 2);

You can then retrieve that string with lscache.get():

alert(lscache.get('greeting'));

You can remove that string from the cache entirely with lscache.remove():

lscache.remove('greeting');

You can remove all items from the cache entirely with lscache.flush():

lscache.flush();

You can remove only expired items from the cache entirely with lscache.flushExpired():

lscache.flushExpired();

The library also takes care of serializing objects, so you can store more complex data:

lscache.set('data', {'name': 'Pamela', 'age': 26}, 2);

And then when you retrieve it, you will get it back as an object:

alert(lscache.get('data').name);

If you have multiple instances of lscache running on the same domain, you can partition data in a certain bucket via:

lscache.set('response', '...', 2);
lscache.setBucket('lib');
lscache.set('path', '...', 2);
lscache.flush(); //only removes 'path' which was set in the lib bucket

For more live examples, play around with the demo here: http://pamelafox.github.com/lscache/demo.html

Real-World Usage

This library was originally developed with the use case of caching results of JSON API queries to speed up my webapps and give them better protection against flaky APIs. (More on that in this blog post)

For example, RageTube uses lscache to fetch Youtube API results for 10 minutes:

var key = 'youtube:' + query;
var json = lscache.get(key);
if (json) {
  processJSON(json);
} else {
  fetchJSON(query);
}

function processJSON(json) {
  // ..
}

function fetchJSON() {
  var searchUrl = 'http://gdata.youtube.com/feeds/api/videos';
  var params = {
   'v': '2', 'alt': 'jsonc', 'q': encodeURIComponent(query)
  }
  JSONP.get(searchUrl, params, null, function(json) {
    processJSON(json);
    lscache.set(key, json, 10);
  });
}

It does not have to be used for only expiration-based caching, however. It can also be used as just a wrapper for localStorage, as it provides the benefit of handling JS object (de-)serialization.

For example, the QuizCards Chrome extensions use lscache to store the user statistics for each user bucket, and those stats are an array of objects.

function initBuckets() {
  var bucket1 = [];
  for (var i = 0; i < CARDS_DATA.length; i++) {
    var datum = CARDS_DATA[i];
    bucket1.push({'id': datum.id, 'lastAsked': 0});
  }
  lscache.set(LS_BUCKET + 1, bucket1);
  lscache.set(LS_BUCKET + 2, []);
  lscache.set(LS_BUCKET + 3, []);
  lscache.set(LS_BUCKET + 4, []);
  lscache.set(LS_BUCKET + 5, []);
  lscache.set(LS_INIT, 'true')
}

Browser Support

The lscache library should work in all browsers where localStorage is supported. A list of those is here: http://www.quirksmode.org/dom/html5.html