lscache

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.

Methods

The library exposes these methods: set(), get(), remove(), flush(), flushExpired(), bucket(), setExpiryMilliseconds().

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lscache.set

Stores the value in localStorage. Expires after specified number of minutes. Will not expire if time is not set.

Arguments

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

Returns

boolean : True if the value was stored successfully.

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lscache.get

Retrieves specified value from localStorage, if not expired.

Arguments

1. key (string)

Returns

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

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lscache.remove

Removes a value from localStorage.

Arguments

1. key (string)

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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.bucket

Use buckets to make lscache partition data in to different buckets. i.e. Flushing a bucket only removes items in that bucket.

Arguments

1. bucketName (string)

Returns

Bucket : A Bucket instance.

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lscache.setExpiryMilliseconds

Sets the number of milliseconds each time unit represents in the set() function's "time" argument. Sample values:

• 1: each time unit = 1 millisecond
• 1000: each time unit = 1 second
• 60000: each time unit = 1 minute (Default value)
• 3600000: each time unit = 1 hour

Arguments

1. milliseconds (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();

You can also check if local storage is supported in the current browser with lscache.supported():

if (!lscache.supported()) {
  alert('Local storage is unsupported in this browser');
  return;
}

You can enable console warning if set fails with lscache.enableWarnings():

// enable warnings
lscache.enableWarnings(true);

// disable warnings
lscache.enableWarnings(false);

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 want to namespace some storage, so that all items can be flushed at once. Or you have multiple instances of lscache on a page, use the bucket system. Buckets have the same API as the top level lscache (lscache is actually a global bucket).

lscache.set('response', '...', 2);
const bucket = lscache.bucket('mythings');
bucket.set('path', '...', 2);
bucket.flush(); // only removes 'path' which was set in the `mythings` bucket

Bucket returns it's own instance so you can chain one command.

lscache.bucket('mythings').set('path', '...', 2);

The default unit for the set() function's "time" argument is in minutes. A shorter time may be desired, for example, in unit tests. You can use lscache.setExpriryMilliseconds() to select a finer granularity of time unit. This affects all buckets.

asyncTest('Testing set() and get() with different units', function() {´
  var expiryMilliseconds = 1000;  //time units is seconds
  lscache.setExpiryMilliseconds(expiryMilliseconds);
  var key = 'thekey';
  var numExpiryUnits = 2; // expire after two seconds
  lscache.set(key, 'some value', numExpiryUnits);
  setTimeout(function() {
    equal(lscache.get(key), null, 'We expect value to be null');
    start();
  }, expiryMilliseconds * numExpiryUnits + 1);
});

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

Developing

• Run npm install to install all the dependencies.
• Run npm run build to build the library using rollup.
• Run npm test to run the tests.